Lectrosonics DMTH4 User manual

DMTH4
Digital Telephone Hybrid
REFERENCE MANUAL
Rio Rancho, NM, USA
www.lectrosonics.com
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DMTH4
LECTROSONICS, INC.
2

Digital Telephone Hybrid
Rio Rancho, NM 3
Important Safety Instructions
This symbol, wherever it appears, alerts you
to the presence of uninsulated dangerous
voltage inside the enclosure -- voltage that
may be sufficient to constitute a risk of shock.
This symbol, wherever it appears, alerts you
to important operating and maintenance
instructions in the accompanying literature.
Please read the manual.
When using your telephone equipment, basic safety
precautions should always be followed to reduce the risk
of fire, electrick shock and injury to persons, including
the following:
1) Read these instructions.
2) Keep these instructions.
3) Heed all warnings.
4) Follow all instructions.
5) Do not use this apparatus near water.
6) Clean only with a dry cloth.
7) Do not block any ventilation openings. Install in ac-
cordance with the manufacturer’s instructions.
8) Do not install near any heat sources such as ra-
diators, heat registers, stoves, or other apparatus
(including amplifiers) that produce heat.
9) Do not defeat the safety purpose of the polarized
or grounding-type plug. A polarized plug has two
blades with one wider than the other. A grounding
type plug has two blades and third grounding prong.
The wider blade or the third prong are provided for
your safety. If the provided plug does not fit into your
outlet, consult an electrician for replacement of the
obsolete outlet.
10) Protect the power cord from being walked on or
pinched particularly at plugs, convenience recepta-
cles, and the point where they exit from the appara-
tus.
11) Only use attachments/accessories specified by the
manufacturer.
12) Use only with the cart, stand,
tripod, bracket, or table specified
by the manufacturer, or sold with
the apparatus. When a cart is
used, use caution when moving
the cart/apparatus combination to
avoid injury from tip-over.
13) Unplug this apparatus during
lightning storms or when unused for long periods of
time.
14) Refer all servicing to qualified service personnel.
Servicing is required when the apparatus has been
damaged in any way, such as power-supply cord or
plug is damaged, liquid has been spilled or objects
have fallen into the apparatus, the apparatus has
been exposed to rain or moisture, does not operate
normally, or has been dropped.
15) WARNING -- TO REDUCE THE RISK OF FIRE OR
ELECTRIC SHOCK, DO NOT EXPOSE THIS AP-
PARATUS TO RAIN OR MOISTURE.
16) The AC mains plug, or appliance coupler shall be
readily available to the operator as a means of
power disconnection, if applicable.
17) Unit shall be connected to a MAINS socket outlet
with a protective earthing connection.
18) Do not use this product near water for example, near
a bathtub, washbowl, kitchen sink or laundry tub, in
a wet basement or near a swimming pool.
19) Avoid using a telephone (other than a cordless type)
during an electrical storm. There may be a remote
risk of electric shock from lightning.
20) Do not use the telephone to report a gas leak in the
vicinity of the leak.
21) Use only the power cord and batteries indicated in
this manual. Do not dispose of batteries in a fire.
They may explode. Check with local codes for pos-
sible special disposal instructions.
22) “CAUTION: To reduce the risk of fire, use only No.
26 AWG or larger (e.g., 24 AWG) UL Listed or CSA
Certified Telecommunication Line Cord”
SAVE THESE INSTRUCTIONS

DMTH4
LECTROSONICS, INC.
4
Introduction
Lectrosonics is proud to introduce the DMTH4, our most advanced digital telephone hybrid. The DMTH4 is an integral
part of our DM Series of digital audio processors and interfaces easily with a standard telephone line to add telecon-
ferencing capability to any sound system. Sophisticated featues, fast setup, easy operation and high sound quality
make the DMTH4 an ideal choice for board rooms, conference rooms, talk radio and countless other applications.
Product Highlights
 •Built-inDSP-basedacousticechocancellerfortelephoneandcodecinterfaces
•Internallystoredphonebookholdsupto32dialstrings
•DTMFdialingcapability
•Simultaneous2-wire/4-wirebridgingfor3-siteconferencing
•Serialinterfaceforintegrationwithcontrolsystems
•Automatictelephoneansweranddisconnect
•AUXinterfacecanbeusedforautomatictaperecording

Digital Telephone Hybrid
Rio Rancho, NM 5
Table of Contents
Important Safety Instructions................................................. 3
Introduction ............................................................................ 4
Product Highlights.................................................................. 4
Echo and Echo Cancellation .................................................. 6
ERL........................................................................................ 6
ERLE...................................................................................... 6
Return Loss Enhancement .................................................... 6
General Overview .................................................................... 8
Digital Matrix .......................................................................... 9
LecNet2 Software .................................................................. 10
Input Processing ................................................................... 10
Input Gain ............................................................................ 10
Filters ................................................................................... 10
Input Compressor ................................................................ 10
Output Processing ................................................................ 11
Output Source Select........................................................... 11
CODEC and AUX Output Channels..................................... 11
Output Signal Processing Stages ........................................ 11
Output Gain and Level Indicator .......................................... 11
Filters ................................................................................... 11
Output Compressor and Limiter........................................... 11
Dynamic Compressor ........................................................... 12
DMTH4 Controls and Features ............................................. 13
Front Panel........................................................................... 13
Rear Panel ........................................................................... 13
Control Panel Overview ........................................................ 14
Special Key Functions.......................................................... 15
DMTH4 System Setup Tabs................................................... 16
Activity Tab ........................................................................... 16
Inputs Tab ............................................................................... 18
In Filters Tab (Input Filters)................................................... 20
In Comp Tab (Input Compressor) ......................................... 22
Matrix Tab ............................................................................... 24
Out Source Tab (Output Source) .......................................... 26
Out Filters Tab (Output Filters)............................................. 28
Out Comp/Lim Tab (Output Compressor/Limiter)............... 30
Outputs Tab ............................................................................ 32
Rear Panel Control Tab.......................................................... 34
Tel/Codec Tab......................................................................... 38
Serial Interface and Control Commands ............................. 43
DMTH4 General Commands ............................................... 43
DMTH4 Audio Input Commands .......................................... 46
DMTH4 Matrix Crosspoint Commands ................................ 48
DMTH4 Audio Output Commands ....................................... 49
DMTH4 Input Compressor Commands................................ 51
DMTH4 Output Compressor Commands............................. 52
DM Output Limiter Commands ............................................ 53
DM Programmable I/O Commands...................................... 54
DMTH4 Rear Panel Control Commands.............................. 57
DMTH4 Telephone Interface Commands............................. 59
DMTH4 Codec Interface Commands................................... 63
DM Command Grammar Rules............................................. 65
FCC Part 68 Compliance....................................................... 66
FCC Part 15 Compliance....................................................... 66
Specifications and Features................................................. 67
Service and Repair ................................................................ 68
Returning Units for Repair ................................................... 68

DMTH4
LECTROSONICS, INC.
6
Telephone
Interface
Local
loudspeaker
Local
microphone
Far-end
Local
sound system
Digital echo cancellation is another method of reduc-
ing the echo delivered to the far-end. The concept,
described in very simple terms, is to have the DSP
recognize the far-end audio and subtract it from the
transmitted audio to remove any echo they might hear at
the far-end. Sounds simple, but in a sound system with
multiple microphones and loudspeakers, it is not easy
to identify the far-end audio in the complex mix of local
sound, local noise and the effects of the room on the
far-end audio delivered by the local loudspeaker system.
When there is no sound or noise in the local room, the
DSP can do a decent job of identifying the far-end audio
and subtracting it from the transmitted signal, but this is
rarely the case in full duplex teleconferencing. People
talk, laugh and make noise, and air conditioners and
projectors make noise, etc.
The fundamental problem with microphone/speaker
acoustical coupling is illustrated below. Far end audio
is delivered by the loudspeakers in the room and the
microphones pick it up and return it to the far end. The
delay through this process creates an echo heard on the
far end.
There are several methods used to reduce or eliminate
the echo heard on the far end of the conversation:
• Optimaldesigninthesoundsystemtominimizethe
coupling between loudspeakers and microphones.
• Mix-minusmatrixrouting.
• Automaticmicrophonemixing.
• Digitalechocancelling.
Matters become more complex when the sound system
is required to provide both teleconferencing and sound
reinforcement. A gain proportional automatic mixing
process is widely recognized as the optimum solution for
sound reinforcement, but it places significant demands
on an acoustic echo canceller used for teleconferencing.
The matrix mixer enables complex signal routing and
level controls without limitations. The matrix mixing
allows “mix-minus” zoning of microphones and loud-
speakers to decouple them and reduce or eliminate
acoustic feedback and echoes. NOM attenuation is ap-
plied by the DSP at the crosspoints in the matrix, which
essentially provides 24 separate automatic mixers, each
with its own NOM mixing bus. Four different mixing
modes can be selected at the crosspoint for each input,
so each input can participate differently in each output
mix.
The automatic mixing process uses a seamless algo-
rithm that eliminates gating and its ill-effects. Gain is
proportioned among all inputs assigned to a particular
output channel in a seamless and continuous manner
based upon microphone activity. The algorithm oper-
ates in a natural, transparent manner and incorporates
an adaptive AutoSkew™process to eliminate artifacts
such as comb filtering and abrupt gating that occur with
conventional automatic mixing schemes. Audio from the
far-end of a conference participates in the local mix-
ing algorithm just like a microphone in the local sound
system.
Two digital acoustic echo cancellers are provided in the
DMTH4 to further reduce the return of local signals to
the far-end. One operates on the telco connection and
the other is dedicated to the video codec connection. In
conjunction with the automixing process, echoes are
minimized and not heard at the far end.
ERL
ERL (echo return loss) refers to the natural attenuation
of the far-end audio signal as it circulates from the far-
end through loudspeakers and microphones in the local
sound system and back to the far-end. Good design in
the local sound system will reduce the acoustic coupling
between loudspeakers and microphones using physical
placement and mix-minus matrix routing. Depending
upon room size and acoustics, it is often impossible to
achieve adequate decoupling to avoid an echo heard by
the far-end during a teleconference. Thus, other types of
processing are needed to further reduce the return echo.
ERLE
ERLE (echo return loss enhancement) refers to ad-
ditional circuits and processes used to further increase
ERL. Common methods are to use automatic mixing
and digital echo cancellation.
Return Loss Enhancement
The gain proportional automatic mixing algorithm* in the
DM Series processors not only provides seamless mix-
ing for local sound reinforcement without abrupt gating,
but it also contributes significantly to ERLE. The addi-
tional contribution is plotted in the following graph.
Echo and Echo Cancellation

Digital Telephone Hybrid
Rio Rancho, NM 7
In a simple sound system arrangement, the local micro-
phone can be muted when nobody is talking in the local
room. A simple gated mixer can provide this function.
With no open microphones locally, there is obviously
no return echo signal. This requires that a threshold
level be set high enough to keep the microphone from
being opened by background noise, but low enough
to allow it to open when someone speaks. When the
local microphone is open, a return echo path is created,
which is when a DSP echo canceller is needed. Given
the wide variety of human voices and the dynamics of
noise in a meeting room, a gated mixer is often not the
best choice.
Using a dedicated DSP echo canceller on each input of
the local mixer (referred to as “distributed echo cancel-
lation”) is an expensive but effective approach to reduc-
ing the return echo. The process requires the algorithm
to “converge,” which is to identify the far-end audio
and subtract it from the signal sent to the far-end. This
requires at least a brief moment when there is very little
local sound or noise, with significant far-end audio pres-
ent in the room. If nobody moves and there are no gain
changes made to local microphones and loudspeakers,
it is possible (in theory) to effectively remove return
echo, but this is not a very realistic situation.
The theory behind distributed echo cancelling is that
once the DSP has converged, it can continue to sub-
tract far-end audio even when the local microphone is
open and far-end audio is present at the same time. If
there are any changes in gain, noise or acoustics in the
local space and equipment, the DSP must re-converge,
which requires another brief moment with little or no
local noise or sound, and significant far-end audio pres-
ent.
A gated automatic mixer does not change the gain
when the microphone is open, it just turns the channel
off and on abruptly. This helps with distributed echo
cancelling since the microphone is completely muted
when not in use, but it is very “choppy” sounding in the
local sound reinforcement system.
A gain proportional automatic mixer applies the most
gain to the most active microphone with smooth,
continuous changes. This makes it extremely effec-
tive for local sound reinforcement, but the continuous
gain changes make it difficult for the echo canceller to
remain converged and effectively reduce the echoes at
the far end.
The DMTH4 in conjuction with a DM Series processor
offers a unique approach to the problems with simulta-
neous teleconferencing and sound reinforcement. The
patented adaptive gain proportional mixing algorithm
works in conjunction with a centralized echo canceller
to address a variety of issues. The automatic mixer pro-
vides seamless allocation of gain to local microphones
through a mix-minus matrix to reduce background noise
and decouple loudspeaker and microphones, while a
very fast converging DSP echo canceller operates on
the composite transmitted signal being sent to the far
end. This combination of processes is possible only
with the latest DSP technology.
The auto mixing algorithm adapts to changes in back-
ground noise continuously, and unlike a gated mixer
there are no threshold levels to adjust. A sum of all
channels is the reference signal, each channel level
is compared to this reference and the individual chan-
nel gain is adjusted to apply NOM attenuation. Gain
is adjusted continuously to eliminate audible artifacts
that gating and abrupt level changes can cause. As the
common mode noise in the room changes, all channels
are affected equally. The end result is seamless, adap-
tive auto mixing that requires no calibration or threshold
adjustments.
Each individual output of the matrix operates as a sepa-
rate NOM bus, so a particular input can be assigned to
multiple outputs with mix parameters adjusted differently
for each output. In other words, gain and mix mode are
configured independently for each matrix crosspoint, re-
sulting in great flexibility. Four mix modes are supported:
Auto,Direct,OverrideandBackground.
The echo canceller converges continuously when the
level of the far side signal exceeds a minimum level,
and the ratio of the far side signal to local room sound
exceeds a minimum ratio. This dynamic control pre-
vents divergence during periods of silence from the
far side room or in “doubletalk” situations. The con-
vergence takes place very quickly to keep up with the
changes made by the automatic mixing algorithm and
other changes that occur in the room. Setup is greatly
simplified and any adjustments, such as level changes
made with a remote control system, are accommodated
automatically.
The convergence speed is adjustable in the control
panel GUI to fine tune it to a particular situation. Faster
convergence times can track changes in the room al-
most instantaneously, but the depth of echo cancellation
will be reduced. Slower convergence times take a bit
longer to fully converge, but produce greater echo can-
cellation. The ERLE value achieved by the echo cancel-
ler is displayed on the GUI and the effects of altering the
convergence rate will be immediately visible and audible.
An important final note on the DMTH4 is the fact that the
echo canceller will never “diverge” (lose convergence).
This unique algorithm will also converge on a continuous
sine wave, which is especially important when DTMF
tones are present in the room. Since the echo canceller
will never diverge, there is no need for a “panic button”
(as is used in other designs) to generate a noise burst to
help the echo canceller re-converge.

DMTH4
LECTROSONICS, INC.
8
General Overview
The DMTH4 integrates telephone lines, video codecs
and external audio sources into the digital bus structure
of DM Series processors, allowing these audio signal
sources to operate as though they are another micro-
phone or audio input in the local sound system. This is
a complete DM Series digital matrix processor, with a
3-in/24-out digital matrix, automatic mixing and compre-
hensive signal processing on every input and output.
The latest generation DSP microchips and microproces-
sors are the core of the engineering of the DM Series
and the DMTH4 is no different. The focus and purpose
is to meet the requirements of modern applications and
also the demands for convenience and automation.
The DMTH4 is configured through the DMTH4 Control
Panel which is part of LecNet2™, a user-friendly, yet
powerful control program. The Control Panel offers
quick configuration and full command of the system
througheitheraUSBorRS-232compatibleinterface.
Once configured, it operates independently.
All models in the DM Series offer the same signal pro-
cessing functions, and vary only by the number of audio
inputs and outputs available. The DM’s basic structure
consists of three stages: Input, Matrix and Output. (See
DMTH4 Signal Flow Block Diagram and DMTH4 Func-
tional Block Diagram.)
Each input channel includes a high quality 24-bit A-D
converter. Extensive digital signal processing is pro-
vided on each input channel. Each input channel is
processed and filtered as needed and the signal is deliv-
ered into the matrix.
The digital matrix mixer distributes each input signal
to any selected combination of mix busses, with level
control at each crosspoint. The matrix processes the
signals and communicates them to other devices in the
system. Each output receives signals from the mixing
matrix, the pink noise generator or the tone generator
as needed for setup, diagnostics or operation. Each of
the 3 outputs includes extensive signal processing to
optimize the mixed signal for the intended purpose, such
as sound reinforcement, recording or teleconferencing.
The DMTH4 is integrated into a system of DM Series
automixers using the Digital Audio Network Interface
(DANI). DANI connects the digital audio outputs of the
units through standard RJ45 connectors.
When DM Series automixers are stacked, mixing data
and the digital audio are passed between the slave units
and the master unit through the DANI. Multiple units can
be stacked in a master/slave configuration to expand the
number of inputs to hundreds of channels. The DMTH4
is designed to be the end slave unit of a DM Series
stack.
The audio and data from all units in the stack is gath-
ered in the matrix in the master unit, which is where the
final mix signals are generated. The first 12 final mix
signals from the master are back propagated through
the DANI to each slave. (See DM Series Back Propaga-
tion Diagram.)
DMTH4 Signal Flow Diagram
3 by 24
Automatic
Mixing
Matrix
Aux
Input
Processing
TEL
Input
Processing
Codec
Output
Processing
Tone
Generator
1 kHz, 0 dBu
Pink Noise
Generator
0 dBu
Codec
Input
Processing
24 Output Submixes
2 Expansion Submixes
26 Automixing Aux. Data
2 Expansion Signals
26 Automixing Control Data
24 Output Submixes 12 Backpropogated Final Mixes
2 Expansion Submixes
26 Automixing Aux. Data
2 Expansion Signals
26 Automixing Control Data
DTMF
Generator
0 dBu
TEL
Output
Processing
Aux
Output
Processing
DM1624
Master
DM1624
Slave
1
24
Outputs
DM1624 Master
with
DM1624 and DMTH4 Slaves
DM1624
Master
DM1612
Slave
1
24
1
12
Master and
Slave Outputs
Outputs
DM1624 Master
with
DM1612 and DMTH4 Slaves
12
13 Master only
Outputs
DM1612
Master
DM812
Slave
1
1
12
Outputs
DM1612 Master
with
DM812 and DMTH4 Slaves
12
Outputs
1
Outputs
12
Outputs
1(13)
12(24)
DMTH4
Slave
Codec
AUX
Outputs
TEL
DMTH4
Slave
Codec
AUX
Outputs
TEL
DMTH4
Slave
Codec
AUX
Outputs
TEL
DM Series Back Propogation Diagram

Digital Telephone Hybrid
Rio Rancho, NM 9
Digital Matrix
The digital matrix provides signal routing and commu-
nication with other devices in the system, and applies
automatic mixing and level control. (See Digital Matrix
Functional Block Diagram.)
Automixer Cell
The Automixer Cell is the core of the matrix. It is where
level control for the automatic mixing algorithm, mixing
mode and crosspoint gain is applied to data gathered
from other channels and devices. The cell receives data
from the master unit in a multiple unit stacked configura-
tion and from the slave units farther down in the chain.
Power of the Mix
The Power of the Mix is the reference used to determine
the gain to be applied to each individual output channel.
In a multi-unit stacked configuration, this data is sent to
the slaves from the master unit.
Crosspoint Gain
Crosspoint Gain is the gain selected with the control
panel that determines the level at the output.
Mixing Mode
The automatic mixing algorithm applies a
patented gain proportional algorithm (US
Patents #5,414,776 and #5,402,500) al-
lowing each input assigned to a particular
output to behave differently relative to the
other inputs assigned to the output.
Four different mixing modes are available:
Auto - In automatic mode the input applied
to the crosspoint is mixed into the output
channel using the the Adaptive Proportional
Gain automixing algorithm in the normal
manner. This is the most common setting.
Direct - In Direct mode the automixing algo-
rithm is bypassed.
Override - Override mode is selected when
it is required that the input applied to the
crosspoint always dominates the output
channel when it becomes active.
Background-Backgroundmodeisse-
lected when it is required that the input
applied to the crosspoint dominates the
output channel only when all other inputs
are inactive.
Digital Matrix Functional Block Diagram
(Typical Matrix Crosspoint)
+
Automixer
Cell
Submix
Submix
Crosspoint Gain
-70 to 20 dB
1 dB steps
Mixing Mode
-Auto
-Direct
-Override
-Background
Audio Input
Power of the Mix
Power of the
Submix
Power of the
Submix
DMTH4 Functional Block Diagram

DMTH4
LECTROSONICS, INC.
10
Input Processing
Each input channel provides individual stages for gain,
filtering and compression.
Input Gain
The input applies software controllable gain with a level
indicator and clipping indicator.
Filters
Up to six filters can be implemented at each input and
each output to idealize the signal equalization.
The filter types include:
Low pass
High pass
Bandpass
Parametric EQ
Low shelving
High shelving
Filterslopescanbeselectedwith6or12dBperoctave
ButterworthorBesselparameters.Multiplelterscanbe
assignedtocreatersteeperslopesin6dBsteps.
Input Compressor
The compressor implementation is a unique “soft knee”
type based on an RMS level detector controlled by a
single time constant parameter. This is a new design
which responds to varying rates of change in the signal
level by dynamically adjusting the attack and release
times for best performance. Adjustment is simplified by
entering a single value (half of the desired release time).
The attack time is then applied by the DSP to vary with
the signal.
The default value is 100 ms, which sets the release
time at about 200 ms. The attack time is signal con-
trolled and varies from about 2 ms to about 100 ms as is
needed to handle the signal dynamics. See page 14 for
a closer look at this unique and effective compressor.
Compressor adjustment parameters include:
Threshold
Time Constant
Compression ratio
Makeup gain
LecNet2 Software
Software is included with the DMTH4 and available
for download from the website at: www.lectrosonics.
com. The software is used primarily for setup, with the
configuration saved on file and into the unit’s memory
for actual operation. Once configured, the DMTH4 runs
without a host computer.
The software is user-friendly, with a variety of screens
provided for each section of the signal flow and system
design. The software runs under Windows® XP, Vista
and 7 operating systems using a familiar tabbed layout.
A few sample screens are shown below.
*Windows is a registered trademark of Microsoft Corp.
Six Filter
StagesCompressor
Level Meter
Clipping
Detector
Gain Reduction Indicator
Fine Gain & Polarity
-10 to 10 dB,
1 dB steps
Threshold
Comp. Ratio
Time Constant
Off, LP , HP, BP,
PEQ, LS, HS
6 or 12 dB /oct.
ButteRworth or Bessel
when applicable
One of 3 Input Signal Processing Blocks
Indicator
Indicator
Activity Indicator
A/ D
Input Gain & Polarity
-10 to +60 dB
1 dB steps
Coarse Gain
0 to 12 dB ,
3 dB steps
Typical Input Signal Processing Blocks

Digital Telephone Hybrid
Rio Rancho, NM 11
Output Processing
Output Source Select
The TEL, CODEC and AUX outputs can each be set to
receive a signal from the pink noise generator, the tone
generator, the expansion outputs or from the master unit
outputs. The pink noise source can be used for sound
masking during operation, and for equalization during
setup. The tone generator is used for level adjustments
and signal routing diagnostics. The DTMF generator is
used to create the DTMF tones for initiating calls.
In normal operation the digital matrix delivers the audio
signals to the outputs, which consist of the final mixes
backpropagated from the master unit in the system
via the Digital Audio Network Interface (DANI), with 12
mixes from the main matrix and 2 mixes from the expan-
sion matrix.
CODEC and AUX Output Channels
These outputs include an attenuator to reduce the out-
put level from line to mic level. The passive attenuator
does not change the signal to noise ratio of the signal,
butsimplyappliesuserselectable20dBor40dBofat-
tenuation to reduce the signal level.
Output Signal Processing Stages
Each output channel provides six filters plus a compres-
sor and limiter to idealize the channel for its function in
the sound system. (See Typical Output Signal Process-
ing Block)
Output Gain and Level Indicator
Theoutputlevelcanbeadjustedfrom-70dButo+20
dBuin1dBstepstoperfectlymatchtherequirements
of the device being fed by the channel. A bar graph is
provided by the on screen GUI to accurately indicate the
output level as it operates and is adjusted.
Filters
Up to six filters can be implemented at each input and
each output to idealize the signal equalization.
The filter types include:
Low pass
High pass
Bandpass
Parametric EQ
Low shelving
High shelving
Filterslopescanbeselectedwith6or12dBperoctave
ButterworthorBesselparameters.Multiplelterscanbe
assignedtocreatersteeperslopesin6dBsteps.
Output Compressor and Limiter
A versatile compressor and limiter are provided at each
output to control the average level and dynamics of the
audio signal, and restrict the maximum output level to
optimize the channel for its purpose. Compression is of-
ten needed when the channel is feeding a recorder, and
limiting is often used to protect a loudspeaker system
and reduce distortion and amplifier overload.
The compressor implementation is a unique “soft knee”
type based on an RMS level detector controlled by a
single time constant parameter. This is a new design
which responds to varying rates of change in the signal
level by dynamically adjusting the attack and release
times for best performance. Adjustment is simplified by
entering a single value (half of the desired release time).
The attack time is then applied by the DSP to vary with
the signal.
The default value is 100 ms, which sets the release
time at about 200 ms. The attack time is signal con-
trolled and varies from about 2 ms to about 100 ms
as is needed to handle the signal dynamics. See the
following page for a closer look at this unique and very
effective compressor.
Compressor adjustment parameters include:
Threshold
Time Constant
Compression ratio
Makeup gain
Limiter adjustment parameters include:
Threshold
Time Constant
Typical Output Signal Processing Blocks
Activity Indicator
Output Gain
-70 - +20 dB
1 dB steps
Six Filter
StagesCompressor Limiter
Level Meter
Off, LP , HP, BP,
PEQ, LS, HS
6 or 12 dB /oct.
Butterworth or Bessel
when applicable
Threshold
Comp. Ratio
Time Constant
Indicator
Activity Indicator
Gain Reduction Indicator
Threshol d
Time Constant

DMTH4
LECTROSONICS, INC.
12
0 0.02 0.04 0.06 0.08 0.1
0
10
x/x0 [dB]
time [sec]
Dynamic Compressor
Signal dynamics vary widely even in simple audio
conferencing applications, requiring different attack and
release timing for optimum performance. The DMTH4
provides a new DSP-based compressor that addresses
this issue very effectively, and at the same time simpli-
fies the setup. A true RMS level detector is used to
assess signal levels, so the values are accurate and
consistent regardless of the waveform of the signal.
Whether the audio signal is normal voice, loud bursts, a
sine wave for testing or random pink noise for adjusting
levels, the compressor will track the signal and apply
optimum attack and release times.
An RMS detector handles signal dynamics in much the
same way human ears and eyes handle abrupt tran-
sients. As a loud peak hits the ears, or a bright flash
of light hits the eyes, the muscles contract very quickly
to suppress overload. As the signal decays, muscle
tension reduces gradually as the senses recover. The
DMTH4 compressor effectively acts in the same manner.
A dynamic, variable attack time is applied by the DSP to
apply an optimal rate as the signal transient arrives. The
release time then follows a longer time constant as de-
nedinthesystemsetup.Bothattackandreleasetimes
are dynamically controlled to track the RMS value of the
signal level as it passes.
To simplify setup, a single time constant value is entered
to set the desired release time. There is no need to
specify separate attack and release times and try to pick
good average values. A value of about one half of the
desired release time is specified, and the attack time is
taken care of dynamically by the DSP.
In the example shown here, a value of 100 ms (the
default) is specified, which correlates to about 200 ms of
equivalent release time (a good value for most applica-
tions). The DSP controlled attack time varies to follow
an optimum curve as the signal level rises. At first, it is
the equivalent of about 2 ms, then as the signal nears
full level it slows to to about 100 ms. The release time is
also dynamically controlled and follows the defined rate
as set in the control panel. The overall envelope is very
transparent with no audible artifacts.
The RMS detector approach is used throughout the
DMTH4 for processes such as automatic mixing and
level metering in addition to the compressor action.
DSP controlled, variable attack time is based upon values
generated by an RMS detector to follow the ears natural
handling of signal transients.
TRMS = 100 ms
T
AT T = 67 ms
T
AT T = 40 ms
0 0.5 1 1.5 2
0
10
20
30
40
50
x/x0 [dB]
time [sec]
TREL = 200 ms
TRMS = 100 ms
The RMS detector dynamic release time set at 100 ms is
approximately the same as a fixed time constant of 200 ms.

Digital Telephone Hybrid
Rio Rancho, NM 13
TELEPHONE
SET LINE
EXPANSION TEL RC
1: GND
2: IN 1
3: IN 3
4: IN 5
5: IN 7
25
13
14
1
6: IN 9
7: IN 11
8: GND
9: OUT 1
10: OUT 3
11: OUT 5
12: OUT 7
13: GND
14: +5V
15: IN 2
16: IN 4
17: IN 6
18: IN 8
19: IN 10
20: +5V
21: OUT 2
22: OUT 4
23: OUT 6
24: OUT 8
25: +5V
PROGRAMMABLE
INPUTS / OUTPUTS
RS-232
LecNet 2
USB
POWER
90-240V 50/60Hz 15W
AB
TELEPHONE HYBRID
MADE IN U.S.A.
TX RX
RX TX
CODEC RC
CODEC AUXCODEC AUX
U.S. Patent Number: 5,414,776
LINE
INPUTS
MIC/LINE
OUTPUTS
LECTROSONICS, INC.
Complies with Part 68 FCC rules.
Registration Number:4J3BR09BDMTH4
Ringer Equivalence: 0 . 9 B
This device complies with
Part 15 of the FCC rules.
Operation is subject to the
following two conditions: (1)
This device may not cause
harmful interference, and (2)
this device must accept any
interference received, including
interference that may cause
undesired operation.
POWERUSBSTATUSMODE
TELEPHONE HYBRID INTERFACE
R
DMTH4 Controls and Features
POWER On/Off Switch
The Power On/Off Switch is used to turn the DMTH4 on
(1) or off (0).
USB Port
StandardUSBconnectorforthesetupandcontrolofa
DMTH4 from a Windows-based computer system with
USBinterface.
Status LED
The DMTH4 is normally configured as the end slave unit
in a DM Series stack. In this configuration, the Status
LED glows steadily after the power on boot sequence
is complete and a Master unit has been detected. The
Status indicator will blink to indicate that it has not found
the Master unit or some other error exists.
Note: The DMTH4 is shipped from the factory
configured as a Master unit.
Mode Switch
If a unit configured to run in slave mode doesn’t detect
a master unit on powerup, it will refuse to boot and the
Status LED will blink rapidly. In this case, pressing the
Mode switch will force the unit to boot up temporar-
ily in Master mode to allow settings to be examined
or changed. Cycling the power returns the DMTH4 to
Slave mode.
Pressing the Mode switch while powering up the
DMTH4, places the unit in Software Update mode. The
Status LED blinks slowly once a firmware update is ini-
tiated, then extinguishes when the process is complete.
Cycling the power exits Software Update mode.
Front Panel
Rear Panel
POWER Plug
Standard NEMA 5-15 three-pin power plug capable of
accepting 100-240 VAC, 47-63 Hz.
RS232 Port
A serial RS232 interface provided for setup and control
vial computer serial port, or for connecting RS-232 serial
control devices from other companies.
USB Port
StandardUSBconnectorforthesetupandcontrolofa
DMTH4 from an Windows-based computer system with
USBinterface.
Programmable Input and Output Ports
Programmable inputs are provided to enable external
control over a variety of parameters. Each input can re-
spond to a contact closure, a DC voltage source, or the
variable voltage outpout from a potentiometer. Program-
mable outputs are used to indicate the current state of
a programmable input, monitor activity on telephone or
codec interfaces, and monitor active preset changes.
DANI
DANI, or Digital Audio Network Interface, is used to con-
nect the digital audio outputs of the master and slave
units in stacked configurations.
TEL/CODEC RC
These interfaces are used to connect RCWTH4 remote
control units.
Audio Inputs and Outputs
Analog audio input and output connectors for Codec and
Aux inputs and outputs only.
Telephone Set/Line
RJ11 jacks for connecting standard telephone line and a
single line telephone set to the DMTH4.
POWER Plug RS232 USB Programmable
Input and Output
Ports DANI
Tel RC Line Inputs
(Codec/Aux)
Telephone
Set/Line
Mic/Line Outputs
(Codec/Aux)
Codec RC

DMTH4
LECTROSONICS, INC.
14
Control Panel Overview
The DMTH4 is a powerful digital Telephone Hybrid that
integrates telephone lines, video codecs and external
audio sources into the digital bus structure of DM Series
processors so these audio signal sources operate as
though they are another microphone or audio input in
the local sound system. The unit is much more than
just a telephone interface. Instead, it is a complete DM
Series digital matrix processor, with a 3-in/24-out digital
matrix, automatic mixing and comprehensive signal
processing on every input and output.
The control panel is the primary tool for configuring the
DMTH4 for service, prior to installation on site. It or-
ganizes the multitude of DMTH4 features and settings
efficiently so that device setup can be as straightforward
as possible. Setups and macros can be saved to disk file
so that they can be rapidly “cloned” in multiple units.
The control panel also acts as a controller for the
DMTH4 for simple operational scenarios or trouble-
shooting purposes. Every function of the device can be
monitored and adjusted in real time. The control panel
canconnecttotheDMTH4overaserialport,aUSB
port, or a network connection.
The control panel itself uses the familiar tabbed layout
with the first tab providing “at a glance” monitoring of
input and output levels, as well as sliders to adjust input
channel gains, when a PC is connected to a DMTH4.
Telephone calls may also be initiated or accepted from
this tab.
The next tabs in the control panel contain settings for
a particular signal processing block. These tabs are
arranged left to right in the order of signal flow in the
DMTH4
These are followed by a tab containing settings for the
Rear Panel Control interface.
The final tab in the control panel contain settings for the
telephone and codec interfaces.
A status bar at the bottom of the control panel displays
connection status information, including the serial
number of the DMTH4 and the number of the preset
last recalled.
Menu Bar
Tabs
Status Bar

Digital Telephone Hybrid
Rio Rancho, NM 15
Special Key Functions
F12
When the control panel is connected to a DMTH4, the
F12 key can be pressed at any time to mute all outputs.
This provides a “panic button” on the PC keyboard to
shut the system down if a settings change has caused
harmful feedback to develop.
Ctrl-F5
When the control panel is connected to a DMTH4, the
key combination Ctrl-F5 can be pressed at any time to
suspend the “polling” of the device by the control panel
program. This is an advanced feature that is useful in
situations where it is desirable to have both the control
panel and a 3rd party control system (e.g. Crestron or
AMX) connected to the DMTH4 at the same time. The
polling of the DMTH4 by the control panel program for
real-time status data on one communication port tends
to keep the device so “busy” that commands received on
the other port may be ignored. The solution is to sus-
pend polling by the control panel program for those pe-
riods of time when the 3rd party control system is being
operated. When polling is suspended DMTH4 settings
may still be modified using the control panel,
but the screen is not refreshed as usual when levels
change, etc. To resume polling, press the Ctrl-F5 key
combination again.
Name Description
MenuBar Aseriesofpull-downmenusforinteractionsbetweenthecontrolpanelandthehostcom-
puter, and for functions such as device setup, macro creation and preset select.
Tabs A series of tabs which follow signal flow used for setting up the DMTH4.
StatusBar Connection status:
Offline: Displayed when the control panel is opened but the computer is not connected to a
DMTH4. Menu options for features that require a connection to a DMTH4 will be disabled
in this mode.
Connected: Displayed when the control panel is opened with the computer connected to
the DMTH4. All features requiring a connection to the DMTH4 are enabled.
Connection type:
 USB:Displayedwhenthecontrolpanelisopenedandthecomputerisconnectedtothe
DMTH4viatheUSBport.
Network: Displayed when the control panel is opened and the computer is connected to
the DMTH4 via a Gateway PC running the LecNet2 “Net Server” program..
Extron ILP T S: Displayed when the control panel is opened and the computer is con-
nected to the DMTH4 via an Extron ILP T S ethernet control interface.
Serial: Displayed when the control panel is opened and the computer is connected to the
DMTH4 via the computers RS-232 serial port.
Port ID:
 USB:NotUsed.
Network: Displays TCP/IP address of Gateway PC.
Extron ILP T S: Displays TCP/IP address of Extron unit.
Serial: Displays COM Port Identifier if computer connected to a serial port.
Device ID: DMTH4
Device Serial Number: Embedded serial number of DMTH4.
Device Firmware Version: Current firmware version level.
Rank in processing order: Indicates the “rank” or position of the unit in a system of DM
series processors. For instance, “2 of 3” means that the unit is 2nd in a three unit system.
The first unit is always the master, with all others configured as slaves.
Last preset recalled: Number of last preset recalled. May not be the same as the preset
loaded when the unit was initially turned on.

DMTH4
LECTROSONICS, INC.
16
Activity Tab
The Activity Tab provides an “at a glance” monitoring of
input and output levels, in addition to input gain controls
(enabled when DMTH4 is connected to computer sys-
tem) and the ability to initiate and accept telephone calls.
DMTH4 System Setup Tabs
The tab is divided into four functional sections: Input
Channel Activity/Gain, Output Channel Activity, Tele-
phone Interface/Acoustic Echo Canceller and Codec
Interface/Acoustic Echo Canceller
Input Channel
Activity/Gain
Output Channel
Activity
Telephone
Interface/Acoustic
Echo Canceller
Codec Interface/
Acoustic Echo
Canceller
Input Channel
Activity/Gain
Output Channel
Activity
LEDs, level meters and slider controls for monitoring and controlling the telephone, codec
and auxiliary input channels.
C/C LED - Glows yellow if the input channel compressor has become active, or red if
the input channel audio is clipping.
LevelMeter-TrueRMSlevelmeterdisplayingtheinputaudiolevelindBuasmeasured
after application of the main and rear panel input channel gains, but before the
signalisprocessedbytheinputltersandcompressor.Meterrangeis-40dButo
+20dBu.
MU LED - Glows red when channel is muted.
LEDs and level meters for monitoring the activity of the telephone, codec and auxiliary
channels.
C/L LED - Glows yellow if the output channel compressor has become active, or red if
the output channel audio is in limiting.
LevelMeter-TrueRMSlevelmeterdisplayingtheoutputaudiolevelindBuasmea-
sured after application of the main and rear panel output channel gains, but
before the signal is processed by the output filters, compressor/limiter, or mic level
attenuator.Meterrangeis-40dButo+20dBu.
MU LED - Glows red when channel is muted.
Name Description

Digital Telephone Hybrid
Rio Rancho, NM 17
Telephone Interface/
Acoustic Echo
Canceller
Codec Interface/
Acoustic Echo
Canceller
Used to initiate telephone calls from the control panel through the DMTH4 when computer
is connected to unit. Section includes telephone interface controls and level meters to
indicate level of ERL or ERLE applied.
Ringing indicator - blinks yellow if an incoming call is present.
Connect/Disconnect button - Toggles the connection state of the interface. Enabled
only when the control panel is connected to a DMTH4.
Connection indicator - glows red to indicate the telephone interface is connected.
Privacy button - Toggles the privacy state of the interface. Enabled only when the con-
trol panel is connected to a DMTH4 and the interface is in the connected state.
Privacy indicator - glows red to indicate that privacy is engaged.
Redial button - Used to redial the last number called.
Phone Bookbutton-UsedtoaccesstheDMTH4’sinternallystoredPhoneBook.
Telephone Number list - Used for typing in the telephone number to be dialed when
the Dial button is clicked. It is enabled only when the control panel is connected to a
DMTH4. This control has a “memory”. Once a number has been dialed, additional num-
bers may be typed in without losing previously entered numbers. To recall a previously
entered telephone number, click the “dropdown” button to display the list and and click
on desired number to select. Double click a number to cause it to be dialed immedi-
ately.
Dial button - Click to dial the telephone number displayed in the telephone number list.
It is enabled only when the control panel is connected to a DMTH4. If the telephone
interface is not connected to the line, it will be automatically connected before the
number is dialed.
Tone pad - complete DTMF dial pad for dialing telephone numbers through the control
panel.
Flash button - used to momentarily place the telephone interface in an “on-hook” con-
dition. Enabled only when the interface is in the connect state.
AEC Enabled - glows to indicate that AEC is enabled.
ERL - “Echo Return Loss” meter displays dB of attenuation and represents the echo
return loss resulting from the acoustic path(s) in the room.
ERLE - “Echo Return Loss Enhancement” meter represents the additional echo return
loss achieved by the action of the AEC and is displayed as dB of attenuation.
Used to control the Codec Interface and Codec Acoustic Echo Canceller when the com-
puter system is connected to the DMTH4.
Connect/Disconnect button - Toggles the connection state of the interface. Enabled
only when the control panel is connected to a DMTH4.
Connect indicator - glows red if codec interface is connected.
Privacy button - Toggles the privacy state of the interface. Enabled only when the con-
trol panel is connected to a DMTH4 and the interface is in the connected state.
Privacy indicator - glows red to indicate that privacy is engaged.
AEC Enabled - glows to indicate that AEC is enabled.
ERL - “Echo Return Loss” meter represents the ratio between the telephone transmit
signal and the reference signal presented to the AEC. Displayed as dB of attenuation
and represents the echo return loss resulting from the acoustic path(s) in the room.
ERLE - “Echo Return Loss Enhancement” meter represents the additional echo return
loss achieved by the action of the AEC and is displayed as dB of attenuation.
Name Description

DMTH4
LECTROSONICS, INC.
18
Inputs Tab
The Inputs Tab is used to setup and control the inputs to
the DMTH4. The tab is divided into three functional sec-
tions: TEL, CODEC and AUX.
TEL Input Section CODEC Input Section AUX Input Section
TEL
CODEC and AUX
LEDs, level meters and slider controls for setup and controlling of the telephone input chan-
nels.
Comp/Clip LED - Glows yellow if the input channel compressor has become active, or
red if the input channel audio is clipping.
Level (dBu) meter-TrueRMSlevelmeterdisplayingtheinputaudiolevelindBuas
measured after application of the main and rear panel input channel gains, but before
thesignalisprocessedbytheinputltersandcompressor.Meterrangeis-40dButo
+20dBu.
Gain (dB) control - used to set the input channel gain. Gain can be set in one of four
ways:
Use the mouse to drag the control slider up or down
Click above or below the gain slider
Click the up or down arrow next to the Gain Control input box
Type the value directly into the Gain Control
Mute - mutes input channel when checked.
Invert - inverts input signal phase when checked.
Same as TEL section.
Gain Control
Gain Control Slider
Name Description

Digital Telephone Hybrid
Rio Rancho, NM 19

DMTH4
LECTROSONICS, INC.
20
This tab contains a notebook of the six available input
filter settings, with a separate page for each input chan-
nel. If connected to a DMTH4, changes made in this tab
take effect immediately. As always, the changes are not
Filter Set Notebook Contains information about input filter selection and setup for each input channel.
Filter Type - Used to set up input filters. Up to six input filters can be setup per input
channel. Tabs at the bottom of the Filter Set Notebook select the filter for setup.
The following types of filters are available:
Parametric (PE) filter with adjustable center frequency, boost/cut, and bandwidth
parameters.
Low Shelving (LS) filter with adjustable center frequency and boost/cut param-
eters.Rolloffrateisselectableas6dBor12dBperoctave.
High Shelving (HS) filter with adjustable center frequency and boost/cut param-
eters.Rolloffrateisselectableas6dBor12dBperoctave.
Low Pass (LP) filter with adjustable center frequency parameter. Rolloff rate is
selectableas6dBor12dBperoctave,andresponseisselectableasButterworth
orBessel.
High Pass (HP) filter with adjustable center frequency parameter. Rolloff rate is
selectableas6dBor12dBperoctave,andresponseisselectableasButterworth
orBessel.
Band Pass (BP) filter with adjustable center frequency and bandwidth param-
eters.
Filter Type
Filter Number
Input Select
Frequency
Response
Graph
Graph Scale
Filter Set
Notebook
permanent unless the DMTH4 settings are stored to a
preset in nonvolatile memory.
Name Description
In Filters Tab (Input Filters)