BRUEL & KJAER 2972 User guide

Gate Deltlv
Tape Signal Gate
Type 2972
A mains operated instrument for
the elimination of tape splice noise
or other unwanted signals on
pre-recorded tape loops . Although
primarily intended for use with
multi-track tape recorders it may
also be used with
digital recorders.
For the analysis of recorded
signals it
will be found especially
useful for ensuring that only the
desired part of the signal is
analyzed.

TAPE SIGNAL GATE
TYPE 2972
November 1972

CONTENTS
1. INTRODUCTION ..................... . ........................... . ........ . ..... 5
2. CONTROLS ...... ............. .............. .. ...... ........ .... .. .. .. ......... ... . 6
2.1. Front Panel ........................................................... 6
2.2. Rear Panel
.. . .. .. .... ......... ... ....... . .................. ..... ...... 10
3. OPERATION ............................................................. . .... ..... 12
3.1. Preliminary ............................................................ 12
Mounting .............................................................. 12
Mains Voltage Selection ......................................... 12
3.2 . Operation With Tape Recorder . .......... . .. .... ...... . ....... . 12
Use with the 8 & K FM Tape Recorder Type 7001 . .. .. . 13
Use with other types of Tape Recorders .................... 19
3.3. Operation With Digital Recorder ...... ... ... . ...... .... .. . ..... 19
Use with the 8 & K Digital Event Recorder Type 7501 20
3.4. Uses of Auxiliary Output .. .. ...... ..... .. ..... . ........ .. ........ 23
Remote Reset facility .................. .... ...... .... ... ........... 24
TIL Gate Pulses ..... ................... ..... .............. .. ........ 25
4. OPERATING CHARARACTERISTICS .............. .. .................. 27
4. 1. Triggering .. .. .. . .... . ..... ... .. . .. . ............ ... .... .. . ............. 27
4.2.
Gate Delay ... .. ...... ..... .. ....... ........ .. .......... ..... . .. ....... 29
4.3. Gate Width ....... . ......... .. .... .. ...... ... ....... .. .. .. .. ........... 30
4.4 . Signal Gate ...... ........ ................. .. ..... ... ......... .. ....... 31
4.5.
Power Supply ..... ..... .. ... .. ... . .. ... .... ...... ........ . .. .. ....... 32
4.6. Effect of Gating .. ............ ... ....... ..... .. .......... ..... ...... .. 33

5. FREQUENCY ANALYSIS ............................................... ..... 35
5.1 . Impulse Analysis .. .. ............ ..... ............................... 35
5.2 . Spectrum Strobing ... .. .. ........... .. ......... . ........... ..... .. . 41
6.
SHORT TAPE LOOP ADAPTOR TYPE WA 0030 ....... . ........ ... 44
6.1. Fitting the Short Tape Loop AdaP,tor .... ....... ..... ...... .. . 44
6.2. Fitting Automatic Speed Sensing On - Off Switch . .. ..... 47
7. SPECIFICATIONS ...................................................... ... ...... 49

1. INTRODUCTION
The Tape Signal Gate Type 2972 is a mains operated instrument for the
elimination of tape splice noise or · other unwanted signals on prerecorded
tape loops. Although primarily intended for use on multitrack tape
recorders such as the B & K Type 7001 it may also be used with digital
recorders. For frequency analysis of recorded signals it will be found
essential for ensuring that only the desired part of the signal is analyzed.
The 2972 has a frequency response from DC to 40kHz and a dynamic
range of 50 dB. It is capable of gating a signal so that only the part of
interest is passed to its output, whilst unwanted signals or noise are rejected
by more than 50 dB. The length of the signal passed to the output (gate
window) as well as its relative position in relation to the rest of the signal
may be adjusted from 0.1 msec. to 30 sec. using the GATE WIDTH and
GATE DELAY controls provided. To accurately set the position of the gate
window, the 2972 has two trigger modes whereby it may be triggered from
a sinusoidal signal burst recorded on a tape loop or a TTL pulse from a
digital recorder. For monitoring the output of the 2972 in order to see that
the correct part of the recorded signal is gated, a separate output is available
for triggering an oscilloscope.
5

2.1. FRONT PANEL
Power
Switch-+-___,.
Gate
Delay
DC
Equalizer
Trigger
Input ___ _,
2. CONTROLS
Trigger
Mode
Gate
width
Signal
~---"'---+-Displacement
Gate
'--------Output
Fig.2.1. Front panel of the 2912.
POWER SWITCH:
POWER INDICATOR:
GATE DELAY:
6
On/Off switch for the internal
connection of a mains supply connected
at the POWER
INPUT socket.
An opal indicator lamp which lights
when the instrument is connected to
an AC mains supply and the POWER
SWITCH
is set to "On".
Two controls, one switched (outer
knob), the other continuously variable
(inner knob) providing gate delays (time
taken for gate to be triggered after an
external trigger signal has been applied

DC EQUALIZER:
SIGNAL INPUT:
TRIGGER INPUT:
TRIGGER MODE: ·
to the TRIGGER INPUT socket) of
0. 1 msec. to 30 seconds.
The inner control provides continuous
adjustment of the gate delay over the
delay range selected using the outer
control.
Potentiometer control for adjustment of
the DC level of the gated signal (outside
gate
window, only) available at the
GATE OUTPUT socket. The range of
adjustment is ± 2 V referred to 0 V
chassis level.
Coaxial
input socket accepting the
B & K plug JP 0101 for application of
the signal to be gated. The socket is
connected to the BNC SIGNAL INPUT
socket on the rear panel of the
instrument and has an input impedance
of 1 MQ in parallel with 50 pF. The
maximum input voltage is 2 V peak.
Coaxial input socket for application of
an external trigger signal (burst or pulse
depending on setting of TRIGGER
MODE switch) to trigger the gate. The
trigger signal is delayed internally for
the time selected using the GATE
DELAY controls.
The TRIGGER INPUT has an input
impedance of approximately 18 kQ and
accepts
the B & K plug JP 0101.
Two position switch with the following
functions:
"Burst". With this mode selected a
sinusoidal signal burst with amplitude of
0.3 to 10 V and length greater than
30 msec. may be applied to the
7

GATE WIDTH:
SIGNAL DISPLACEMENT:
GATE MODE:
8
TRIGGER INPUT socket to trigger the
gate.
"Pulse". In this mode a TTL (transistor-
transistor logic)
compatible pulse (i.e. 0
to 0.5 V for logical 0, and 2.5 to 5 V for
logical 1) with length greater than
20 psec. may be applied to the
TRIGGER INPUT socket to trigger the
gate.
Two controls, one switched (outer
knob), the other continuously variable
(inner knob) providing gate widths
(period for which gate is open when the
GATE MODE switch is set to "Normal"
or period for which gate is closed when
the
GATE MODE switch is set to
"Inverted") of 0.1 msec. to 30 seconds.
The inner
control provides continuous
adjustment of the gate width over the
gate
width range selected using the
· outer contro I.
Potentiometer control for adjustment of
the DC level of the gated signal (within
gate window only) available at the
GATE 0 UTPUT socket. The range of
adjustment is ±2 V referred to 0 V
chassis level.
Two position switch governing the gate
mode.
The modes are :
"Normal"". In this mode the gate is open
for the period selected using the GATE
WIDTH controls i.e. the signal applied
at the GATE INPUT socket during this
period is passed to the GATE OUTPUT
socket. During the period immediately
before and after the gate is opened the
input signal is blocked.

TRIGGER OUTPUT:
GATE OUTPUT:
"Inverted". In this mode the gate is
closed for the period selected using the
GATE WIDTH controls i.e. the signal
applied to the GATE INPUT socket
during this period is blocked. During the
period immediately before and after the
gate is closed the input signal is passed
to the GATE OUTPUT socket.
B & K coaxial output socket producing
a + 14 V to -14 V pulse with period
corresponding to the GATE DELAY
selected. By using the positive and
negative slopes of the pulse to trigger an
oscilloscope the gate window and delay
may be monitored.
The TRIGGER OUTPUT accepts the
B & K plug JP 0101 and has an output
impedance of less than 200 n, but
should not be loaded by less than
10 kn.
Coaxial output socket for the gated
signal. The socket accepts the B & K
plug JP 0101 and is connected in
parallel with the BNC GATE OUTPUT
socket on the rear panel of the
instrument. It has an output impedance
of 10 n, but should not be loaded by
less than 2 kS1. The maximum output
voltage is 2 V peak.
9

2.2. REAR PANEL
Signal
Input
Gate
Output
Auxiliary
Output
SIGNAL INPUT:
GATE OUTPUT:
Fig.2.2. Rearpanel of the 2972
Voltage Selector
and Fuse
Power
Input
Chassis
Ground
Coaxial input socket accepting a BNC
plug (B & K Type JP 0035) for
application of the signal to be gated.
The socket
is connected to the B & K
coaxial SIGNAL INPUT socket on the
front panel of the instrument and has an
input impedance of 1 MD in parallel
with 50 pF. The maximum input
voltage is 2 V peak.
Coaxial output socket for the gated
signal. The socket accepts BNC plug
(B & K Type JP 0035) and is connected
in parallel
with the B & K coaxial GATE
OUTPUT socket on the front panel of
the instrument. It has an output
impedance of 10 n, but should not be
loaded by less than 2 kD. The
maximum output voltage is 2 V peak.
AUXILIARY OUTPUT:
8-pin Dl N socket accepting the B & K
plug JP 0802 for connection of a peak
reset
facility to a 2607 Measuring
Amplifier or a 2114 Third-Octave and
Octave Spectrometer. Also available at
10

VOLTAGE SELECTOR
AND FUSE:
POWER INPUT:
CHASSIS
GROUND:
the socket are gate width and gate delay
pulses and a + 14 V supply rail. For
connections see section 3.4.
Voltage selector for selection of the
correct mains voltage. The central knob
of the selector contains a 200 rnA fuse.
To remove the fuse or to set the selector
to the correct mains voltage refer to
section 3.1.2.
Input socket accepting European or
American power cables (B & K Types
AN 0005 and AN 0006 respectively),
for connection of an AC mains supply.
Socket accepting a Banana plug (B & K
Type JB 0002) for grounding the chassis
of the 2972 to the earth of a mains
supply.
11

3. OPERATION
3.1. PRELIMINARY
3.1.1. Mounting
The Type A metal cabinet of the 2972 is the Modular Unit KK 0025.
The unit may be used free standing, or with the Metal Frame KK 0014 for
rack mounting or with the Mahogany Case KA 0027 it may be mounted
with a number of other KK modular units. For further information on the
Bri.iel & Kjcer Module System refer to the KA, KK, KO, KS Assembly
Instructions.
3.1.2. Mains Voltage Selection
Before each use of the 2972 check that the VOLTAGE SELECTOR on
the rear panel of the instrument corresponds to the mains voltage to be
used. If not, remove the central knob of the selector containing a 200 rnA
fuse,
by simply pressing the knob in and turning it counter clockwise. Half a
turn should be sufficient to release the knob which may then be pulled
straight out. Behind the knob are some slots which with the aid of a coin
may be used to turn the selector so that the white line points to the correct
voltage indication .
3.2.
OPERATION WITH TAPE RECORDER
The 2972 is primarily designed for use with the B & K FM Tape
Recorder Type 7001, but may also be used with most other types of
multi-channel tape recorder which like the 7001 may be fitted with tape
loops. However, because of the dissimilarities between various types of
recorder this section gives separately operation with the 7001 and other
types of recorder.
12

3.2.1. Use With The B & K Tape Recorder Type 7001
The B & K Tape Recorder Type 7001 has four tape speeds ranging from
1.5 to 60 inches/sec, as well as two FM recording channels and one direct
recording voice channel. The FM channels have a frequency response
ranging
from DC up to 20 kHz depending on the tape speed selected, and
are used for recording signals for measurements or analysis at a later date.
The voice channel has a frequency response ranging from 300 Hz up to
3kHz and is used for recording comments and identifications of recordings
made using the measurement channels. For triggering the 2972, so as to
ensure that the opening of the signal gate can be accurately positioned, the
recorder's 440 Hz marker signal may be recorded on the voice channel.
Fig.3. 1. Controls of the FM Tape Recorder Type 7001
The controls of the 7001 are shown in Fig.2.1. To gate signals recorded
with the 7001 using the 2972 proceed as follows:
1. With the 7001 set up for reel to reel operation, locate the portion
of tape on which the signal to be gated has been recorded.
13

14
2. Erase any information which has been previously recorded on the
voice track of the portion of tape selected. The voice track may be
monitored by setting the recorder's MONITOR SELECTOR switch
to "Voice" and connecting the dynamic microphone supplied with
7001 to the MIC socket of the instrument. On play-back with the
MONITOR VOLUME control turned fully clockwise the
microphone will act as a small loudspeaker. To erase the voice track
press the push-button on the microphone. Whilst erasure is in
progress
the microphone's diaphragm should be muffled to prevent
any back ground noise from being recorded.
3.
Form the portion of tape selected into a tape loop and fit it to the
7001 using the Tape Loop Adaptor UD 0011 or WA 0030. The
adaptor UD 0011 is adjustable and accepts tape loops of between 8
and 25ft. (2.4 and 7.5 m) in length. Adaptor WA 0030 accepts
short tape loops of 14 ± 0.25 inches (362 ±5 mm) in length.
Instructions for mounting the Adaptor UD 0011 and fitting tape
loops to it are given in the Instruction manual for the 7001. Those
for the Adaptor WA 0030 are given in Chaper 6 of this manual.
4. Check that the tape loop is fitting properly by selecting a suitable
low tape speed and setting the recorder to play back. If necessary
adjust the tape tension.
5. Using the 440 Hz marker facility of the recorder, record a short
marker burst on the voice track of the tape for triggering the 2972.
The marker facility is activated by momentarily turning the
MONITOR SELECTOR switch from "Voice" to its "Marker"
position.
The marker burst should be recorded so that on replay at the tape
speed to be used for analysis, the portion of signal to be gated
occurs between 25 msec. and 25 sec. later. The length of the
marker burst should be kept as short as possible.
In
order that the marker burst may be correctly positioned, on the
tape, it can be recorded at one tape speed setting lower than that
which is to be used for analysis of the gated signal.
6. Reset the recorder for play-back at the tape speed at which the
gated signal is to be analyzed anq calculate the approximate
repetition time of the tape loop using the relation:

X
Tr =--,=-
s
where Tr, is the repetition time (sec.), X is the length of the loop
(inches, or mm x 0.0394 to convert to inches) and Ts is the tape
speed selected (inches/sec.).
7. With
the MONITOR OUTPUT sockets of the recorder connected to
the TRIGGER INPUT socket of the 2972, connect the TRIGGER
OUTPUT socket of the 2972 to an oscilloscope as shown in Fig.3.2.
8. Set the time base of the oscilloscope so that the length of the trace
on the display screen corresponds to a value equal to or less than
the repetition time of the tape loop calculated in item 6.
9. Set the remaining controls on the oscilloscope to:
CHANNEL SENSITIVITY
CHANNEL COUPLING
TRIGGER MODE
TRIGGER COUPLING
TRIGGER SLOPE
"1 0 V/division"
"DC"
"External"
"DC''
"Negative"
10. On the 2972 set the TRIGGER MODE switch to "Burst" and
adjust the GATE DELAY controls to give a delay time which with
the TIME BASE setting selected in item 8 corresponds
approximately to two divisions on the oscilloscope display screen.
~~~.,....,.~-- 7001 FM Tape Recorder
Monitoring
Microphone
+ UD 0011 or WA 0030
Loop Adaptor
Measurement
Channel
Trigger
Output
Oscilloscope
External
Trigger
472104
Fig.3.2. Instrument arrangement for checking ' that the 2972 is correctly
triggered by a marker burst recorded on the 7001 Tape Recorder
15

11. With the MONITOR SELECTOR on the recorder still set to
"Voice" and the dynamic microphone connected to the MIC
socket, set the MONITOR VOLUME control fully clockwise and
then gradually turn it counter clockwise until the signal burst only
just triggers the 2972. When correctly triggered the oscilloscope will
display the output waveform of the gate delay monostable of the
2972. The
waveform should appear as a single rectangular pulse and
will be displayed once (and only once) every time the 2972 is
triggered by the signal b~rst, which may be monitored using the
dynamic microphone of the 7001. The position of the pulse on the
display screen should remain constant.
12.
With the Tl ME BASE of the oscilloscope set to approximately
0,1 msec/division, check to see if the 2972 remains triggered when
the
GATE DELAY controls are set to 0.1 msec. If not then readjust
the
MONITOR VOLUME.
~~~.,.......~-- 7001 FM Tape Recorder
+ UD 0011 or WA 0030
Signal to be gated
Signal Burst
Measurement
Channel
Trigger
Output
Oscilloscope
External
Trigger
Gated Signal
472105
Fig.3.3. Instrument arrangement for gating a signal recorded on the 7001
Tape Recorder using the 29 72
16
13. Once correctly triggered reconnect the instrumentation as shown in
Fig.3.3 and
set the controls on the 2972 to:
TRIGGER MODE
GATE MODE
DC EQUALIZER
SIGNAL DISPLACEMENT
"Burst"
"Normal"
fully counterclockwise
central
position

14. With the CHANNEL SENSITIVITY of the oscilloscope set to
1 V/division and the Tl ME BASE set as in item 8 adjust the GATE
DELAY and GATE WIDTH controls of the 2972 so that the
waveform displayed on the oscilloscope is as shown in Fig.3.4.
15.
On the 2972 set the GATE WIDTH controls to the approximate
width of the signal portion of interest (if this is sufficiently long
then it should be seen in the gate window) and set the TRIGGER
SLOPE selector of the oscilloscope to "Positive". The oscilloscope
will
now be triggered at the end of each gate delay period so only
the gate window will de displayed.
, __
Gate Width
-i
or Window
-
r--
f"'
_j
(',
l 1 J
lJV
~
Gate
1,.. Delay v
...
472106
Fig.3.4. Oscilloscope display of the output from the GATE OUTPUT
socket of the 2972 when the "Normal" GATE MODE is selected
and the oscilloscope is triggered from the start of the gate delay
pulse (negative slope), which
is available at the TRIGGER
OUTPUT socket of the 2972
16. Set the Time Base of the oscilloscope so that the gate window
almost entirely fills the display screen. The displayed waveform
should now appear as shown in Fig.3.5.
17.
From the "0.1 msec." position, adjust the GATE DELAY controls
so that the signal portion of interest appears in the gate window. If
necessary readjust the GATE WIDTH controls if the gate window is
17

Gate Width
... 1-------or Window ______ __..,.
472107
Fig.3.5. Oscilloscope display of the output from the GATE OUTPUT
socket of the 2972 when the uNormalu GATE MODE is selected
and the oscilloscope is triggered by the positive reset slope of the
gate delay pulse available
at the TRIGGER OUTPUT of the 2972
~~--------Gate Width ____ ---11-..J~
or Window _,
472108
Fig.3.6. Oscilloscope display of the output from the GATE OUTPUT
socket of the 2972 when an oscillatory waveform with an
exponential decay
is gated. The zero crossing of the signal where
the opening
and closing of the gate window must be set for correct
gating are shown as dots on the waveform
18