Grimm Audio CC1v2 User manual

Please read this manual before operating the
unit.
Jitter is the Achilles heel of digital audio. A stable clock
is of vital importance for high quality sound.
CC1: without doubt The Final Word.
Please read this manual before operating the unit.

Manual CC1 © Grimm Audio
2
Table of Contents
1. Introduction 3
2. Important Safety Instructions 4
3. Installing 6
4. Operation 8
5. Setting up the studio 14
6. Jitter and PLL’s explained 18
7. Specications 23
8. Grimm Audio Limited Warranty 25
Table of Contents

Manual CC1 © Grimm Audio 3
1. Introduction
Thank you for choosing the Grimm Audio CC1 Central
Clock distributor for your production environment. This
product features a rich set of distributor functions,
based upon an ultra low jitter clock oscillator. It
embodies our company philosophy of providing the
most transparent signal chain possible, enabling you to
achieve the best possible results sonically and
artistically. Large parts of the CC1 circuitry, like the
oscillator and its power supplies, use a discrete design.
Because of this a CC1 has more than 700 components.
During construction 125 seperate elements are tested.
The CC1 can be used in a wide variety of applications
such as:
• House Sync generation in audio and/or video studios.
Sixteen outputs permit the use of the reliable “star”
distribution scheme. The extremely low jitter of the CC1
clock outputs maximises the sound quality potential of
the attached devices. In case you need to slave your
system to video clock, please insert a Grimm Audio VCC,
‘video to word clock converter’ before the clock input.
• Clock stability improvement, and hence improved
sound, of digital live desks and digital snakes.
• Clock distribution in broadcast studios.
• Improvement of your home audio system. For instance
in conjunction with word clock equiped devices or as
reclock unit in between a transport or media system
and a DA converter.
This manual describes how to set up the unit in your
studio, live or home environment as well as important
tips on how to get the best performance from the CC1.
In addition, some background information on the unit’s
operation is provided.
We hope this investment will bring you many years of
creative enjoyment and help you achieve your goals.
1. Introduction

Manual CC1 © Grimm Audio
4
2. Important Safety Instructions
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volledig begrijpt. Als u hier moeite mee heeft dient u contact
op te nemen met Grimm Audio. Op verzoek sturen wij u een
vertaling toe.
Grimm Audio nimmt an, dass Sie diesen Englischen Text
völlig verstehen. Wenn notwendig, nehmen Sie bitte Kontakt
auf mit Grimm Audio. Auf Wünsch wird Ihnen eine
Übersetzung zugeschickt.
Grimm Audio suppose que le lecteur comprend
parfaitement le texte en Anglais ci-dessous. En cas de doute
s.v.p. contacter Grimm Audio. Si necessaire, on pourra vous
envoyer une traduction.
Grimm Audio da por supuesto que el texto en versión
Inglesa no ofrece ninguna duda de interpretación y se
entiende integramente. Si este no fuese su caso rogamos
contacte con Grimm Audio quien, a petición, se encargaría
de enviarle la correspondiente traducción.
Please follow these precautions when using
this product:
1. Read these instructions.
2. Keep these instructions.
3. Heed all warnings.
4. Follow all instructions.
5. Dangerous voltage is inside this apparatus. Opening
is only allowed by qualied service personnel.
6. Verify line voltage before use.
7. 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 a third grounding
prong. The wide blade or the third prong are
provided for your safety. When the provided plug
2. Important Safety Instructions

Manual CC1 © Grimm Audio 5
does not t into your outlet, consult an electrician
for replacement of the obsolete outlet.
8. Protect the power cord from being walked on or
pinched, particularly at plugs, convenience
receptacles, and the point where they exit from the
apparatus.
9. Unplug this apparatus during lightning storms or
when unused for long periods of time.
10. Do not use this apparatus near water.
11. Do not use this apparatus outside.
12. Do not expose the apparatus to dripping or
splashing. Do not place objects lled with liquids
(ower vases, drink cans, coee cups, etc) on the
apparatus.
13. Clean only with a dry, soft, non-uy cloth. Do not
spray any liquid cleaner onto the cabinet, as this
may lead to dangerous shocks. Do not spray any
liquid cleaner onto the faceplate, as this may
damage the wooden front panel.
14. Install in accordance with the manufacturer’s
instructions.
15. Do not install near any heat sources such as
radiators, heat registers, stoves, or other apparatus
(including ampliers) that produce heat. Avoid
exposure to direct sunlight.
16. Use only attachments or accessories specied by
the manufacturer.
17. Use only with a cart, stand, bracket, or table
designed for use with professional audio or music
equipment. In any installation, make sure that injury
or damage will not result from cables pulling on the
apparatus and its mounting. If a cart is used, use
precaution when moving the cart/apparatus
combination to avoid injury from tip-over.
2. Important Safety Instructions

Manual CC1 © Grimm Audio
6
18. This apparatus typically runs slightly warm when
operated. Install in a normal ventilated area. If the
product will be used in a rack, make certain there is
sucient air movement within the rack.
19. Refer all servicing to qualied service personnel.
Servicing is required when the apparatus has been
damaged in any way, such as when the 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.
20. WARNING: To reduce the risk of re or electric
shock, do not expose this apparatus to rain or
moisture.
3. Installing
Unpacking and Inspection
Your CC1 was carefully packed at the factory and the
carton it came in was designed to protect it from the
trials and tribulations of shipping. Keep the box and all
packing materials, so that in the unlikely event that you
need to return the CC1 for servicing, you can do so
safely.
Mounting the CC1
The CC1 can mount in any standard 19” rack. The CC1
does not produce RF elds nor is susceptible to them.
You can position it near other digital gear such as
computer and disk recorders without worry. In general it
is a good idea to keep some distance between monitors
(LCD and CRT) and audio and word clock cables because
of risk of induced low level noise due to stray magnetic
elds.
3. Installing

Manual CC1 © Grimm Audio 7
Grimm Audio products have a real wood face plate that
provides a beautiful and vivid appearance. To maintain
the outstanding looks, one is advised to take some
precautions:
- Do not place the CC1 in humid nor very dry
environments. The wood might crack.
- Do not use chemical or alcohol based cleaner on the
wood.
AC Power hookup
The CC1 has a linear power supply that needs to be
factory set for your local line voltage. Make sure to
check the noted line voltage on the back below the IEC
cable entrance (‘wired for ... V’) and verify that this
complies with your locally supplied line voltage. Grimm
Audio cannot be responsible for problems caused by
using the CC1 with improper AC wiring or voltage. Since
the CC1 does not have a power switch on the front
panel, a convenient way to power down the unit is to
use a power strip equipped with a switch.
3. Installing

4. Operation
Signal Connections
Manual CC1 © Grimm Audio
8
AES-in
lock
status
Impe-
dance
group 1
(75 Ohm
or Lo-Z)
Impe-
dance
group 1
(75 Ohm
or Lo-Z)
Impe-
dance
group 2
(75 Ohm
or Lo-Z)AES-out
behavior
(thru or
mute)
Word
clock
input
AES-in
(also for
re-
clocking
purpose)
AES-out
(follows
AES3-in
when
available
and
selected,
otherwise
group 2)
Word
clock
outputs
group 1
Word
clock
outputs
group 1
Word
clock
outputs
group 2
Unlock
be-
havior
(hold or
mute)
Phase of
output
11, 12,
13, 14
(normal
or 180°)
Group 2
sample
rate multi-
plication
factor
Group 1
sample
rate multi-
plication
factor
Master
44.1/48 kHz
and Slave
4. Operation
Indication of
factory set
line voltage
value

Manual CC1 © Grimm Audio 9
Master mode
The leftmost front panel push-button selects between
two base clock rates: 44.1kHz or 48kHz. The two other
push-buttons determine the actual clock frequency
transmitted across the 16 BNC outputs. They control in
two groups. Group 1 are channels 1-12. The second
push-button sets the multiplication factor for this group
to 1x, 2x or 4x. Group 2 are channels 13-16, and their
multiplication factor is set by the third push-button.
Setting ‘48kHz’ using the leftmost push-button, ‘1x’ on
the middle selector and ‘2x’ on the rightmost selector
will set the clock rate on group 1 BNC’s to 48kHz and on
group 2 BNC’s to 96kHz.
The AES3 output has two functions, depending on the
setting of the dip switch marked ‘aes’ on the rear. It
determines whether the audio content found at the
AES3 input is passed through to the output (‘thru’) or not
(‘mute’). In mute mode the AES3 data will be black and
run at the rate selected for group 2. In thru mode, the
AES3 output will obviously run at the same rate as the
AES3 input. On loss of input signal, the output will
remain at the same sampling rate.
The exact behaviour of the AES3 output in master mode
depends on the use case:
1. No signal connected to the AES3 input. The AES3
output is a black signal (no audio) with the same
sampling rate as the Group 2 BNC outputs. The channel
status bits indicate “Grade 1 reference” if the CC1 has
gone through the optional frequency calibration,
otherwise “Grade 2 reference”. The AES lock indicator on
the rear is o.
2. A signal is applied to the AES3 input that is not
synchronous to the CC1. The AES3 output is a black
signal with the same sampling rate as the Group 2 BNC
outputs. The AES lock indicator on the rear blinks.
3. A signal is applied to the AES3 input that is
synchronous to the CC1, but the ‘aes’ dip switch is set to
‘mute’. The AES3 output will still be black and
synchronous. The presence of the aes signal has no
4. Operation

Manual CC1 © Grimm Audio
10
eect. In master mode with the ‘aes’ dip switch set to
‘mute’, any signal at the AES3 input is basically ignored,
except that the ‘aes lock’ light on the rear will be on to
indicate the presence of a synchronous input.
4. A signal is applied to the AES3 input that is
synchronous to the CC1, and the ‘aes’ dip switch is set to
‘thru’. The AES3 output fully copies audio and subcode
data (but not the jitter!) from the AES3 input. In this
mode the AES3 output will never indicate ‘grade 1’ as
AES11 requires a black signal. The ‘aes lock’ light is on.
5. If the input signal is lost from case 4, the sampling
frequency setting is retained, as sudden loss of signal is
most likely accidental. This is dierent from case 1
where an AES3 input signal was never present. If the
removal of the AES3 input was on purpose, changing the
group 2 multiple on the front will return to case 1, and
the AES3 output will once again be synchronous to the
selected multiple of group 2.
Slave mode
Slave mode is the third setting on the leftmost selector.
In slave mode, the CC1 will look at the word sync input
as well as the AES3 input for a valid signal. Slave mode
permits the following use cases:
1. Word sync only. Upon selection of slave mode, the
CC1 selects the word sync input. The middle and right
selector once again set groups 1 and 2 to a multiple of
the base rate. The AES3 output will be black,
synchronised to group 2 and the channel status is
marked “no reference”. Upon loss of lock, the output
frequency will be held constant, see case 4.
2. AES3 only. Upon selection of slave mode, the CC1
selects the AES3 input. The ‘aes’ dip switch at the rear
selects whether the audio data is transmitted (thru) or
not (mute). In thru mode, the AES3 output will obviously
run at the same multiple as the AES3 input. In mute
mode it will be synchronous to group 2, audio is black
and marked “no reference”. Upon loss of lock the output
4. Operation

Manual CC1 © Grimm Audio 11
rate will be held constant, see case 4. The ‘AES lock’ light
on the rear will be on as long as the CC1 PLL is locked.
3. Both AES3 and word sync are present. Word sync will
take precedence over the AES3 input. In thru mode the
AES3 output will produce a jitter-free copy of the AES3
input, provided the input signal is synchronous. In mute
mode the AES3 input will be ignored altogether. Upon
loss of the AES3 input the AES3 output will revert to
black. Upon loss of the word sync input the output
frequency will be held constant (within 1 ppm of the last
detected frequency) and the front slave light will blink to
indicate an error. The fault condition is cleared either by
reattaching the word sync or by cycling the source
selector through the two master modes back to slave
mode to force the CC1 to lock to the AES3 signal instead.
The CC1 will never automatically switch from word sync
to AES3 to prevent clock loops.
4. No valid signal on either word sync or AES3 input. The
response is now determined by the dip switch named
‘unlock’. When set to “thru”, the clock frequency will be
held constant within 1 ppm of the last valid measured
frequency. When set to “mute”, the outputs will be
muted. In both cases the slave light will blink to indicate
an error. The mute mode is advantageous at for
instance post production where the clock usually is out
of sight in a machine room. Upon loss of lock, all clocks
disappear and your software automatically provides a
warning. The thru mode has an advantage in for
instance live recording or Public Address. Here sound
should never mute, whatever happens.
Lock indication
The CC1 detects whether the input signal is a multiple of
44.1kHz or 48kHz, and indicates this on the LEDs above
the source push-button. The frequency multiple settings
of groups 1 and 2 remain as set by the user. While the
CC1 acquires lock, the slave light will blink. This can take
up to 20s, on account of the CC1’s extremely slow PLL
design (see chapter 7, “Jitter and PLL’s explained”). Once
frequency lock is achieved, the slave LED will come on
continuously. The PLL will then settle to 0ophase lock
with the input in an even slower mode. The smallest
achievable phase error depends mostly on the stability
4. Operation

Manual CC1 © Grimm Audio
12
of the incoming clock. The static (average) phase error is
factory-aligned to be within 50ns.
The ‘lock’ LED on the rear indicates the AES3 receiver
status:
1. O: No AES3 signal present.
2. On: AES3 signal present and found to be synchronous
with the CC1.
3. Blinking: An AES3 signal is present but it is out of sync
with the CC1.
Impedance
The ‘impedance’ DIP switches on the rear allow each
individual output to be set to low-impedance (25 ohms
typical) or characteristic (75 ohms) impedance. The
switches are factory set to 75 ohms. Should the
receiving device have problems recognising the word
sync signal, you might try the low impedance setting.
Word sync connections are usually 5V square waves
transmitted over 75 ohm (video) coax cables. Although
the word clock frequency itself is relatively low, the
transitions are steep. Taken as a whole, word sync is a
wideband signal, necessitating characteristic
termination at least at one end of the cable. Single-
terminated lines are usually terminated on the source
end. Better still is series termination on the transmit
end and parallel termination on the receiving end. This
minimises reections even when the cable impedance is
not exact. Double termination like this produces a 6dB
loss, resulting in a 2.5V signal. All Grimm Audio products
are designed to work in this manner.
There is no standard dening word sync connections. As
a result, impementations vary across makes. Outputs
may be series terminated or low impedance, DC or AC
coupled. Inputs may be parallel terminated or high
impedance. This yields 8 permutations, all of them
encountered in the wild. The CC1 input will correctly
interpret all variants without user manipulation, but the
impedance of the CC1’s outputs may need to be set low
in some cases. A parallel-terminated input expecting a
4V input signal will only work with the CC1’s output
impedance set low. A fair number of products were
found to have parallel terminated input but stop
4. Operation

Manual CC1 © Grimm Audio 13
working around 2.5V which is why the CC1 puts out a
slightly higher-than-normal voltage in order to allow
correct operation in the factory preset mode.
Nevertheless, some equipment will only respond
correctly when the output impedance DIP switch is set
to low.
Phase
Channels 11 to 14 can be polarity-reversed (180ophase
shift) to cater for equipment that uses the falling edge
to synchronise on instead of the rising edge. Should you
encounter operational problems with equipment, even
though it is locked, check its manual for any mention of
clock phase and try the 180oswitch setting.
AES and unlock switches
The ‘aes’ and ‘unlock’ switches can be set to ‘’mute’ or
‘thru’. The behavior of the CC1 in these modes is
described in the sub-chapters ‘Master mode’ and ‘Slave
mode’ above.
Key Lock mode
If your CC1 is used in one setting most of the time you
can use ‘Key Lock’ mode to prevent clocking errors by
accidental pressing of control buttons on the CC1’s front
panel. In this mode the CC1 does not respond to the
push of any button. ‘Key Lock’ mode is engaged by
holding the ‘44.1/48/Slave’ button during power up. It
will then lock the mode that was set before the last
power down. To leave ‘Key Lock’ mode, hold the
‘44.1/48/Slave’ button during power up again.
4. Operation

Manual CC1 © Grimm Audio
14
5. Setting up the studio
In a digital studio all equipment will need to be
synchronised. If all there is are two boxes, one sending
and one transmitting, all that is needed is a digital signal
and the recipient will lock to that. When multiple
sources are feeding into one recipient (e.g. a digital desk
connected to several digital recorders and converters),
all sources will need to be synchronous. This is what a
house sync (aka master clock) is for.
Several formats are in use. The AES promotes a
standard called AES11 that uses an AES3 link, but most
equipment tends to use a word sync signal on a BNC
connector. The AES11 standard is to be amended to
include this practice. We think this is a good move, since
the purity of a clock recovered from an AES3 link is
rather susceptible to issues like cable length and
bandwidth limitations. The AES3 output on the CC1 is
provided to cater for the few equipment that needs
AES11 over AES3. When the output is not used for this
purpose, it doubles as a jitter removal function for any
AES3 signal (provided it is synchronous with the CC1).
Most studios that have introduced a house sync have
noticed a change in the overall sound quality of their
equipment. Usually for the better, sometimes for the
worse. At issue is jitter (akin to wow and utter, but then
applied to the sampling clock). Chapter 7 “Jitter
explained” will go into more details. The important point
to remember is that jitter aects audio quality only at
points in the chain where an actual time-based
conversion is taking place, typically AD and DA
converters but also in asynchronous sampling rate
converters.
Connection
The CC1 will usually be located in the machine room.
Each device connected to the CC1 is set to ‘slave to word
sync’. The best connection scheme is ‘star distribution’
with each output of the CC1 connected to one device.
The sixteen outputs will normally suce. Daisy chain
connections are best avoided, as are T-junction con-
nections. The latter will most likely fail anyway because
of the lack of standardisation of termination impedance
5. Setting up the studio

Manual CC1 © Grimm Audio 15
and signal level, not to mention mechanical failure of T-
junctions themselves.
Try to prevent circulating ground currents through the
coax cables (ground loops). As coax cables go, they are
unbalanced and currents owing through the cable
shields will impose an error voltage on the signal,
causing jitter at the receiving end that wasn’t there at
the transmitting end. One common cause of circulating
currents is leakage current through the power supply of
an ungrounded product. Another is voltage drop
between the earth ground connection between devices
that are located far apart. Minimizing the loop area
encircled by the entire loop, comprising the word sync
cable and the mains cords is usually a good idea. By all
means, never defeat the safety earth connection of any
device that is designed to have an earth connection.
Lifting grounds may produce a lethal shock hazard.
The CC1 itself has a low-impedance reference plane at
the rear, insuring that it will never be aected by
circulating currents. Not all equipment is designed in
this manner unfortunately.
AES3 reclocking
Apart from the word sync input and sixteen outputs, the
CC1 also sports an AES3 input and output. The output
can carry an AES11 DARS signal but it may also be used
to transmit a re-clocked (de-jittered) copy of the signal at
the AES3 input. This may be useful when driving a jitter
sensitive device lacking a word sync input, most typically
an outboard DAC in the monitoring system or a digital
input loudspeaker.
The reclocking function is available in all congurations,
that is, master, slaved to word sync or slaved to the
AES3 input itself. Apart from the latter trivial case this
implies that the source of the AES3 signal should itself
be somehow synchronised to the CC1. Setting the ‘aes’
switch to ‘thru’ activates the reclocking mode, passing
maximally jitter-free signal to e.g. a DAC. In most cases
this will improve replay quality signicantly.
In consumer use, most equipment features RCA S/PDIF
connectors in stead of XLR AES3 connectors. To use the
CC1 in such a hi setup, special S/PDIF to AES3 and AES3
5. Setting up the studio

Manual CC1 © Grimm Audio
16
to S/PDIF conversion cables are needed with build-in
impedance and signal level adaptation circuitry. Grimm
Audio has these cables available in its web shop.
Clock Loop
A clock loop is a runaway situation where two devices
are slaved to each other, for instance the CC1 slaved to
a recorder which in turn is slaved to the CC1. Neither
device is the clock master and the result will usually be
that the clock frequency ends up at an extreme of the
tuning range of one of the devices. Before setting the
CC1 to slave, insure that the clock input originates from
a master.
PLL setting in attached devices.
Audio devices capable of locking to an external clock use
a phase-locked loop to do so. Details of this are covered
in our white paper “PLL and clock basics” that can be
found in the downloads section of the CC1 page on our
website. An important thing to know for a CC1 user is
that some products have two modes, usually called
‘fast/wide’ or ‘slow’. The slow setting has the best
suppression of incoming jitter, and is the optimum
choice when the incoming clock is polluted. The
downside of the slow setting is that the jitter in the
recovered clock is fully dominated by the clock oscillator
that forms part of its PLL. If the incoming clock happens
to be much stabler than that, the slow mode will
actually end up adding jitter. The latter scenario is
almost guaranteed to be the case with the CC1, which is
signicantly cleaner than any clock oscillator we’ve
found in commercially available audio products so far.
As a rule, in this case it is usually better to select the
‘fast’ mode. In fast mode, the PLL will track the incoming
clock more closely and if the incoming clock is extremely
clean, the same will go for the regenerated clock.
Products that have a xed slow PLL, such as those from
Apogee, dCS, Lynx or Prism Sound, will improve
comparably little compared to products with a
switchable or relatively fast PLL like those from Avid,
Lavry Engineering, MOTU, Mytec, SSL or RME.
5. Setting up the studio

Manual CC1 © Grimm Audio 17
Syncing to Video
Before complicated digital studios became practical,
video studios have been using house syncs for decades.
As soon as more than one camera is used, all sources
need to be synchronised. Apart from a time base
reference there is also a need to know the actual timing
of a frame. This information is provided by time codes.
Although the time code signal is of course synchronous
with the video sync, it is usually transmitted separately.
In some cases the ‘LTC’ code is used as main video sync,
but fortunately it is going out of fashion. LTC is probably
the least stable time reference around.
Enter digital audio. The audio track now needs to be
synchronised to the picture. Word sync has a better
time resolution than video sync, but still the most
common practice is locking the audio clock to the video
clock. Even modern digital video production practice
tends to lock audio to an analogue video signal. This is
unfortunate because low jitter is not exactly a design
spec for a video sync generator.
The CC1 cannot slave to video sync directly. In order to
avoid direct crosstalk between the black-burst generator
or receiver and the audio clock, we have omitted video
sync functionality. Our best advise is to use the CC1 as
the audio master and slave all video to the audio word
sync. If you nevertheless do need to slave the audio
system to video sync you can either buy our VCC ‘Video
to Word Clock’ converter or use any other generic video
to word clock converter. Connect the retreived word
clock to the CC1’s word clock input and have the CC1
clean up the sync and distribute clock to the audio
devices as usual. The extraordinary stability of the CC1’s
PLL will make sure the result comes as close as possible
to making the audio chain master.
Hints
• Select decent quality 75 Ohm coax cables and certainly
do not skimp on the quality of the connectors.
• Make cables no longer than necessary.
5. Setting up the studio

Manual CC1 © Grimm Audio
18
• Cables add propagation delay. It is exceedingly rare for
dierences in clock phase to aect the interoperability
of connected audio equipment, but if it happens, using
equal length cables throughout helps.
• Disconnect unused gear.
• Ideally do not leave the word clock input connected
when the CC1 is master. Although unlikely (the CC1 has
a relay to actively disconnect the input when not in use),
there is a possibility of cross-talk from the input clock to
the outputs. Likewise, do not leave an AES3 cable
attached if not needed.
6. Jitter and PLL’s explained
Jitter is an instability in the timing of a clock signal.
Ideally the rising or falling edges of a clock signal are all
separated by exactly the same amount of time. In
reality, the timing is more uncertain. Figure 1 below
shows the time-domain view of the problem.
We can also look at the problem spectrally. An ideal
clock has only a single frequency components (and
harmonics). All energy is concentrated on an innitely
narrow frequency band, see the top graph in gure 2.
When jitter is present, side bands occur. Some spectral
energy is located away from the clock. The faster the
timing chatters, the further away from the main
frequency you’ll nd energy.
Figure 1. A stable and a jittery clock signal
6. Jitter and PLL’s explained

Manual CC1 © Grimm Audio 19
The red curve in the bottom graph shows only random
jitter. Sometimes jitter is periodical and then you
wouldn’t see a smooth slope but sharp peaks. An
important note is that frequency accuracy has
absolutely nothing to do with jitter at all. The red graph
shows a jittery clock with an exactly correct frequency,
say 44100.0000000Hz, the blue one shows a clock that’s
somewhat o but otherwise brilliantly stable. A
frequency error can be annoying from a practical
perspective, but as far as signal quality is concerned
you’re better o with a stable clock at an inexact
frequency.
Jitter is not a problem in fully digital processes. As long
as it isn’t so large that a processor can’t distinguish the
previous bit from the next, all-digital processes are
completely indierent about jitter. The problem occurs
when you go from the analogue domain into the digital
domain or back. Digital audio presumes uniform
sampling. That way, given a string of numbers and
knowledge of the sampling interval, you can perfectly
reconstruct a sampled signal. With jitter that theory falls
apart.
Imagine the blue analog signal top left in gure 3. The
AD converter samples it at neatly uniform intervals.
Bottom left, the DA converter reconstructs the correct
values but it gets the timing wrong. The dierence
between the red and blue curve shows the error. Or
take the converse in the right column: the DAC is ne
Figure 2. Spectra of clock signals
6. Jitter and PLL’s explained

Manual CC1 © Grimm Audio
20
but the ADC took samples at the wrong time. The eect
is much the same. What’s worse in this case is that the
recorded numbers are now wrong. In the previous case
we could use a better DAC, with a jittery ADC the
damage is unrecoverable.
PLL’s
Converter chips need a high frequency clock signal,
usually something around 22MHz. It is always made by
a local oscillator (anything from a simple RC oscillator in
a receiver chip to a crystal oscillator) regardless of
whether the unit is operating in master or slave mode.
This local oscillator is indispensible: external sync
signals may be AES/EBU, a sample rate frequency word
clock or even a video signal, neither of which are of any
direct use to the AD/DA. Instead, the local oscillator is
sped up or slowed down to make it run in step with
(“locked to”) the sync signal. A system that uses a local
oscillator and pulls it in sync with an external signal is
called a Phase Locked Loop (PLL). A ‘phase detector’
compares the local clock with the external sync signal
and puts out a signal when the local oscillator runs too
fast or too slow.
Now, the PLL will track the external sync closely, but not
too closely. Unwanted uctuations (jitter) of the external
sync signal are to be ignored but if the local oscillator
drifts o it should be slowly pulled back into step. So
6. Jitter and PLL’s explained
Figure 3. Jitter in AD (top) and DA (bottom) converters