Synchronization

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Sample rate, Word Clock, PLL and more: An overview for a better understanding

With the continuing change from analog to digital technology a new part of digital audio confuses more and more users: the synchronization. But which kind of synchronization? In this overview we don't take a look on MIDI Time Clock or SMPTE, but on the problems that come up when connecting digital audio devices.

CD to DAT

CD-Player and DAT Recorder make up good examples and are suitable as help in understanding, as their principle of operation is easy to understand and well known to most users.

In the analog domain one can connect any device to another device, a synchronization is not necessary. Digital audio is different. It uses a clock, the sample frequency. The signal can only be processed and transmitted, when all participated devices share the same clock. If not, the signal will have wrong samples, distortion, crackle sounds and drop outs.

In practice digital appliances are master or slave. The master is the clock reference, all slaves follow the master.
 
Not all appliances are able to work as slave and master! For example usual CD-player only work with their internal clock. They are not able to synchronize themselves to an external clock, thus are only useable as master.

Each device which is able to receive digital audio signals is able to work as slave, else no error free recording would be possible. A DAT for example includes a special circuitry, that extracts the clock reference out of the digital datastream. A part of this circuit is the so called PLL (Phase Locked Loop.)

Fig. 1 shows the way these devices work together. The digital output of the CD-player sends the clock inside the SPDIF signal to the DAT, which locks (using a PLL) on this signal and switches off its internal clock reference. Thus the DAT is able to record the data from the CD-player without any error.

Fig. 1

Master - or what?

So each digital input can be used as reference clock source. This feature is often restricted to internal usage. As digital devices should also work without any input signal present they include an internal fixed clock, a quarz crystal. Because of this the DAT is working as master in playback mode.
 
Usual DAT's will only work as master in playback mode, because - like shown in the diagram - in record mode the PLL, and in playback mode automatically the internal clock is active.

Confusing (and not yet covered through specific names) is the difference between a master and a 'complete' master. The 'complete' master is able to allow recordings without getting its clock out of the input signal, instead using its internal clock (this may be explained as master usage in Full Duplex mode.)

When a digital device allows recordings without getting its clock out of the input signal there should be much disturbance, as there is no synchronization between the external and the internal clock. But if the combination and cabling of the used devices is done in a special way it is no problem.

Digital Mixing Desk and ADAT

Let's take a look at an example, where this kind of usage happens every day: a digital mixing desk connected to an ADAT recorder. The mixing desk is master, sends its data to the ADAT.

The ADAT acts different compared to 'normal' DAT's, because the clock reference used can be switched between internal (quarz) and external (input), independent of record or playback mode (Fig. 2.) While a DAT would change its clock at playback automatically to internal, the ADAT allows to get the clock even then out of the input signal. This makes it possible to change its clock with the sample frequency fed, or in other words to synchronize him to the input signal regardless of his actual state.

Fig. 2

It doesn't matter if in playback, record or input monitor mode, the ADAT will always work with the clock received from the master (in this case the mixer), and send its data with the same clock back to the mixing desk.

Such a behavior may be described as synchronizable. But this will make professionals think of timecode related control, for example to shuttle forward and backwards in sync to another tape machine. But what we are talking about here is not related to time domain or absolute position, it's just the synchronization to a sample clock with sample accuracy.

To prevent disturbances due to misaligned sampling at the digital input of a mixing desk in master mode, the input signal has to be in total synchronicity to the signal send out from the desk. Note that in this case the inputs will not use the PLL to sync on the input signal, but are in sync with the internal clock of the desk. And that's exactly what's happening here: the desk sends out a digital signal controlled from its internal clock, the ADAT receives it, locks on it (using its PLL), sends out a digital signal with exactly this clock, which in turn could be received without any error from the digital mixing desk.

More interesting is the other way round: the desk works as slave and uses an external clock (the input signal), the ADAT is master. The ADAT now uses the modes 'Int 44,1' or 'Int 48'. Compared to a DAT the whole loop would crash when a recording is started. But the ADAT is able to use the internal clock even in record mode, so everything works fine.
 
A recording is only possible because the mixing desk locks itself on the clock from the ADAT and sends it data with exactly this clock as record source back to the ADAT. With many digital appliances this wouldn't be possible, as playback mode often forces the internal clock to be used.

DIGI32 Series

The principle of operation of all DIGI32 cards can be seen in Fig. 3. All three versions are equipped with internal quarz clocks, enabling them to playback data in master mode at the fixed sample frequencies 32 kHz, 44,1 kHz and 48 kHz (DIGI32 PRO additional 64 kHz, 88,2 kHz and 96 kHz), without any input signal needed.

Fig. 3

Thanks to a PLL in the input circuitry they will also sync to any input signal in the range of 25 kHz to 60 kHz (DIGI32 PRO 25 kHz to 105 kHz.) As opposed to the DAT shown in picture 1 DIGI32 can use the input synchronization also in playback mode. With 'AutoSync' active and a valid input signal available the card will switch off the quarz and use the clock extracted out of the input signal. 

This automatic behavior has many advantages, and additional allows true varispeed usage, because the card will follow every change in speed detected at the input. Conclusion: the DIGI cards may work as master or slave and can be synchronized even in playback mode from the input signal.

The DIGI32 series was designed as Hi-Speed interface without compromises. Because of this input and output share the same memory, which leads to a sensational low loop delay of only 2 samples in Record while Play mode. With AutoSync active input and output are truly 'in phase'.

The difference to a 'complete' master (like the ADAT) is the ability to be used as master in a loop mode. A special input buffer is needed, that is not available in the DIGI cards because of the Hi-Speed interface. Picture 3 shows this as missing record mode when using the internal clock.

Loop Mode

This missing mode leads to a restriction when using the loop mode. In principal DIGI's are not able to be used in loop mode. This is always true when the other participating devices are also no 'complete' master. A typical application would be to use a digital effects device in loop cabling with the DIGI32. Digital effect devices work in most cases exactly like the DIGI32, thus can be synchronized from their input signal, but won't use their own clock instead. Playing back audio data to the effects device and immediately returning the processed data back to the DIGI (which now records this signal) is therefore impossible. Despite Full Duplex this can't be done because both devices in the loop try to synchronize themselves on their input.

Fig. 4 shows a digital mixing desk and a digital effects device in loop mode. The loop is closed regarding the audio signal (in fact feedback may occur), but interrupted inside the desk regarding the clock signal.

Fig. 4
   
Fig. 5 shows an example for a loop mode that won't work using the DIGI32. There is no break in the loop, both effects device and DIGI try to extract their clock via PLL out of the input signal. Because of a missing clock (master) reference the sample rate will immediately drift away and lead to a break down of the loop.

Fig. 5

Yes it works...

When 'AutoSync' is deactivated and the DIGI is only used in playback mode it works without any problem in a loop configuration, because the DIGI will not try to sync to the input signal. This is the case when used with a DAT in loop mode. As the DAT is no Full Duplex device (hence plays back or records, but not both at the same time), and a recording with the DIGI makes no sense when the DAT is also in record mode, the double PLL will not happen.

The loop mode with a simple effects device requires an external clock fed into the loop. There are several possibilities for this like an effects device that is useable as master (if the device is equipped with a word clock input it is.)
 
If the device is not a 'complete' master a third 'complete' master should be added to the loop. The audio data should bypass it unchanged. In most cases this device will be a mixing desk, because these are always a 'complete' master and have internal and external sync at hand.

If you don't have a digital mixing desk a special clock generator (there is no common name for it, so we just name it Loop-Clock) will help. The Loop-Clock is made out of a simple digital circuit and works as clock master for loops of devices that all get their clock out of their inputs. We will publish a circuit diagram of this device as Tech Info in the next time.

Another solution is to use a sample rate converter, like the Behringer SRC-2000. An SRC automatically breaks up every loop and feeds in his own clock. The SRC-2000 resamples the audio data in such a high quality that it is impossible to notice any difference.

DIGI32/8 and ADAT

The ADAT interface of the DIGI32/8 acts exactly like written before. Therefore a loop mode of ADAT and DIGI32/8 works without any problems as long as the ADAT is master (Int) and the DIGI32/8 is slave (AutoSync active.)

Many users asked us why DIGI32/8 has no word clock input. Well, simply because it is not needed! What's more: a word clock based ADAT system will not work with the same synchronization quality as with the bitclock PLL we use. All people asking us this question never looked at the back of their ADAT: yes, there is no word clock input or output. Guess why...

DIGI32/8 needs no word clock input, because it synchronizes perfect to its ADAT input signal. And our special Hi-Speed hardware guarantees phase locked input to output, so the signal at the output is always synchronous to the input signal.

Of course one of the used devices has to be the master. This can be a digital mixing desk with ADAT interface or an ADAT machine connected to the DIGI32/8. The master sends his clock along with the audio data through the optical cable (lightpipe) to the DIGI32/8.

DIGI32/8 uses a special Bitclock PLL. Thanks to an extreme fine resolution this circuit is able to follow the complete varipitch range of an ADAT XT without loosing a sample. This can't be achieved with a normal word clock circuit! A word clock is based on a fraction of the real clock needed. For example SPDIF: 44,1 kHz word clock (a simple square wave signal) has to be multiplied by 128 inside the device with a special PLL (to 5,6 MHz.) This signal then replaces the one from the quarz oscillator. Big disadvantage: because of the high multiplication the signal has great deviations called jitter. The jitter of a word clock is normally 15 x higher as if using a quarz based clock.

What's more: in ADAT mode when changing the sample rate (speed) fast, some bits are already wrongly sampled, before an impulse from the word clock circuit arrives to correct the frequency established by the PLL! The Bitclock PLL does not suffer from these effects.

The other kind of Synchronization

At last we again want to make clear that this Tech Info only covers the sync needed and used between digital appliances to send, receive and transmit digital audio data between them. There is another kind of synchronization, providing an absolute time reference of the actual position. For example when starting a PC based hard disk recording software to play back not at the beginning of a track, but somewhere in the middle. Then some kind of timecode is needed (MTC or SMPTE) and some kind of external interface (like the ADAT BRC) that now controls the ADAT recorder(s) to jump to this position, stop, enter playback and get close to the actual position of the already playing PC software via varipitch (this is called Chase Lock Sync.) This is not a question of word clock or Bitclock, but again the Bitclock PLL will - because of the dramatic speed changes in Chase Lock Sync mode - work much better than a simple word clock.
 
Copying single tracks from the ADAT to the PC, processing them, and copying them back to the ADAT to the exact same place on the tape is only possible when synchronizing PC and ADAT using additional hardware and some kind of Timecode (MIDI Time Clock, SMPTE.)

But even then the whole system does not act like a 'normal' tape recorder. For example the buffers in the software needed for record and playback result in a delay, that must be corrected by a manual applied offset to the software. In Chase Lock Sync mode another correction factor is needed, because the software calculates its absolute position based on ideal 44,1 kHz sample frequency, which of course is not given in reality.

Glossary

Bitclock PLL: Special kind of PLL with very accurate regulation based on bit domain instead of word domain
Full Duplex: The ability to simultaneous record and playback (Record while Play)
In Phase: When two signals are absolute synchronous in time domain
Jitter: Deviation of a signal in time domain
Loop: Signal fed from one device to another and back
Loop Clock: Device that allows to feed an external clock into a looped combination of devices
Loop Delay: The delay that occures in combined Full duplex and loop mode between playback and record. (The direct loop back mode using DIGI32's out to in is only possible using a Loop-Clock)
Master: Clock source, feeds all slaves
PLL: Phase Locked Loop. Extracts a simple clock signal out of any complex signal
Quarz: Electrical device; used as oscillator with minimal deviation and very low jitter
Sample frequency: Number of probes taken from the analog signal per second
Slave: Follows the clock reference (master)
SPDIF: Sony/Philips-Digital Interface, consumer version of the digital audio signal
SRC: Sample Rate Converter, converts any given sample frequency to a fixed one
Timecode: Constant coded signal, allows absolute positioning on time domain
Word clock: Pure sample frequency without any audio data, used to synchronize multiple devices to one common clock

Copyright © RME®, Matthias Carstens, 1998.

All entries in this Tech Infopaper have been thoroughly checked, however no guarantee for correctness can be given. RME cannot be held responsible for any misleading or incorrect information provided throughout this manual. Lending or copying any part or the complete document or its contents is only possible with the written permission from RME.
 

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