Synchronization II (DIGI96 Series)

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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.

DIGI96 Series

The principle of operation of all DIGI96 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, 48 kHz, 64 kHz, 88,2 kHz and 96 kHz, without any input signal needed.

Thanks to a PLL in the input circuitry they will also sync to any input signal in the range of 25 kHz to 105 kHz. As opposed to the DAT shown in picture 1 DIGI96 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.

Fig. 3

This automatic behavior has many advantages, and additional allows true varispeed operation, because the card will follow every change in speed detected at the input. So all DIGI cards may work as master or slave, and can be synchronized from the input signal even in playback mode. As opposed to the DIGI32 series the DIGI96 series' memory is divided, and input (record) and output (playback) are no longer one circuit. Because of this the input circuit will always use its PLL to lock on the input signal, and the output circuit can be controlled by PLL or quarz. With this the DIGI96 series is enabled to behave like a 'complete' master device.

The DIGI96 series was designed as Hi-Speed interface without compromises. Because of this input and output share the same memory, which results in a sensational low loop delay of only 2 samples in Record while Play mode. With AutoSync active input and output are truly 'in phase'.

Loop Mode

Thanks to their 'complete' master technology the DIGI96 series is able to work in loop mode. The DIGI32 is not able to be used in loop mode without additional help. A typical application would be to use a digital effects device in loop cabling with the DIGI32. Digital effect devices in most cases operate 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 DIGI32 (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. When using a DIGI96 series card simply switch the card to 'Master' (setting 'Clock Mode') and everything works as expected.

DIGI96/8 and ADAT

The ADAT interface of the DIGI96/8 acts exactly like written before. Therefore a loop mode of ADAT and DIGI96/8 works without any problems as long as one of both devices is set to master mode.

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 occurs 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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