Digidesign Universal Slave Driver

Digital devices need synchronisation just as much as analogue ones, and this new offering from Digidesign aims to be a one‑box solution for most audio and visual sync requirements. Mike Collins says it's about time...

All digital audio systems, whether workstations like Digidesign's Pro Tools, mixers like the Yamaha 02R, or outboard like the Lexicon PCM90, have internal quartz crystal oscillators which control the speed of the digital audio playback or recording — just as the motors in analogue tape machines control the speed of the tapes. If you want to link two or more analogue tape machines together and have them play back or record in sync with each other, you need a machine synchroniser which will control the speeds of the motors on the analogue machines to keep them in step. The machine synchroniser typically reads SMPTE timecode and 'tach' pulses coming from each machine, compares these with the synchroniser's internal clock or with an external clock feeding the synchroniser, and speeds up or slows down the motors of the analogue machines accordingly. SMPTE timecode is also used to provide location information, so that locate points can be entered into the synchroniser and the machines will wind forward or backwards to the correct points. Some of you may have come across 48‑track recording systems in professional studios, where two 24‑track tape machines are linked and controlled in this way, while others may be more familiar with linking a Sony U‑Matic VCR with a Fostex B16 multitrack tape recorder to put music to picture.

What you may not have fully realised is that when you connect digital audio systems together, their clocks need to be synchronised too — even if they are mixers or other outboard devices. Digital audio signals are similar in some respects to video signals, in that they carry sync information within the signal. So, whether you connect your digital devices using S/PDIF, AES/EBU or ADAT optical connectors, sync information will be available which the receiving device can 'lock' to. Most professional digital devices will also have a separate sync connection, which would be a so‑called 'wordclock' signal via BNC connectors for most equipment, a Digidesign 'Super Clock' (wordclock x 256) in the case of Digidesign equipment, or possibly an AES/EBU sync signal via XLR. Another option is to use a video sync signal via BNC connectors and lock each device's internal clock to a video sync pulse, or 'house sync' signal. This video sync signal is often called a video 'black burst' signal, as it carries no picture information and would appear black if you tried to view it on screen. By analogy, the AES/EBU sync signal, which carries no audible information, is sometimes referred to as a digital 'black' signal.

There are various ways in which you can hook up all your digital devices, and which one you choose depends on the task at hand. If you want to achieve the highest quality at all times, combined with the greatest flexibility, ideally you need to use the most accurate house sync generator you can afford, as your ultimate timing 'master', and feed all your other devices from this. The other devices are said to be 'slaves' to the timing master, and synchronisation is achieved by adjusting the speed of the slave clocks — that is, the playback sample rates of the slave devices — to match that of the master clock. Varying the speed of the clock varies the playback sample rate and causes the pitch of the audio to go up and down — and if you varispeed the master clock, the slaves will all follow.

A New Master

The supplied USD setup software for the Macintosh.

Previously, Digidesign sold two different sync boxes: the SMPTE Slave Driver and the Video Slave Driver. The SMPTE Slave Driver would lock Pro Tools positionally to external LTC (Longitudinal Time Code) and resolve the digital audio timing to a clock reference derived from the incoming LTC, so the Digidesign audio system would know where it should be and how long each frame should last. The Video Slave Driver would lock Pro Tools' digital audio timing clock to an external video or black burst signal, so Pro Tools would run at the same speed as a VCR or other devices resolving to this same master clock source. The Universal Slave Driver integrates the functions of both these units and adds many other functions, such as the ability to read and write VITC (Vertical Interval Time Code) and to 'burn' SMPTE into a window within a video signal.

The USD has its own internal clock so that you can operate it in stand‑alone mode, using its internal clock as the master for all the devices in your system, and it can produce most of the sync signals you're likely to need in a digital audio project studio. It can generate LTC, VITC and MIDI Time Code, Word Clock, Digidesign Super Clock, and AES/EBU 'Null Clock'. It also has six General Purpose Interface (GPI) outputs and four GPI inputs. GPI is often featured on radio broadcast equipment for powering studio record lights or controlling audio cart machines. GPI features have been announced for future versions of the Pro Tools software — so Digidesign are clearly aiming their products at radio studios and for other broadcast work.

Control Zone

The USD's front panel features one button to switch between the clock reference sources (video sync, wordclock or whatever) and another to let you select the positional reference (internally generated timecode or externally‑generated LTC, VITC or Bi‑phase). A third button lets you set the frame rate, with six choices available (30, 30 drop‑frame, 29.97, 29.97 drop‑frame, 25 and 24 frames per second). In the centre of the front panel there's an LED display which normally shows timecode, with four associated buttons to let you program the various functions from the front panel, in which case the timecode display shows the parameters being adjusted. You can choose between 44.1 and 48kHz sample rates, and both pull‑up and pull‑down rates are supported.

A better way to control the unit is using the supplied USD Setup software for the Macintosh. In this case you can disable or 'lock' the front‑panel controls on the USD if you wish, which can be useful if you want to prevent anyone from altering the controls by accident. Using the Variable Speed Override (VSO) feature, you can vary the speed of the USD's internal clock, and consequently the pitch of Pro Tools and any other device receiving its clock reference from the USD. You raise or lower the pitch of your audio material using two faders (one each for semitones and cents) provided in the software's Setup window, and these let you vary the sample rate between 35 and 50kHz (this gives you a range of approximately ‑4 to + 3 semitones at 44.1 kHz, for instance).

By the way, don't forget that if you're controlling the USD from a Mac using the USD Setup software, you're occupying one of the Mac's serial ports. The other port will almost certainly be connected to your MIDI interface — so where can you connect your Sony 9‑pin machine control? This normally connects to a Mac serial port, but now you're using both. Fortunately, the USD can pass machine control and other RS422 serial information to and from the 9‑pin sockets on the back of the USD. The USD does not offer any 9‑pin control itself — the 9‑pin sockets simply pass the information through the USD to and from the Mac's serial port, so to use 9‑pin control you will need the optional Machine Control software for Pro Tools.

Video & Film Features

If you're working to picture, you will normally want to sync both the USD and your VCR to a common video sync source. The USD has a video reference input to allow it to slave to house sync, and it even goes one better by providing a standard video input. This means that you can lock to the timing signals within a standard video signal coming from a VCR — which you can use if you don't have house sync, or if you need to synchronise Pro Tools to a consumer‑grade VCR or to an inexpensive PC‑based video editing system that does not have a video sync input. And the timecode features are particularly well‑implemented. You can use both LTC and VITC, and there is an 'Auto‑Switch' mode which lets you automatically switch between these depending on which is the better choice at the time. VITC can be read while the tape is crawling or paused, so you can Auto‑spot regions in Pro Tools (which you cannot do using LTC, as the timecode disappears when the tape is paused). But VITC cannot be read at fast winding speeds, while LTC is OK with this, so Auto‑Switch uses LTC when winding fast, and reads VITC when moving slowly. By the way, the USD's VITC features are far superior to those in the MOTU Digital Time Piece. For instance, you can specifically select which pair of lines in the incoming video signal to read, or you can use the 'Auto' setting, which will search for the valid line pair automatically. You can also select which pair of outgoing video lines you want to generate VITC onto.

The USD also has great features for people working in film. For instance, you can synchronise to Pilot Tone, which is used on Iocation film shoots to provide sync between a film or video camera and a portable quarter‑inch analogue tape machine such as a Nagra. Pilot Tone is derived by clock‑referencing the camera to the locaI 50Hz or 60Hz AC line frequency and using this same frequency to clock‑reference the tape machine, so that both the camera and the audio tape will run at the same speed. Pilot Tone is falling out of use because it contains no positional information, unlike LTC, which is far more popular today. Nevertheless, Pilot Tone is still used and it is good to see that Digidesign have provided for this.

Film soundtracks are frequently recorded and mixed using special magnetic film recorders, or edited using flatbed editing systems, which typically use Bi‑phase or Tach pulses for synchronisation purposes. These timing pulses encode both speed and direction, though in slightly different ways. The Universal Slave Driver can 'count' both the speed and direction of a stream of these pulses, so it can use a bi‑phase/tach source to deduce positional information from a starting 'address point'. Again, Digidesign are to be applauded for providing a sync solution for film sound.

Summary

If you need a synchroniser for your Digidesign Pro Tools or Avid AudioVision system, and particularly if you are working to picture with video or film, or working in broadcast, this unit has the features you need. I was particularly impressed with the control provided for the VITC facilities, and this is the only unit I am aware of which provides a varispeed function for Pro Tools. It is not as versatile as the MOTU Digital Timepiece and it costs half as much again, so is it worth it? If you're working in film or broadcast, the answer is probably yes. If you work in music recording, however, the DTP is possibly a better choice.