Time For SMPTE (Part 2)
Kendall Wrightson rounds off his introduction to SMPTE with a look at its application in modern music production as exemplified by a synchronisation unit - the Friend Chip SRC.
Kendall Wrightson rounds off his introduction to SMPTE with a brief look at its applications in the electronic instrument field as exemplified by a synchronisation unit - the Friend Chip SRC.
In Part One of Time for SMPTE last month, it was explained that SMPTE - named after the Society of Motion, Picture & Television Engineers - is a digital timecode with three international standards:
25 frames per second - Europe TV
30 frames per second - USA TV
24 frames per second - Film
The widespread problems of syncing due to incompatible equipment were also summarised, and we hinted that a sync box of the SMPTE variety might provide a better solution than the conventional clock pulse synchroniser. This month we examine why...
Although conventional sync boxes such as the (old) Garfield Dr Click or Korg KMS-30 can convert between various clock and trigger standards, the problems of timing errors (known as 'driving delay') between the master and slave machines remain. These are commonly overcome by syncing all instruments to a tempo-related click track laid down on tape. However, there are several drawbacks:
1) Much messing about with digital delay lines to delay the code, as equipment with different response times will still run fractionally out of time with each other.
2) Tempo is always fixed to the frequency of the recorded code.
3) The tape must always be re-started from the beginning to get the drum machines, sequencers etc in sync with the track.
The information stored in SMPTE code, however, relates to the time elapsed in hours, minutes, seconds and frames, and is not merely tempo information. Instead of the regular clock pulse, every moment of SMPTE code is unique - designating time elapsed and (therefore) an exact position on the tape.
Enter Friend Chip, a small German synth company who early on spotted SMPTE's potential for syncing electronic musical instruments and in 1981 introduced their revolutionary SMPTE Reading Clock. Unlike conventional sync boxes, the SRC eliminated driving delay problems by acting as a master controller for all machines connected to it.
The SRC generates SMPTE code which is recorded onto the edge track of the multitrack tape (in real time) at the start of the recording session - an operation referred to as 'striping' the tape. At this stage, no decision is needed regarding tempo or duration because the start time is programmed into the SRC and may be changed as and when necessary. In fact, the start time can be changed by just one frame (1/25th of a second) to overcome timing response differences. The usual technique is to stripe the entire length of tape because the SRC can be reprogrammed for the start of any songs that follow on the tape.
The SRC also stores 'cue' points to mark, for example, the start of the first verse, chorus etc. If the tape is relocated at a new position, the SRC activates its multi-standard clock outputs when it receives the next cue time which removes the necessity to always begin recording at the start of the track.
The convenience and saving in studio time afforded by the SRC have assured its status as the studio standard. Recent hardware and software additions to the unit have ensured that it remains at the top, and have provided it with, amongst other things, a MIDI clock output...
As mentioned in Part One, the MIDI specification incorporates System Real Time messages to enable synchronisation via MIDI. These messages include a Clock, Stop/Start/Continue information and a message called the Song Position Pointer.
On receipt of a Song Position Pointer message, a MIDI drum machine or sequencer will move to the measure number supplied in the message. If the device transmitting the positional message is itself synchronised to tape via SMPTE, then the tape can be started anywhere, and connected machines will automatically start at the correct measure, without user intervention. This removes the need to programme cue points, providing the user with a fully automated set-up.
This MIDI/SMPTE tape synchronisation system is the idea behind the Roland SBX-80 Sync Box. It works fine with Roland's own TR-909/707 drum machines and MSQ-700/100 sequencers. However, as mentioned last month, the MIDI specification does not insist that manufacturers implement every possible message and thus we end up with the unfortunate situation where, for example, the Yamaha RX11/15 drum machines do not understand the Song Position message. How strange that Yamaha did not foresee the usefulness of this position pointer message.
There is now a trend amongst manufacturers to provide a SMPTE generating/reading ability as a built-in feature on various new devices. It would therefore seem to be advantageous to do away with the old sync converter box - one less piece of equipment to buy and learn how to use. However, the sync box has other reasons to justify its future existence.
Firstly, though moving the SMPTE start time by one frame gives a resolution of 1/25th of a second, there is still a need for finer accuracy. With conventional clock pulses, this is achieved by delaying the click through a digital delay line. This is a time-consuming activity which involves recording delayed code backwards, then re-delaying it with the tape round the right way. The SRC achieves fine resolution through its built-in variable delay module, providing accuracy to within one millisecond. Another SRC feature is the ability to produce a human feel to the tempo by slowly modulating clock pulses backwards and forwards - and there are a lot of producers around who will testify to the need for facilities of this kind.
Secondly, despite the incredible pace at which new products appear, old machines are not instantly discarded - a product may have a market life of only one year, but it will be grinding away for many more in the studio.
There is another drawback to the current breed of machines with a built-in SMPTE facility. One might assume that should several such devices all be fed with a common SMPTE code from tape that they will synchronise together perfectly. That is, after all, the whole idea!
Unfortunately, our story takes a rather nasty turn at this point. The fact is that at present, two SMPTE-reading machines such as the SRC may not stay in time with each other for much longer than about 30 seconds. The reason for this disturbing fact being due to a thing called round-off error.
A machine that is designed around 8 bit technology will usually store tempo data as one byte (8 bits). This means that the possible tempo range is 00000000 to 11111111 (binary) - ie. 0 to 255 units (usually beats per minute).
In other words, the tempo resolution is such that it cannot accommodate smaller tempo divisions like 120.5 bpm.
The physical consequence of this is that, should two 8 bit SMPTE machines reading the same SMPTE code be set to run at 115 bpm, one might actually run at 115.1 bpm and the other at 115.9. The machines will start off together OK, but will gradually drift apart.
This drift is called round-off error because the tempo data has been rounded off to the nearest whole number.
There will also be round-off error in a 16 bit machine, of course, but it will be significantly smaller, with the effect that machines will stay in time for hours rather than seconds.
Presently, we are caught in the transition between 8 and 16 bit technology but until 16 bit technology becomes as widely available as 8 bit, round-off errors will be significant enough to cause loss of sync.
In conclusion of this brief series then, it's clear that SMPTE/MIDI tape synchronisation of electronic musical instruments provides numerous advantages: mixdown automation, slaving of audio tape recorders, and synchronisation of audio to film/video. However, the introduction of SMPTE in the electronic musical instrument field will, like MIDI, undoubtedly have its teething troubles. But what it is beginning to offer is definitely worth having.
If this introduction to SMPTE has whetted your appetite, detailed information is available in an eminently readable book called the 'Time Code Handbook' from (Contact Details)
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Feature by Kendall Wrightson
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