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Whatever happened to MTC?

'Whatever happened to MIDI Timecode?' asks Chris Smith of XRI Systems.


Before plunging into the audio industry, I used to work in telecommunications. My job was to plan international telephone networks (sounds great, eh?), which mainly involved studying reams of long-winded specifications and regulations for loads of different gear, to make sure that it would all interface and work together properly. Encoding, encryption, multiplexing, regeneration, echo suppression and error detection - you have no idea what your voice has to go through to get to America!

All of this equipment was made by different manufacturers from all over the world, with different interfaces, threshold levels, data rates, connectors, voltage supplies, noise and safety regulations, and so on. Consequently, it was a nightmare to match up the system on paper, and when the gear was out in the field the problems really started. By comparison, the collection of black boxes and cables at the heart of a contemporary studio is a very simple setup.

Now, I am not saying that MIDI and SMPTE systems do not get complicated, but they are universal standards, adopted enthusiastically and successfully by nearly all manufacturers, as in no other industry. With generic editing, sample dump exchanges, standard MIDI sequence files, SMPTE/MIDI conversion and The Music Network (TMN), the applications of MIDI have expanded way beyond its original concept. This benefits manufacturers, technicians, musicians, SOS journalists, and eventually the record-buying public.

With so many areas of the MIDI specification being successfully exploited, why is it that the area of the spec with the greatest potential for total system synchronisation, in music and video recording, has been so totally ignored? This aspect is known as 'MIDI Timecode' (MTC), and one reason for its apparent rejection is the general lack of knowledge and concern in the recording world about what it is and what it can do. Let's see if I can't put that right here and now.

At the moment we have two main forms of sync code: SMPTE/EBU timecode recorded on audio or video tape, and MIDI Clocks sent between MIDI devices. MTC is a new standard that has been specified to unite these two forms into one by encoding the time-of-day (hours/minutes/seconds/frames) information of the SMPTE signal into a MIDI format, and thereby dispense with the conventional MIDI Clock/Song Position Pointer format altogether. New MIDI devices that accept MTC can, in effect, read raw SMPTE via their MIDI In socket and derive tempo, SPP and cue points for themselves. This method has numerous advantages:

- Simplicity. The SMPTE/MTC conversion can be performed with total accuracy by a single 'dumb box' with few (or no) controls. The task of deriving musical tempo and displaying SMPTE time and/or musical bars is passed on to the MIDI sequencer itself. Tempo variations and cue points are saved to disk as part of the sequence data.

- Accuracy. Tempo resolution is now entirely governed by the MIDI devices responding to MTC. MTC updates the SMPTE time every two frames (regardless of tempo) and allows for cue points to be specified to one hundredth of a frame resolution (ie. within a third of a millisecond). Lock-up times can be in fractions of a second.

- Universal compatibility. As long as each device converts the MTC into tempo and song position in the same way, there should never be system synchronisation problems, no matter what equipment is used. So you can change your sequencer (or sequencing software), drum machines, tape machines, videos, SMPTE/MTC convertors, etc at any time during a production, without creating any problems - a dream come true for most studio engineers.

- Advanced features. As well as 'raw SMPTE', the MTC format allows for a whole range of messages to be sent between devices via the MIDI cable. High speed 'thinned out' SMPTE is transmitted for fast forward/rewind of tape machines, allowing MIDI equipment to chase to tape, or sequencers to control tape machines. Punch in/out points, event starts/stops, cue points, offsets and setup codes can all be enabled, numbered, time-tagged and even named within the protocol. Even the SMPTE user bits (now used to identify tape spools) can be encoded into MTC for display as ASCII characters on remote MIDI devices.

One common misunderstanding is that MIDI Timecode might compromise an already overstretched MIDI bandwidth, so let's look at this in more detail: MTC data is sent in two byte chunks - called Quarter Frame (QF) messages - at regular intervals, each identifying a different element of the SMPTE time. Once eight QF messages have been sent, the processor in the MTC receiving device is able to assemble and interpret the full SMPTE time, ie. once every two frames (15 times per second at 30fps rate).

The MIDI data transmission rate is 31.25 Kbits per second - so one bit lasts for 32 microseconds. Including stop and start bits, one byte (10 bits) lasts for 320 microseconds, so one whole QF message (two bytes) takes 640 microseconds, okay? Now, there are four QF messages per frame, which means 120 QF messages per second at 30fps, or one every 8.33 milliseconds. Quarter Frame messages therefore take up a total of 0.0768 seconds (= 120 x 0.00064s) every second, or 7.68 percent of the MIDI bandwidth - a reasonably small proportion.

Other elements of MTC, such as tape location messages and cue point times, are sent as System Exclusive data of 10 or 15 byte blocks. These are sent far less frequently than QF messages, and even in the busiest of systems should require only one or two percent more bandwidth. There should therefore be plenty of capacity left for performance (ie. music) data, even if conventional MIDI Clocks and Song Position Pointers are sent simultaneously with MTC.

So where is MTC? Despite all this 'techno praise' about the inherent advantages, I have yet to see a working system in which MTC is implemented. More manufacturers are now beginning to provide MTC facilities in their equipment, but are slow to point out the benefits to users, so no-one is taking advantage of them. It seems that because we already have a system of MIDI Clocks and SPP that works well, people are reluctant to move to a new standard, especially one about which precious little is commonly known.

So just how common is MTC support on equipment? Steinberg's new Cubase sequencer is compatible with MTC, although they don't make much of the fact. So is Passport's Master Tracks Pro (which is, incidentally, my favourite sequencer), and Passport themselves are supporting the standard in America by offering SMPTE/MTC conversion hardware. Other hardware companies (J.L. Cooper, Adams-Smith, Tascam) list MTC as a feature on some of their equipment, but say little more about it.

My opinion is that it will take some time yet, but manufacturers, software writers and users will eventually realise the vast potential of MTC. Sequencers, effects units, audio and video tape machines, and countless other devices will all work together in unison, jam-synced, chase-locked and event-controlled by that simple 5-pin DIN socket. Imagine a MIDI-equipped video recorder that you can plug straight into your sequencer — it is not that far away...



Chris Smith is involved with the design and development of new MIDI-based equipment and systems, and is Head of Sales at XRI Systems, England.



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Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Oct 1989

Donated by: Mike Gorman, Bird201

Scanned by: Mike Gorman

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