Quietly but surely, synchronisation codes have become an essential part of MIDI recording, but what's all this SMPTE and FSK stuff really about? Chris Many looks at the different sync codes and why we need them.
These days we tend to take equipment synchronisation for granted, but just how do sequencers, drum machines and tape recorders stay in time?
IN THE MIDI era, it's not enough that a musician or composer knows how to create a haunting melodic phrase, killer dance groove or the next Top Ten hit. It's not even enough to know the Ins and Outs of synthesisers or how to get a good, clean sample, much less the real difference between 12-bit and 16-bit samplers (besides the price tag, that is). We all know that the length of the list of subjects a musician is required to know is long and extends beyond mere matters of music. Synchronisation usually tends to be low on the list, and rightly so. After all, it's just a matter of turning on the tape and locking up a sequencer. Or is it?
LET'S GET A couple of definitions out of the way first. What do we mean by "synchronisation"? My pocket dictionary defines "synchronise" thus: 1. Occur at the same time; agree in time. 2. Move or take place at the same rate and exactly together.
Orchestras normally require a conductor to synchronise their performances, getting all of the musicians to play together and agree on a common tempo, interpretation, dynamics and so on. A computer or dedicated sequencer actually takes on the conductor's role when used to perform music, causing different tracks to start at the same time and perform exactly together at the same tempo.
But what would happen if our conductor was unable to communicate his directions to the musicians in the orchestra? Then we have chaos. Fortunately, musicians have a common language in music. Whether or not individual performers within an orchestra speak French, Italian or English, the language of a conductor setting the pace and dynamics of a piece is understood the world over.
This analogy holds up well when we apply it to the field of synchronisation. Let's say we have a 24-track tape recorder, a 3/4" video deck and a sequencer, all made by different manufacturers, all working happily, but independently, and we want to connect them together to form an audio-visual composition suite. By imposing a senior communication protocol on all these machines we can get them all working in sync with one another.
THERE ARE TWO main categories into which synchronisation languages fall: click or pulse types, and time reference types. Click types are basically electronic pulses occurring at a regular rate that can be read by two machines so that they can run at the same speed. In other words, codes that ensure two machines start at the same time and run at the same speed, and so stay in sync. The problem with this type of sync is that there's only one reference point to work with, namely the starting point.
Many sequencers use this kind of sync signal, known as FSK (Frequency Shift Keying). It's an audio signal in which the frequency of one sine wave tone is modulated between two distinct values by a square wave (a little like like simple FM synthesis). For example, in Roland FSK on the MC500 Mkll, the frequency of the primary tone is modulated between 1.3kHz and 2.1kHz. These modulations, or shifts between frequencies, are performed a certain number of times per quarter note - 24, 48 or 96 are the standard frequencies - and so drum machines and sequencers are ideally suited for this type of sync. Of course, each modulation, or pulse, is identical to every other pulse, so there's no way to identify any specific part of your music.
FSK is one example of a click-type sync, but there are several others - Roland's Sync 24 (or DIN sync), Oberheim's "O" sync, and a few other proprietary sync codes adopted by various manufacturers. There's also another type of pulse or click sync called Control Track, that is used extensively in video applications. Control Track is a series of electronic pulses that are recorded on the bottom part of a videotape, separate from the two audio tracks. These pulses are used for editing purposes in conjunction with a Control Track Editor, allowing clean edits using video tape. By finding a blank space between selected video frames (called the vertical interval), a Control Track Editor lets you make electronic splices without causing visible jumps. Again, the pulses used with Control Track are all identical, so such editing is not 100% accurate because there is no distinction between different points on the tape. The machines being used have to count the pulses, so they are only accurate to within two or three frames.
And synchronisation doesn't end with pursuading one machine to run in sync with another - the same principle can be applied to running machines in sync with tape. A simple example of a click-type tape sync signal is the Click track used by musicians when multitracking. Although the tempo can change and a different audio pitch could be used to delineate the start of a new series of clicks (click, click, click, click . . .) the clicks are essentially identical. Once again, there's no location reference using a click track (the music might give you a reference as to where you are in a song, but the click itself doesn't).
All the electronic click-type sync codes are suitable for recording onto tape to allow sync'ing to tape. However, the click type of synchronisation does provide an agreed-upon method for locking the performance of two machines together, as long as they both start from the beginning of the music each time you run them. By counting and comparing the electronic pulses, two or more machines can be reliably sync'd to each other.
ALTHOUGH CLICK-TYPE synchronisation is a workable system, it's inconvenient to have to rewind to the beginning of a recording every time you want to slave one machine to another especially if you're working with a long piece of music or video. Enter SMPTE timecode, an audio signal developed by the Society of Motion Picture and Television Engineers (hence its name). SMPTE code is based on the internationally accepted concept of time (Hours: Minutes:Seconds) and provides a great number of pointers or location references. SMPTE is not based on counting pulses per quarter note, but instead it sets the tempo against a real time standard. So when you record SMPTE timecode onto video tape, each frame is stamped with its own individual reference point in time. This is true for audio tape as well - every point on the tape is uniquely marked with a precise identity (Hours, Minutes, Seconds, Frames and Sub Frames). This means that moving to any location is simply a matter of requesting the machine to find that identity.
There are two kinds of SMPTE code: Longitudinal Time Code (LTC) and Vertical Interval Time Code (VITC, pronounced "vit-see"). LTC, commonly used by most musicians for sync purposes, is the audio signal recorded on audio or videotape. VITC is recorded in the vertical interval between video frames on video tape. One of the big advantages of VITC is that, when using the appropriate hardware, you can read code while in very slow motion or in pause mode. For the most part, LTC cannot be read at very low or high speeds because there are frequency changes due to playback speed.
Now, if you didn't know that there was a blank space between video frames, you're not alone. VITC, although used daily in a wide variety of video applications, is largely unknown in the music field. Why? Because VITC cannot be recorded on audio tracks. Therefore LTC must be used when running audio hardware, such as a multitrack tape machine or a sequencer. In order to synchronise these different types of machines (video and audio) we must find the common ground between them, and LTC is it.
Just to make things a little more complicated, there are four different types of LTC: Non-drop frame (30 frames per second, or fps), Drop frame (29.97fps - it requires the code to "drop" or skip a frame number once in a while to stay in sync), arid 25 and 24-frame SMPTE. Simple, eh? Twenty five-frame is the standard used by the European video community and 24-frame just happens to coincide with the standard film (as opposed to video) frame rate. American video uses the drop frame-rate of 29.97 fps. The 30-frame rate is used for audio applications, such as synchronising sequencers to multitrack tape recorders. In order to synchronise things using LTC then, you must make sure that the type of code used is the same for all machines, otherwise you'll wind up with some very confused machines.
ANOTHER TIME-BASED synchronising code can be found in the MIDI specification. Song Position Pointer (SPP) is one method used by sequencers, drum machines and assorted MIDI machines of locating a particular point in a song. Using a code based on numbered beats in a song to identify specific points, SPP is useful in providing a simple cueing system for MIDI instruments.
MIDI Time Code (MTC) was recently adopted for use in similar applications, and uses the same method as SMPTE timecode (Hours, Minutes, Seconds, Frames and Sub Frames) for location reference. The drawbacks of MTC are the additional information it adds to the MIDI data stream, the need for special conversion boxes to change true SMPTE code to MTC, and less accuracy than SMPTE itself. However, it does currently provide the best method of time-based synchronisation available for MIDI users without SMPTE. However, MTC by itself cannot be used for synchronising MIDI gear to multitrack tape machines or VCRs since it cannot be recorded onto tape.
Recently, yet another form of synchronisation code has been developed, which is a bridge between the pulse and time methods: Smart FSK. Essentially, Smart FSK is an FSK-type clock with MIDI Song Pointer embedded within the signal. This allows devices designed to read and write this kind of code to use an inexpensive but reliable type of pulse code (FSK) that incorporates one of the main strengths of time-based code (locatability). JL Cooper's PPS1 and Synhance's MTS1 are the only two machines that use this kind of code, so it's quite a way from becoming an industry standard. Consequently, it precludes itself from being widely used to lock up VTRs and MTRs (Video or Multitrack Tape Recorders). However, it is an alternative synchronising method worth considering if you're on a budget.
MIDI itself is a communication protocol, designed to work with the microprocessors in synths, signal processors. sequencers and so on, but not meant to be recorded on audio tape. As a result, additional conversions to other communication languages (FSK or SMPTE) are required in order to use MIDI with other non-MIDI machines such as VTRs and MTRs. Even with the latest advances (like Fostex's R8, a multitrack machine that syncs to MIDI clock), there are still hardware and software requirements to translate the different languages into one common communication protocol.
TO SUMMARISE, THERE are two categories of synchronising languages click or pulse types, and time formats. The click/pulse type of sync code is characterised by the fact that each pulse is identical to every other pulse, requiring you to start from the beginning of the recorded data every time to get an accurate coordination between machines. All that's being communicated by electronic (frequency modulation) or audible (click track) means is continuous stream of countable pulses each one following the previous in predictable, timed fashion.
Time reference code, such as SMPTE allows for electronic information to be recorded on audio or video tape that uniquely identifies each point on the tape These signals represent the passage of time in Hours, Minutes, Seconds, Frame: and Sub Frames. This allows machines to locate to any given point on a piece tape and is used to instruct the machine to move to a specified time stamp.
Feature by Chris Many
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