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MIDI By Example (Part 6)

Syncing Again


Armed only with a tape sync unit, your mission (should you choose to accept it) is to synchronise your sequencer and multitracker...

As anyone who has ever recorded music on a small multitrack system will tell you, however many tracks you think will be adequate when you buy a machine, demand usually exceeds supply and all too often you're left needing an extra couple of tracks. Let's assume that you have a 16-track recorder for a moment; by the time you've recorded all of your sequencer tracks to tape - synth lines, chords, drums, percussion, bass lines etc. - how many tape tracks will you have left for vocals and real instruments? Three, maybe four? Now consider what happens if you only have a four or eight track recorder.

More often than not, in order to get all of the sequenced music onto tape, it's necessary to combine a number of different instruments into one or two stereo pairs - a compromise solution which can often compromise the final result.

Is there an alternative? Yes - run the sequenced instrument lines directly from the sequencer and only record the non-MIDI instruments to tape. If you're using a four track recorder, this would provide you with three tracks for vocals and other instruments; with an eight track machine, the number would be increased to seven. Whither the extra track? Well, the sequencer and tape recorder have to be made to play in time with one another and this requires a sync code to be recorded onto one of the tape tracks.

In Part 5 of MIDI By Example, we looked at how MIDI synchronisation made it possible to control the playback operation of a drum machine. But of course, when referring to a drum machine, what we're actually talking about is a sequencer dedicated to the recording and playback of drum and percussion sounds. Hence, much of what applies to the drum machine is also applicable to the sequencer - and that includes synchronisation.

Just as a quick reminder: MIDI Sync comprises a series of messages sent out from a master sequencer in order to keep a slave in time; MIDI Clock is the general timing 'pulse'. MIDI Start ensures both machines commence at the same point, MIDI Stop halts them and MIDI Continue prompts them to carry on from the point at which they paused. In sequencer/sequencer, or sequencer/drum machine arrangements, these MIDI messages are sent out 'live' so to speak. But it is also possible to encode them into a continuous sync 'tone' and record them to tape for subsequent synchronisation of the recorder and sequencer.

Figure 1: A Tape Sync to MIDI Converter transforms MIDI Sync messages to a code on tape and then decodes them on playback.

The simplest type of MIDI sync tone is known as FSK - or Frequency Shift Keying. This is made up of two frequencies an octave apart - one denoting a '0' and the other a '1' in digital language. By producing a tone which moves between these frequencies, a Tape Sync box can convert MIDI Clock, Start and Stop messages into a special audio tone which may be recorded to tape and on playback, converted back into the relevant MIDI messages (Figure 1).

How is such a box used? Figure 2 shows the basic connections and operation is quite straightforward. Finish the entire song on the sequencer - including any tempo or time signature changes (this is imperative as such alterations determine how frequently MIDI Clocks are sent). Now turn on the sequencer's MIDI Clock Transmit function and set one of the outer tracks on the multitrack to record the tape sync tone (the highest numbered track is usually used). Make sure that the level of tone on tape is about -3dB. Begin recording on the multitrack, start the converter, wait for about ten seconds and then commence playback on the sequencer. Once the song finishes, stop both the sequencer and multitrack.

Figure 2: A typical sync set-up for a multitrack recorder and sequencer.


If possible, turn off any noise reduction facility on the track you're using for the tape sync code. Dolby 'B' and 'C' will generally not have an adverse effect; dbx on the other hand may prevent the code from recording correctly - you'll soon find out on playback. Similarly, be very careful what kind of sound you record on the adjacent track as 'crosstalk' (the slight bleedthrough of sound from one track to another), can cause problems.

Rewind the multitrack to the start of the code, set the converter to receive tape sync and transform it into MIDI sync commands, making certain that the sequencer is set to recognise these commands. Start playback on the multitrack and you should find that the sequencer follows in sync - including any tempo or time signature changes.

Shortcomings? Well, you have to run the tape from the beginning of the song each time to ensure that the necessary MIDI Start command is sent to the sequencer. This can be avoided by using a slightly more up-to-date version of FSK - generally known as Smart FSK - which encodes a further MIDI message called Song Position Pointer (SPP) into the sync code during recording. On playback, you can begin the tape at any point; a SPP is sent to the sequencer which then positions itself at the corresponding place in the song and awaits the stream of MIDI Clocks which follow.

Figure 3: Continuing to record MIDI information onto the sequencer while in sync with the multitrack requires a MIDI merge box.

Once your multitrack recorder and sequencer are playing in sync, you may want to continue recording on the sequencer. If this is the case - and you don't like the idea of continually re-routing your MIDI cables - you'll need a MIDI merge box (Figure 3) to allow the additional keyboard (or other performance) information to be merged with the MIDI sync data from the converter. The only exception to this is if the converter has an extra MIDI In and so merges the incoming keyboard data with the MIDI sync data being created inside.

I must also mention a couple of other shortcomings. Firstly, you cannot make any changes to song structure in terms of tempo or time signature changes once the FSK code has been recorded onto tape. Secondly, you have to record the code to tape for each song individually; it is not possible to 'stripe' or record sync code to the entire reel of tape in one pass. Is there an alternative? There always is... See you next month.


MIDI GLOSSARY

Taking a break for terms which are related exclusively to MIDI, this month well look at some of the letters used to denote quantity...

k

k (kilo) denotes a factor of 1000. It is mainly used with non-digital terms. For example, 1 kHz (Hz = Hertz - the measurement of frequency) is equivalent to 1,000 Hz.

K

K (Kilo) differs from the lower case version (above) in that it denotes a factor of 1,024. As such it is used predominantly with computer and digital terms. For example, 1 Kbyte (a byte is a measure of computer storage memory) is 1,024 bytes.

M

M (Mega) can be used in two different ways. For non-digital terms, it's used to denote a factor of 1,000,000. For example: 1MHz = 1,000,000Hz. With digital or computer terms, however, its value is related to that of Kilo prefix: ie, 1 Megabyte is 1,024 Kilobytes or 1,024 x 1,024 bytes or 1,048,576 bytes.

m

m (milli) denotes a factor of one thousandth. For example, 1ms is one thousandth of a second - or 0.001 second.

micro μ

μ, (micro) denotes a factor of one millionth, 1μs, for example, is one millionth of a second - or 0.000001 second.


Series

Read the next part in this series:
MIDI By Example (Part 7)



Previous Article in this issue

Station MIDI Busker

Next article in this issue

Forefront Technology FT8


Music Technology - Copyright: Music Maker Publications (UK), Future Publishing.

 

Music Technology - Feb 1993

Topic:

MIDI

Syncronisation


Series:

MIDI By Example

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8


Feature by Vic Lennard

Previous article in this issue:

> Station MIDI Busker

Next article in this issue:

> Forefront Technology FT8


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