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MIDI Basics

MIDI-To-Tape Synchronisation

MIDI-to-tape sync: why you need it, what it does and how you use it.


If you think sync is the poor man's equivalent to a dishwasher, read on, as Paul White explains how you can reap the benefits of synchronising your MIDI sequencer to your multitrack tape recorder.

On their own, multitrack tape recorders are limited by the number of tracks they provide — and the more tracks you want, the more you have to pay. True, you can bounce or combine recorded tracks and re-record the result onto a spare track, but every time you do this, the sound quality is degraded and you also lose the opportunity to rebalance, pan or add effects to those tracks that have been bounced.

In the early days of home recording, there was no alternative — you just had to live with it. The introduction of MIDI sequencing changed all that. Obviously, you can't use a MIDI-controlled instrument to replace every instrument or sound that you'd normally want to record, but in the context of pop music, you can certainly use MIDI drums, bass and keyboard parts, as well as samples of instruments such as piano, brass, flute, and so on.

But MIDI is of little help when it comes to real performance, such as vocal parts, guitar solos and virtuoso instrumentals, which is where tape still has a definite place. It's true that you can sequence sampled sections of a vocal part or entire guitar phrases, but this requires a lot of memory, which adds up to high cost. Clearly the most flexible and cost-effective solution is to use a system where tape and MIDI sequencing run side by side.

The simplest scenario to imagine is where you create your backing track on a sequencer and then record this to one or two tracks of your multitrack tape machine, leaving the remaining tracks free for vocals and guitar solos. Many people work in this way, but there are several very real disadvantages.

Firstly, the sequenced parts, once mixed, can't be separately EQ'd, panned, balanced or effected — they behave just like tape tracks that have been bounced. Secondly, whenever a mix is recorded to tape, some quality is lost, and with budget cassette multitrackers, this can be quite noticeable.

A far more flexible approach is to have the sequencer running in sync with the multitrack tape recorder, so that the sequenced sounds can be fed into the final mix without ever having been recorded on multitrack at all. If the sequenced MIDI parts come from a single, multitimbral sound source with stereo outputs, then a Portastudio-type device with six or more input channels will be able to mix both the taped music parts and the sequenced MIDI parts, but if the MIDI system uses sound modules with multiple outputs or several different modules at the same time, then you'll need either a multitracker with a lot of mixer inputs or, better still, a separate mixer altogether. Fair enough, but how is this synchronisation achieved and what additional hardware is required?

Sync Systems



We have had drum machines that can sync to tape for many years now, and though the sync facilities they offer are rather basic, they can still be very useful. They employ what is known as an FSK (Frequency Shift Keying) system, which records a series of electronic tones onto a spare track of the tape machine. These tones are related to the tempo of the drum machine, so as the tempo is increased, the electronic sync track follows.

The procedure is as follows:

  • First, program your drum part, making sure that it runs at the correct tempo, and then record the sync code output from the drum machine onto one track of your tape recorder, while allowing the drum machine to play the piece you have just programmed. Some tape machines have dedicated sync ins and outs, while with others you have to use the audio ins and outs.

  • Sync code is normally recorded onto the highest numbered tape track, which will be track four on a four-track machine.

  • Next, the tape output from the sync track you have just recorded is plugged into the Sync In socket on the drum machine, the drum machine switched to Tape Sync, and the tape started from the beginning.

  • The drum machine will automatically start when it receives the sync code from the tape machine and will stay in time with the tape until you stop the tape machine. This allows normal recordings to be made onto the remaining tape tracks (three in the case of a four-track machine) while listening to the drum machine as it plays along.

This method of working is very simple but serves to illustrate tape sync at its most basic level. The main disadvantage is that all but the most sophisticated drum machines use a very basic type of FSK code, the drawback of which is that whenever you stop the tape, you have to wind it right back to the beginning again to establish sync. For short pieces of work, this is not a problem, but if you're working on the last four bars of a 10-minute epic, it can be very frustrating.

Figure 1a: Sync'ing via a drum machine.


TIP

When recording time code, you may need to experiment with the code level, since if recorded too high, it could "bleed' over to the adjacent tracks, making it audible in the final mix. Conversely, if recorded at too low a level, your time code might not read reliably, causing the sequencer to 'hiccup' or stop.

If you are prepared to work in this way, you can add a sequencer to the system simply by setting the sequencer to External MIDI Sync and connecting the MIDI Out from the drum machine to the MIDI In of the sequencer. This arrangement is shown in Figure 1a, but it has the serious disadvantage that you can't record new parts onto the sequencer while you are locked to tape because the sequencer's MIDI In is taken up by the MIDI Out from the drum machine. If you intend to finish programming your drum and sequencer parts before starting work on the taped sounds this is fine, but if you want the freedom to work on your MIDI parts while listening to the tape parts you have recorded, you'll need to add a MIDI merge box to the system in order to combine the MIDI output of your master keyboard with the MIDI Out of the drum machine. Figure 1b shows how a merge box would be patched into the system. Here are a few tips to help you organise your work when you have this type of system.

  • Program the drum part first so you have something to play along to — even if it needs to be modified later.

  • Set the drum machine to External MIDI Sync mode and connect its MIDI In to the sequencer's MIDI Out. Now you can record parts into your sequencer and the drum machine will play along. Most modern drum machines and sequencers use a MIDI facility known as MIDI Song Position Pointers, which enables them to establish sync regardless of where in the song the master device (in this case the sequencer) is started. This is absolutely transparent to the user and requires no knowledge or action on behalf of the user — it just happens! If your sequencer and drum machine don't work with MIDI Song Position Pointers, then you'll still have to take it from the top every time.

  • When all the sequencer parts are written and any last-minute changes made to the drum part, change the MIDI wiring around so that the drum machine's MIDI Out feeds the sequencer's MIDI In. The drum machine should be connected to the Sync In and Out sockets of the tape recorder (or to track four in and out if no dedicated sync connections are provided) and the drum machine should be set to its Tape Sync mode. Set the sequencer to External MIDI Sync. Now the tape machine acts as the master device, with the drum machine slaving to the sync track on tape and the sequencer slaving to the MIDI Out of the drum machine.

  • Once the tape tracks have been recorded, the tape outs are mixed with the outputs from the sequenced MIDI instruments and drum machine. Some cassette multitrackers have additional mixer inputs for this purpose, but a separate mixer is a more flexible option.

Figure 1b: Incorporating a MIDI merge unit.


Smart FSK



A few years ago, the American Company JL Cooper came up with a refinement to the FSK sync system which they called Smart FSK. This still records the code to tape in the form of tones, but it is designed to work with MIDI Song Position Pointers so that once the code has been recorded to tape, the tape machine can be started anywhere in the song and the sequencer will be locked up to it within a second or two. Because the code is generated from the sequencer's MIDI clock, it is related to the tempo of the original sequence, so if any tempo changes have been programmed, these will be reproduced accurately when the sequencer is sync'ed to tape. The only caveat here is that you can't go changing the tempo of your sequence after the code has been recorded to tape, but you are free to change individual musical parts on the sequencer or to write new parts and change sounds. JL Cooper also saw fit to include a MIDI merge facility in their sync box, so that new sequencer parts could be recorded while the sequencer was sync'ed to tape. Figure 2 shows how a system might be set up using a smart FSK box. Note that the drum machine is now running as a slave from the sequencer's MIDI Out.

Figure 2: Sync'ing via a smart FSK unit.


SMPTE



In recent years, the SMPTE (pronounced simp-tee) time code — which was originally developed for soundtrack sync in the film industry — is being widely used to synchronise audio systems. It has been used for many years to facilitate the sync'ing of two multitrack tape recorders, but it is now also commonplace in MIDI studios. Unlike Smart FSK, which is tempo related, SMPTE is based on real time measured in hours, minutes and seconds, with further subdivisions to accommodate individual frames of TV and film material. Because it is independent of tempo, a whole tape can be recorded or 'striped' with code before any recording or programming starts.

Because there is no tempo information inherent in the timecode itself, a conversion has to be done somewhere along the line, either by the computer used to run the sequencing software, or by the microprocessor inside the SMPTE-to-MIDI sync box. The starting tempo of a piece of music and the location and amount of any subsequent tempo changes are stored in the form of a 'tempo map' which must be used alongside the sequencer data whenever the sequence is run sync'ed to tape. On a modern computer-based sequencer such as C-Lab's Notator or Creator and Steinberg's Cubase, dedicated SMPTE sync units are available which handle the creation and storing of tempo maps via the sequencing software itself. This makes life much easier than having to treat the tempo map as an entirely different subject, though some of the newer stand-alone units also automate as much of the procedure as is possible.

So, why go to the bother of using SMPTE? What advantages does it offer? Unless you are planning to work to picture, then the answer is that SMPTE offers little more than a smart FSK system. Both allow you to start at any point in a song and both will look after your tempo changes, smart FSK doing so automatically. Both will follow changes in tape speed caused by general drift or by deliberate varispeeding, but you have to keep in mind the fact that your taped sounds will increase in pitch if you speed up the tape, whereas the sequenced MIDI sounds will remain at their original pitch.

In SMPTE's favour, it does allow you to stripe the tape first, whether you have programmed any sequence or not, while smart FSK requires that you have programmed your sequence, at least as far as its length and tempo goes, before you start work. Furthermore, if you do decide a tempo change is in order, you have to re-stripe with a new FSK code, whereas with SMPTE you simply have to create a new tempo map. But in practice, these differences are fairly minor.

A relatively new variation on the sync code theme is MIDI Time Code, which is essentially a MIDI interpretation of the traditional SMPTE protocol. New sync boxes such as JL Cooper's PPS2 can read and write MTC, but many of the sequencers in common use aren't equipped to make use of it. There's also a dedicated sync code known as Direct Time lock or DTL, which is used exclusively by Mark of the Unicorn sequencing packages.

Mixing



When working with a cassette multitracker, even the better-equipped models tend to be short of mixer inputs unless your MIDI system is relatively simple. A separate mixer offers greater flexibility; the audio quality is often better, there are more comprehensive facilities and you can choose one with as many input channels as you require. The three tape tracks used for audio can be plugged directly into the line inputs of the mixer if the multitracker has separate tape outputs for each track, but all is not lost if there are only stereo outputs. In this case, you can mix the three tape tracks to stereo, using the mixer section of the multitracker, and then feed these into two channels of the mixer panned hard left and right to retain the stereo mix you have set up. Figure 3 shows a typical setup for mixdown.

Figure 3: Using an external mixer.


It's also possible that you can use the Foldback or Pre-fade send of your multitracker to provide a further output allowing you, for example, to send track one to the Left stereo output, track two to the Right stereo output and track three to the Foldback output. The possibilities vary from machine to machine, but a little lateral thinking often produces a way around a seemingly tricky problem.

The MIDI instruments are fed into the remaining line inputs, and whatever effects units are connected to the mixer can be used to process both the taped and sequenced sounds. Because the MIDI instruments have never been recorded onto tape, the quality of the final recording is largely determined by the quality of the mixer and the tape recorder used to record the final stereo mix. In this respect, a budget DAT machine can be used to produce near CD-quality results, even when working with a modest 4-track recorder.

The other benefit of working in this way is that you can always change the sounds on your MIDI instruments, right up to the moment before you mix. This very flexibility has led some record producers back to recording their MIDI instruments onto tape because they find that leaving everything open-ended can invite unnecessary tinkering, which wastes time and money. In the professional studios, where recording quality is good and tape tracks are abundant, this can make a lot of sense, but in the home studio, where time is free and tape tracks are scarce, sync'ing may be all that keeps you afloat!

Time Code And Noise Reduction

Some types of tape noise reduction con affect the reliability of time codes, so it is best to switch off the noise reduction on your sync track if at all possible. If the multitrack has a dedicated sync input and output, these will have been arranged to bypass the noise reduction and any EQ controls, to allow a reliable recording to be made. You may need to experiment with the code level, since if recorded too high, it could 'bleed' over to the adjacent tracks, making it audible in the final mix. Conversely, if recorded at too low a level, it might not read reliably, causing the sequencer to 'hiccup' or stop.

As a very general rule, dbx noise reduction systems have the greatest effect on time code, whereas Dolby C doesn't seem to bother it at all. Unfortunately, some of the very early multitrackers only allow the noise reduction to be switched on or off globally for all four channels, which means that if you want to work with a sync code, you'll have to put up with more tape hiss.


SMPTE

SMPTE stands for the Society of Motion Pictures and Television Engineers, and the time code bearing their name is a standard in the professional film and video industries. Strictly speaking, the code covers only the American TV format of 30 frames per second (fps) and film at 24fps, the European equivalent being the EBU code, which includes 25fps TV. In practice, these are all rolled into one under the title SMPTE/EBU, but the EBU part is often dropped for convenience. Apart from the more common 24, 25 and 30fps formats, the standard also includes 'drop frame' which is used when converting film to TV. The system gets its name because whole frames of picture are periodically discarded to eliminate cumulative timing errors which would otherwise cause noticeable sync problems. Drop frame is not used in audio-only applications.

It is customary to set the SMPTE format to the local TV standard, which in Europe means 25fps, though all will work perfectly well, with 30fps giving, in theory, the best timing resolution. Because individual frames of picture are too coarse a measure for audio used, interpolation is used to provide a finer degree of resolution by dividing individual frames up into smaller units of time. Again, this is all quite invisible to the user, so don't panic if you don't quite understand it all.


Multitrack Sync Facilities

In order to use a sync unit with a multitrack tape machine, it is vital that the machine either has a dedicated sync in and out or that you can access the output from your sync track directly without it having to be added into the stereo mix from the multitracker — time code is very unmusical!

It is sometimes possible to work around the problem of not having a separate sync track output, but this usually involves some form of compromise. For example, with a tape machine that has only a single stereo output, you could pan the three audio tracks to the left and the sync code to the right. The left output would then contain all the audio in mono, while the right output contains only the sync code. Depending on the machine, it may also be possible to route the sync track to the outside world using one pre-fade (sometimes called Foldback) aux send and turning the channel fader right down. Of course, this prevents that particular aux send being used for any other purpose.



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Sennheiser HD25 Headphones

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First Time In The Studio


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

 

Recording Musician - Jan 1993

Donated & scanned by: Mike Gorman

Feature by Paul White

Previous article in this issue:

> Sennheiser HD25 Headphones

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> First Time In The Studio


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