Getting In Sync - Part 1
(Not a reference to that dreadful boy band!)
by
Ken Lanyon (Slider)
In the world of home recording, the question of syncing machine A to machine B comes up often. Especially in a time when everyone and their mother can get cool recording software and DAWs are becoming a common recording system of choice, knowing how to sync things up becomes very helpful and important. For this article I plan on introducing you to the whole concept of synchronization and the different types of sync signals. I will also explain Linear Time Code, a very common form of sync, and explain how to utilize it within your system. In the next issue, I will continue writing about the various forms of sync, including sync for midi and computers, and explain how to make these signals interact happily with your other equipment.
The whole purpose of synchronization is two-fold. It allows multiple systems to start/stop at the same time and keeps their playback speeds consistent so that one machine does not stray ahead of the other. Of course, if you have machines that you need to sync up, then one will become the master while all others in the chain will be slaves. This means that the speed of the master will designate the speed of the slaves, as well as when they all start and stop. The process of forcing a slave to change its playback speed to match that of a master is called resolution. Typically, the "dumbest" piece of equipment becomes the master, because of its lack of built-in sync options. You first need access your equipment for their limitations, and then decide which one will be the master. For example, if you have an analog tape deck that you are trying to sync up with a computer, then the tape deck must become the master unless it has the ability to receive and react to sync from an external source.
Synchronization is accomplished by the use of different sync signals. You can think of these signals as codes or pulses sent from the master that inform the slaves of the overall sync speed so that they can keep up. The different types of sync include: Linear Time Code (known as LTC or SMPTE), FSK, Smart FSK, Midi Sync, Song Position Pointer, Midi Time Code (MTC), Vertical Interval Time Code (VITC), Superclock, Word Clock, and absolute time code. Of these, LTC, Midi Sync, Superclock, Word Clock, and MTC are the most commonly used with audio today. Since VITC is used for syncing up video signals, I won't be covering it here. Suffice it to say that VITC can be converted to most of these other signals by a converter box, and then integrated with your audio systems.
In understanding how each of these signals work and how to use them within your system, I would like to focus first on LTC and the information that it carries, as it will help to explain the other signals in the next article. LTC is a squarewave audio signal that, when visualized on a display of some kind, will show hours, minutes, seconds and frames to give a specific positional reference. Frames represent one second split into a certain number of parts, depending on the frame rate. The frame rate for audio by itself is 30 frames per second, but if you were working with video or film and trying to sync it up with audio information, you would have to change the frame rate. (Just for your own information, the other frame rates are 24fps, 25fps, 29.97 non-drop frame, 29.97 drop- frame, and 30 drop frame).
Timecode carries two pieces of information; Positional reference, as I briefly mentioned, which assigns a unique time value to every consecutive part of the audio signal, and clock reference, which indicates the speed of the machine. As you will see later, not every sync signal has both positional and clock reference, but they will always have at least one of the two.
I stated earlier that linear timecode is an audio signal, which means that it will be laid back onto tape along with the rest of your song tracks. One thing to keep in mind about timecode is that it works great at normal playback speeds and faster, but it is not recommended for use with video when shuttling tape back and forth, since it works bad at slow speeds. In this case, you would use VITC. The timecode signal must be created from a generator of some type, which may be a separate generator box, or maybe you have a software program that can generate timecode and send it out to be recorded onto tape. In either case, you need to find some way of creating LTC. Many of these generation boxes also convert incoming sync signals to another type. For example, a converter is needed to convert LTC to MTC for interaction between a tape machine and your computer. That is why I suggest making sure your LTC generation box does allow for sync code conversions. If you cannot afford a timecode generator, one thing you can do is find someone who has a generator, and have them burn you a disc with timecode for the full length of the disc so you can always use that to print to tape.
Once you are ready to record your signal onto tape, a process called striping, you must set the level you are printing to tape. You don't want to stripe timecode at high levels because it has a tendency to bleed into other tracks and possibly cause crosstalk in your console. This is why you should also print timecode on the highest track, next to one that is expendable, just in case bleed does occur. However, the signal should be hot enough that it can be read clearly by the receiving machine. There is a fine balance here that you may have to try a few times to figure out the best level for your machines, but when using professional machines, -7db is a common level to record timecode at on digital tape while -3 db is good for analog tapes.
The other thing you should do before striping the tape is to set your timecode start time. A common time is 01:00:00:00, which means 1 hour, zero minutes, zero seconds, and zero frames, although the actual start time of the timecode is arbitrary and up to you. I suggest you not use any time under one hour because if you need to rewind your tape and the master goes further back than 00:00:00:00, the slaves may interpret this as a fast- forward to 23 hours. Starting at one hour allows you that 60-minute leeway and won't roll your tape off the reel. Now set the tape machine in record and start generating timecode. The amount of timecode striped to the tape is up to you, depending on if you want the whole tape striped, or just one song. I should mention here that if you plan on making copies of that master tape, the timecode becomes unreliable after two generations. This means that you can copy the master to one tape, and copy that tape to another, but after that, you will have to reuse the master or regenerate the timecode signal because the signal may drop out and not be able to be read by the slave. Many generator boxes can take the incoming signal and regenerate new timecode in sync with the incoming signal. Regenerated time code is often called "Jam Sync.".
Now that you have the SMPTE timecode on tape, you need to think about what machines you are trying to sync up. If you want to sync to a machine that can read LTC, then you would directly connect that timecode tracks output to the timecode in of the slave. However, if your slaves need to receive MTC as with computers, or any other form of sync signal, then you need to patch the output of the timecode track to the converter, and then from the converter to the slave. The converter will take the incoming LTC and generate other sync signals like MTC or VITC to match it. The next thing you need to do is make sure the slaves are receiving and reacting to the incoming timecode rather than their own internal clocks. Consult your slave manuals to do this.
The last thing you need to do is set an offset on your slave. Normally, the slave will start playing when it sees the time 00:00:00:00, but obviously, if you started recording your timecode at one hour, the slave would never see this number. An offset tells the slave to shift its timescale a certain amount of time ahead or behind. In our example, if the timecode on the master started at 01:00:00:00, and the offset on the slave is set to 01:00:00:00, the value from the master now becomes recognized to the slave as the beginning of the song and playback will start. You can think of this as the slave changing its start time to 01:00:00:00 rather than the usual 00:00:00:00. This also works when you playback in the middle of the song. The slave may see 01:03:41:18 from the master, and it will begin playback from the same location.
That is all there is to syncing up LTC to your slaves. It may take a little trial and error, but you will eventually get it if all the right connections are made. Now I realize all of this is a lot of information to swallow, so I will resume the next article by going over the rest of the sync signals like MTC, FSK, Smart FSK, absolute time, and midi synchronization, and explaining how to hook them all up. Until then, happy recording.
(c) 2000, Ken Lanyon,
All rights reserved.
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