Getting In Sync - Part 2
by Ken Lanyon (Slider)
In the last article, I talked about SMPTE time code and it's use in the typical home studio. Now I want to address the other types of sync signals that are used and how to get them to all work together happily. These would be Absolute Time, Word Clock, Superclock, Midi Clock, Song Position Pointer, Midi Sync, FSK, and Smart FSK.
Let's start with Absolute time signals, since they are the easiest to understand, hopefully having now read the first sync article about SMPTE time code. Absolute Time is very much like SMPTE, in that it is a time stamp that represents an exact position of the tape. It displays hours, minutes, seconds, and frames in the form of 00:00:00:00, and having 33.3 frames per second. One of the main differences between SMPTE time code and absolute signals is that time code is used for synchronization between different types or brands of machines, and absolute is for use with that same brand machines. You will see this absolute format used with Tascam DA-38s, DA-88s, DA-98s, and the new DA-78HRs, as well as Alesis ADATs. When you first unwrap a blank digital tape, it needs to be formatted with the sampling rate and absolute time signal. The absolute time signal will allow for you to keep track of tape location without having to also stripe time code. Since the signal is recorded onto a control track, you can use it to lock multiple Tascam or Alesis units to the same location. All you need is to have the proper sync cables hooked from the master to the slaves. Syncing via absolute time signals on Tascam machines is a very handy thing because it doesn't require the use of SMPTE time code which needs a SY-88 card in the DA-88. This is also nice because DA-38s cannot be time code masters, but can be absolute masters. (Note: The DA-98s and DA-78HRs have built in SMPTE sync cards, and can be both masters and slaves.)
The sample rate signal is called the Word Clock, and it accurately controls the sample rate of other gear so that they won't drift apart. To give you more insight into this, when signal is recorded from analog to digital or vice versa, the converter takes "pictures" of the voltages of the signal at a certain rate. A sample rate of 44.1KHz is saying that 44,100 samples of the signal are taken every second. These "pictures" must be evenly spaced between takes to prevent drift and keep an accurate representation of the signal. It is the job of the word clock signal to keep other machines hooked to it sampling at these same intervals.
Super clock is very similar to Word Clock in that it is a sample rate control signal, but is used with controlling the sampling rate of computers. It operates at 256 times the sample rate, which means that if the sample rate is 44.1KHz, the superclock is running at 11,289,600 pulses per second. Don't confuse this with the computer now taking 11 million samples per second. It still samples at 44.1K but the 11 million pulses allow it a finer resolution to keep in sync better. A computer will either generate superclock, or can receive it from a converter that takes the word clock signal, and multiplies that rate by 256 to get the superclock rate.
So now, let's touch on all sync that is MIDI compatible. The most basic type is Midi Clock. This is a timing pulse that is sent out from the master sequencer 24 times per quarter note to keep two or more sequencers locked to the same tempo. Through the Midi Out port, the master will be constantly adjusting the slaves clock so it will stay on tempo. These pulses will be continuously sent out, even when the master isn't playing. This prepares the slave to be ready to start.
Next, we have Song Position Pointer. This is a form of sync that keeps multiple slaves locked to the nearest 16th note of the master. Think of this as the autolocator. When you want to start the master playing in the middle of a song, the master sends out a value, which is the number of 16th notes from the beginning of the song. Now the slave knows where in the song to start playing.
Many older pieces of midi gear can only respond to Midi Clock, but most new equipment can respond to MIDI Clock AND Song Position Pointer, often referred to as "MIDI Sync". The Midi Clock is the tempo controller, and the Song Position Pointer is the positional controller. Of course, for the slaves to receive this info, they must be set to Midi Sync mode. I also want to mention that these three sync components are all example of relative sync. That is, they don't represent exact locations on tape or in the song. Just close estimations. Close enough that you wont notice most of the time. You can use machines having Midi Sync with those using SMPTE by having a converter box in the middle. The tape machine will send the SMPTE to the converter, which will convert it to Midi Sync and send it out to the slaves. Thus, the converter is the tempo and sync master. On the converter, you must specify the song tempo and the offset. The offset is the SMPTE time value at which the converter will send a start message and Midi Clocks to the slave, so it will start playing. The converter will also send out Song Position Pointer pulses for location.
Our next form of sync, Midi Time Code, on the other hand, is a form of sync that IS absolute. It uses a real-time number to represent an exact location of the song at an exact time. No tempo pulses are involved. This number is very much like SMPTE, and thus, MTC is used to interface computers (which use Midi) to SMPTE. The computer is the tempo master because it sends out word clock signals to the tape machine, but the tape machine is the transport master. Of course, for MTC to be compatible with SMPTE, a converter box is again needed to convert one to the other, and you must do two things to set it up. You must specify the SMPTE frame rate it will be converting to/from, and you must set your offset. In this case, the offset is the difference between the actual SMPTE time and the MTC time that the song starts at. For both Midi Sync and Midi Time Code, there are different ways to connect your units to the converter box, so consult your manuals. That is about all there is to the basics of Midi-compatible sync codes.
The last two things I want to discuss are FSK and Smart FSK. FSK stands for frequency shift key, and you will generally find both types of sync in older machines. By itself, FSK is only a tempo-relative sync signal, and gives no indication of location. Therefore, it can only be used in conjunction with Midi Clock signals. When hooked to Midi Clocks through a converter box, FSK responds by alternating a tone from 1.5KHz to 2.5KHz for every clock pulse. The converter, based on a specific Midi Clock tempo from the sequencer, will generate this tone, which will then get recorded onto tape. Then during playback, the FSK signal gets converted back to Midi Clocks to keep the sequencer in tempo. However, since the converter will only respond to the frequencies of 1.5KHz and 2.5KHz, the tape speed cannot change during playback once the FSK signal is recorded, since changes in speed will change the pitches. After initially recording FSK, make sure to leave enough signal before starting to record tracks of your song, because the tape machine may take a few seconds to ramp up to speed. Also, as with SMPTE time code, don't record your FSK levels too hot because they can bleed into other tracks.
Smart FSK is almost identical to regular FSK, except that it can indicate a specific position in the song based on 16th notes. Therefore, Smart FSK can be used with Midi Sync. It is still recorded and played back in the same fashion as FSK, except that you must tell the converter to create Smart FSK instead of just FSK. The tape machine is the master and the sequencer is the slave.
That about wraps it up with the many types of audio sync you will find in home studios. Hopefully I have addressed the common issues people have with getting different machines to sync up. If anyone has questions for me, feel free to drop me an E-mail and I would be glad to help. It was a nightmare for me before I learned it, so I know where some of you are coming from. However, like most things, it's really not that hard to understand once you dive in. So drag out those dusty manuals, hook up your converters, and be afraid no more! Happy syncing!
(c) 2000, Ken Lanyon,
All rights reserved.
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