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HOME RECORDING |
SEPTEMBER,
2000 ISSUE |
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Getting
In Sync* |
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BY
KEN LANYON |
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| 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. |
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*Not a reference
to that dreadful boy band! - kl |
Copyright © 2000, by
Ken Lanyon, ALL RIGHTS RESERVED |
Published in Recording
Engineer's Quarterly and Alexander magazines with permission |
USE OF THIS ARTICLE SUBJECT TO USER AGREEMENT |
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