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What is Timecode?
Timecode is simply a way of
recording a "time" along with either a
visual or audio event. This allows you to say, "The
beginning of the song is at 0 and the cymbal crash
happened at 2 minutes and 53 seconds." The
information is then stored along with the audio
or video medium so that, as it is moved from device
to device, the time information stays the same.
The most commonly used timecode
format was developed by the Society of Motion Picture
and Television Engineers (www.smpte.org). Referred
to as SMPTE (pronounced Simp-Tee), it was originally
developed to synchronize sound for film but has
since been adapted for video as well as audio-only
use.
The other commonly used timecode
format is Midi Time Code (MTC), which was developed
by Chris Meyer and Evan Brooks for Digidesign.
How
does Synchronization work?
Imagine that you and a friend
have the only two watches in the world, so that
you have no time reference other than your own watches.
You need to meet at an exact time later that day.
No watch is exactly accurate – maybe yours
is a bit slow and your friend’s is a bit fast.
Over the day the result is that there is a 10-second
difference between your two watches. While it may
be okay with your friend that you’re 10 seconds
late because he likes you, it would be very bad
for a cymbal crash. In audio, even delays of a fraction
of a second with doubled vocals can be noticeable.
The next day, in order to
circumvent the 10-second time difference, you and
your friend work out a system. You pick one person
as the Master and one person as the Slave (it’s
that kind of friendship). The Master calls the Slave
on his cell phone and tells him what time it is
every few seconds, and the Slave resets his watch
to match the Master’s. While this system may
be an annoying prospect for human beings, it works
quite well for machines. Similarly, in every timecode
set-up you must determine which machine is the master
(there can be only one) and which are the slaves.
Usually the constraints of the machine will define
who is master and who is slave. For example, analog
multitracks usually don't function as slaves, because
many cannot chase timecode.
How
would I use Timecode?
I offer two scenarios for using
timecode, an audio-only application, and a sync-to-video
application.
Audio Only
SMPTE timecode is often used
in an audio-only setting is to sync-mix automation
or sequenced keyboard parts to an analog multitrack
deck. At some point before mixdown, the tape is "striped" with
SMPTE. This means that timecode is recorded as an
audio signal on one of the tracks on the tape. Usually
an outer track is preferred to reduce the possibility
of cross-talk. Many devices provide the ability
to stripe with timecode, typically through a phono-out
that can be patched into the mixing deck. (Some
examples of such devices are the MOTU MicroExpress
and the Emagic Unitor8.) This time is recorded in
real-time like any other audio source, with care
taken to set the proper volume level and no EQ.
The frame rate for audio-only is generally 30 frames
per second (fps), as this matches real-time exactly.
Many midi devices intended for audio-only use only
read or write 30fps. (When wouldn’t you
want to use 30fps? See the section below entitled "Sync-to-video".)
This type of SMPTE is called LTC (for Linear or
Longitudinal Time Code). It is linear in the same
way that tape is linear – what this means is
that, just like audio, if the tape is stopped there
is no timecode being produced, just as there is
no audio. The other type of SMPTE, VITC, is not
used for audio, because it cannot be recorded on
audio tracks.
Mix automation (or keyboard
parts, etc.) can be recorded in a sequencer or in
the mixing board itself (if it supports automation).
Usually when this is done the timecode is translated
into Midi-Timecode (MTC). (SMPTE can be converted
to MTC through a variety of interfaces.) The timecode
allows the analog multitrack to be the Master. Oddly
enough, the multitrack must be the Master because
it is the most unreliable. It is a mechanical device
and therefore susceptible to "wow-and-flutter" (meaning
very small variations in speed). These variations
are generally not perceptible when listening to
playback, but over time would cause the sequencer
to drift out of sync. For instance, the first time
you play back your mix, the space between the kick
drum and the cymbal crash is .98 seconds, and the
next time it’s 1.01 seconds: in both instances,
timecode only allows that the event happened at
0 minutes, 23 seconds and 22 frames. (Yes, we use
frames even when not doing video.) Therefore your
mute always comes off right before the cymbal crash.
Note that even though digital
decks are not susceptible to drift, timecode is
still used to synchronize them to other unlike devices.
When syncing multiple identical devices such as
ADAT’s or DA-88’s, you can often use a
proprietary means of synchronizing that 's simpler
to set up.
Sync-to-Video
It's here that timecode options
get more interesting, because we have several types
of SMPTE timecode. The simplest video sync scenario
works much like the above audio example: a VHS tape
has LTC on one of its audio tracks (it is the Master).
The device to be synced (the Slave) reads that timecode
and chases it. The tricky part is determining which
type of SMPTE to use.
When using SMPTE at 30 frames-per-second
(fps), one second equals one second. Unfortunately,
NTSC color video does not run at 30fps — it
runs at 29.97fps. Why in blazes does it do that,
you ask? Well, let’s just say that video, like
life, was simpler when it was in black-and-white.
But when color was introduced, they decided to cram
that little bit of color information in the space
left by .03 frames. (If you want to know why, or
how the frame-rate of 30fps is related to the fact
that we run power at 120v, you can email me at brent@audioMIDI.com.)
At 29.97fps, a second of material
is no longer a second, but just a hair longer. While
a few hairs may not matter to you and me, they matter
a lot to people in television who make their living
selling seconds, so these people came up with a
solution called "drop-frame". This means
a few frames are dropped so that SMPTE time matches
with real time.
So we have SMPTE at rates
of 30fps, 29.97 "drop" and 29.97 "non-drop".
All that really matters on a basic level is that
you need a tool that can handle all these different
frame rates, and you must know exactly what frame
rate the video you are syncing with is using.
Many midi devices offer SMPTE,
but rarely offer all the frame-rates necessary.
Other SMPTE frame-rates include 24fps for film and
25fps "EBU Time Code" for PAL /SECAM video. For
this you will need a device such as the MOTU Digital
Timepiece that is capable of handling the range
of frame rates. If you're syncing to a sequencer
program or multitracking software, your software
may handle this. One piece of hardware that handles
all frame rates is the DA-88 with the SY-88 add-in
card; other options are the MOTU Midi Time Piece
AV or the new DA-78HR.
One last very important detail
has to do with video that was transferred from film.
Most professional films are still shot on actual
filmstock. But when the film is sent out for post-production,
it is usually transferred to video. As I mentioned
earlier, film runs at 24fps and video runs at 29.97fps.
So when the transfer takes place, the film must
be "stretched" onto the video. And in
order for it to match, the audio must be stretched
as well. This stretching is called "pull-down," and
it increases the length of the audio (with a corresponding
decrease in pitch) by -.01%. So when you are working
with a video tape, you must be working with pulled-down
audio in order for it to remain in sync to the video.
Any audio that you add (e.g. music or production
audio from DAT) must be pulled-down as well. When
the audio is ready to be transferred back to film,
you must either no longer pull it down if you pulled
the audio down yourself, or "pull-up" the
audio if you received it already pulled-down. If
you ever encounter this fairly common scenario,
you will need software or hardware that can handle
this.
If you are interested in more
information regarding the glories of timecode --
or sync issues in general -- you can email me at brent@audioMIDI.com.
I also recommend the book Sound
for Film and Television by Tomlinson Holman (the
inventor of THX).
Part
2: Syncing in the Digital Studio
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