Mackie 8-BUS Series, Руководство пользователя

Страница: 205 / 247

C-2
D i g i t a l 8 • B u s
Synchronization
LTC vs. VITC
There are two common methods for recording
time code, described as follows:
Longitudinal Time Code — LTC can be
used on either audio or video tape. On audio
tape, it is recorded onto one track of the tape.
On video tape, it is recorded on one of the linear
audio tracks of the tape.
This method of recording SMPTE time code
is the most common in the audio industry. The
time code is recorded onto tape as a square
wave. Due to the high-frequency harmonics of
a square wave, some precautions must be exer-
cised when putting SMPTE time code onto
audio tape.
The process of putting the time code onto
tape is called striping the tape . It is best to
record the time code on an outside track, since
it can only bleed through to one adjacent track.
If you’re using an 8-track recorder, this means
recording the time code onto either track 1 or
track 8. If possible, leave the adjacent track
blank to further isolate the time code track
from the audio tracks. This also prevents any-
thing from the audio tracks from bleeding into
the time code track, which can degrade and
interfere with the synchronization.
Typically, time code is recorded at a –6 dBu
(or –6 VU) level onto the tape. For the most
stable signal, record directly from the output of
the time code generator to the input of the re-
corder. (If you
do record through a mixer, don’t
use any equalization or processing on the sig-
nal.) Stripe the entire tape first, before
recording any audio. Allow at least one minute
of running time code before program material
begins. This leader area provides time for the
system to synchronize, and to get beyond any
tape imperfections (dropouts) that may occur
at the beginning of the tape.
Vertical Interval T ime Code — VITC (pro-
nounced “Vit-see”) is used exclusively for video
editing. The SMPTE information is recorded
within the video picture, during the vertical
blanking interval. VITC offers some advantages
over LTC for video editing because it can be
read from a still frame and provides half-frame
accuracy.
Although SMPTE is still primarily used in
video production, it is often used in digital audio
production as the master clock, while MIDI is
used as the interface between the various
pieces of equipment.
MIDI
Before we delve into the MIDI world, let’s
explore the concept of absolute timing versus
relative timing. SMPTE provides
absolute tim-
ing , based on actual hours, minutes, seconds,
and frames. It is tempo-independent, meaning
that even if the tempo changes, the time code
remains constant.
Relative timing is tempo dependent and is
based on beats and bars (measures). If the
tempo speeds up or slows down, the time code
speeds up and slows down along with it.
MIDI Song Position Pointer (SPP)
An SPP message keeps track of how many
16th notes have elapsed since the beginning of
the song. Since it is based on the tempo of the
song, it is considered relative timing.
MIDI Clock Messages
The MIDI clock emits 24 MIDI clock mes-
sages every quarter note. This, too, is relative
timing because it is tempo dependent.
These MIDI sync signals cannot be recorded
to tape because they run at 31.25kHz, too high
for multitrack recorders to handle. They are
primarily used between MIDI devices such as
sequencers, drum machines, and keyboards.
MIDI Time Code (MTC)
MTC is generated from SMPTE by a
SMPTE-to-MIDI converter, which translates
SMPTE time in hours, minutes, seconds, and
frames into SPP data.
MTC contains the same absolute timing in-
formation as SMPTE, but takes the form of a
digital signal traveling down a MIDI cable
rather than an audio signal on an audio path.
This allows MIDI devices to sync to absolute
time if necessary.
Put the kettle on, it’s time for that nice cup
of tea.