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MIDI Timecode or MTC:
Here's a riddle. What happens
when you take SMPTE timecode and try to send it
down a MIDI cable? Well, not much of anything, unless
your software sequencer can recognize MTC. MTC simply
put, is LTC SMPTE that has been converted to travel
down a MIDI cable. This is how DAW and sequencer
software is able to "chase" or sync to video. Many
devices that you may need to sync will only have
MIDI interfaces and will not be able to receive
audio timecode (like LTC). For example in my studio
I run an MPC2000XL that is synced to Nuendo using
MIDI timecode. Thus I can use two different devices
to do what they do well, Nuendo for audio and the
MPC for drum sounds and MIDI. MTC has certain advantages
and disadvantages. These are:
Advantages
1. Is very standard and can
be used with many devices, such as video devices.
2. Is tied to absolute time
(minutes, seconds, frames) and not Bars and Beats,
thus it is more understandable to non-musicians.
Disadvantages
1. It does not send tempo
information. Thus devices that are looking to sync
to tempo rather than a particular moment in time,
cannot use MTC. Example of devices that need tempo
information would be the LFO on a synthesizer, or
an outboard timed delay effect.
If we need to send tempo
information, we would use MTC's more musical cousin.
MIDI clock.
MIDI Clock
If MTC is equivalent to a
clock on the wall, so everyone knows exactly what
time it is, MIDI clock is equivalent to the conductor
of an orchestra. Its job is to set the tempo and
make sure everybody is playing together.
Unlike MTC, MIDI clock understands
music. It sends tempo information (the song is 80
beats per minute) and Song Position Locator information
(we are currently at measure 40, beat 3). It also
contains a Song Selection parameter to choose from
several songs. In my experience its "time to sync" is
also shorter than MTC. More than likely which standard
your device supports will make your decision as
to which protocol to use. Most times you will need
to run one or the other but not both.
Midi Machine Code - or MMC
MMC is the starters gun of
timecode protocols, the simplest and least useful.
It simply transmits basic commands such as "Start", "Stop",
or "Rewind", and does not transmit tempo or time
information. MMC cannot be used to sync devices
unless you know that the devices will naturally
run in sync. Normally MMC is used in conjunction
with another timecode protocol. An example may be
that you are using an analog tape deck as the timecode
master (it would have to be), but you want to use
the controls in your software sequencer. Then the
software could send MMC controls to the tape deck
(if it recognized them) but receive MTC from the
tape. This would allow you to not only synchronize
play or record, but allow you to rewind all devices
simultaneously with one command.
Other and Miscellaneous:
There are some oddball sync
protocols that linger simply because these pre-MIDI
devices are still in use today. An example of this
would be Roland's DIN-sync protocol that was used
to sync the 808/909/303 line of products. Normally
specialized sync-conversion devices must be purchased
to convert MIDI clock or MIDI time code to these
proprietary protocols. However, this allows these
vintage devices to run perfectly in time with modern
devices.
Word Clock: What's in a Name?
Word clock is a sync system
that ensures the transmission of multiple digital
audio tracks from various devices start playback
at the same time -and stay in sync, or locked together.
Whoa, is that confusing at first glance or what.
Well, look at it this way -since digital audio is
represented by ones and zeros, digital audio data
is organized into groups, or words. Thus, word clock
deals with the "clocking" (the timing), of "words" (the
digital audio data) with respect to digital audio
transmission. This differs from timecode (SMPTE/MTC)
that tells you where, or how far along you are,
in the digital material (think -timeline reference).
Gentlemen (and Women)...Start
Your Engines
It is necessary for multiple
audio signals, whether they be analogue or digital,
to start at the same time and stay in sync relative
to one another. Otherwise you have drift and timing
problems that will manifest itself as phasing in
the analogue world, or artifacts (clicks and pops)
in the digital world. On an even smaller scale,
micro timing discrepancies between devices can cause
clock jitter -which can effect stereo width and
detail. The aim of employing word clock is to make
sure these artifacts are avoided by keeping all
the digital devices in the studio on the same page,
so to speak.
Message in a Bottle?
Digital sync can be transmitted
in various ways. Digital audio signals like AES
or S/PDIF transmit sync down the digital audio cable
itself, sending the sync data to the next device
in line. In other words, the digital audio being
sent down the cable is self clocking. An example
of this would be the S/PDIF stereo out of a digital
mixer feeding the S/PDIF stereo in of a DAT recorder
for a final mix. A dedicated word clock connection
hasn't been made, the DAT recorder simply "looks" for
the sync signal embedded in the digital audio traveling
down the S/PDIF cable from the mixer.
Digital devices -like sound
cards, digital mixers, stand-alone digital recorders,
and digital effects boxes can also have dedicated
word clock inputs. These inputs are BNC connectors
and the word clock signal is routed, or daisy-chained
from device to device via a 75 ohm video cable.
The word clock runs at the same sample rate as your
audio (i.e. 44.1 or 48 kHz), or at a dedicated rate
like Digidesign's Superclock (FS x256).
Lastly, word clock can be "distributed" from
a dedicated sync device that has multiple word clock "outs" so
that you can send the word clock signal to multiple
devices without having to daisy-chain. The system
you employ for word clock sync in your studio will
depend on how many digital devices you have that
need to transmit signals simultaneously.
Regardless of whether you
utilize the embedded clock signal in the digital
audio line itself for transmission from one device
to another, or utilize a dedicated word clock distribution
device for transmission to multiple devices, the
relationship between two separate digital devices
must be clear. There has to be a master, and all
other devices must slave, or clock to, this master.
Master and Servant
Anytime you have more than
one piece of equipment a master clock must be chosen
to which all other devices synchronize their clocks.
Take our simple example from above of a digital
mixer's stereo out feeding a DAT recorder's stereo
in. If both clocks were set to internal (meaning
both devices would sync to themselves only), clocking
differences would result in "clock jitter" and clicks
because no two clocks are going to be exactly in
sync independent of one another. In this example
you need to designate the master (the digital mixer
set to internal sync) and the slave (the DAT recorder
set to external sync). This way the DAT recorder
knows to sync itself to the mixers clock.
Even if you start to daisy
chain devices, or distribute word clock to multiple
devices, you must have, and can only have, one master
with the rest of the devices slaving (syncing) to
this master. Which device should be the master?
Which devices should be the slaves? Is there a difference
in word clock quality?
Distribution -Get the Word
(Clock) Out
Just like AD/DA converters,
not all clocks are created equal. Some are inherently
more stable than others. Hence, it is important
to have a high quality clock as your master since
all your other devices will be slaving to this one
clock. Also, if you have multiple digital devices
in your studio it is advantageous to distribute
that master clock from a common source without having
to daisy chain. This is where dedicated sync boxes
like the Lucid
GENx6 or Rosendahl
Nanosync come into play. These boxes have extremely
high quality clocks, and can distribute multiple
feeds of word clock to all your devices simultaneously.
Some, like the Rosendahl, can even generate video
black burst (see below). For an in-depth look at
the Rosendahl device check out Greg Ripes review
here at audioMIDI.com - http://www.audiomidi.com/aboutus/reviews/ripes_nanosyncs.cfm
Are We in Sync?
So, SMPTE is a way of referencing
a timeline, or how far along you are in the program
material, as well as "locking" picture to sound.
This way, two machines can know where they need
to be -if the video has gone 1hr-4mins-23frames
into the film then the audio needs to "chase" the
SMPTE to this exact location in the timeline before
it starts play back. Word clock, on the other hand,
tells multiple digital signals when to start playing,
and to slave themselves to a master so they stay
in sync with each other after they start playback.
Video Blackburst or "House
Sync"
Blackburst is very similar
to word clock and most often will be found in post-production
facilities. A blackburst, or "house sync" signal
is generated, acts as a master, and is fed to video
and audio devices. These audio devices that can
accept video blackburst can then send word clock
to the other audio devices that don't accept video
blackburst. The result is clocking nirvana -devices
in sync with themselves and in relation to those
around them.
Summary
Although syncing can be quite
a chore, normally once it is setup correctly you
should not need to do anything more with. And with
today's modern devices and the prevalence of support
for these well-established protocols, this allows
the digital musician to choose his devices based
upon his musical needs and not his synchronization
ones. With a little patience, you can keep your
studio perfectly in time.
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