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]>
<section id="sn-synchronization_concepts">
<title>Synchronization Concepts</title>
<para>
As soon as you start handling audio on more than one device, it is important
to understand and to think about
<emphasis>synchronization</emphasis>
: how to get the devices to have the same sense of time and speed.
</para>
<para>
However, there are two fundamentally different kinds of synchronization:
</para>
<section id="sample-clock">
<title>Sample Clock</title>
<para>
As outlined in the <emphasis>introductory concepts</emphasis> section,
digital audio is created by taking a "sample" of an analog signal level on
a periodic basis, say 48000 times per seconds (the "sample rate"). A
dedicated clock (the "sample clock") ((actually, an oscillating crystal,
but technology people call such things clocks)) "ticks" at that rate, and
every time it does, a new sample is measured. The way the clock is used to
convert digital audio back to an analog signal (i.e. to be sent to some
loudspeakers) is more complex, but the clock is still an absolutely
fundamental part of the mechanism.
</para>
<para>
Whenever you connect two digital audio devices together in order to move
audio data from one to the other, you <emphasis>must ensure they share the
same sample clock</emphasis> . Why is this necessary? The oscillating
crystals used for the sample clock are generally very stable (they always
tick at the same speed), but there are always minute differences in the
speed that any two clocks tick at. When used by themselves, this makes no
difference, but connect two digital audio devices together and these minute
differences will eventually accumulate over time. Eventually, one of the
devices will be trying to read a sample "in the middle" of the other
device's tick, and the result is a small click or pop in the audio stream.
</para>
</section>
<section id="timeline-sync">
<title>Timeline Sync</title>
<para>
The concept of a timeline comes up over and over again when working with a
digital audio workstation, and also with video editing systems. By
"timeline" we mean nothing more than some way to define a "name" for the
point where certain sounds (and/or visual images) occur. When you work in
Ardour's editor window, the rulers near the top provide one or more
timelines in different units. You can look at the editor window and say
"this sound starts at 1 minute 32 seconds" or "this tracks fades out
starting at bar 13 beat 22".
</para>
<para>
But what happens when you want to share a timeline between two different
devices? For example, you may want to run a hardware video editor in
conjunction with ardour, and always have the visual and audio playback be
at the same point "in time". How do they each know what "in time" means?
How do they know where the other one is? A mechanism for answering these
questions provides <emphasis>timeline synchronization</emphasis> .
</para>
<para>
Timeline synchronization is entirely different from sample clock
synchronization. Two devices can share a sample clock, but never use
timeline information. Two devices can be sharing timeline information, but
run on different sample clocks - they might not even have sample clocks if
they are analog devices.
</para>
</section>
<section id="word-clock">
<title>Word Clock</title>
<para>
"Word Clock" is the name given to a signal used
to distribute the "ticks" of a sample clock to multiple devices. Most
digital audio devices that are intended for professional use have a word
clock connector and a way to tell the device to use either its internal
sample clock (for standalone use), or to use the word clock signal as the
sample clock. Because of the electrical characteristics of the signal, it is
very important that any length of cable used to distribute word clock is
"terminated" with a 75 ohm resistor at both ends. Unfortunately, some
devices include this terminator themselves, some contain a switchable
resistor and some do not. Worse still, the user manuals for many devices do
not provide any information on their termination configuration. It is often
necessary to ask the manufacturer in cases where it is not made very obvious
from marking near the word clock connectors on the device.
</para>
</section>
<section id="timecode">
<title>Timecode</title>
<para>
"Timecode" is a signal that contains positional or "timeline" information.
There are several different kinds of timecode signal, but by far the most
important is known as SMPTE. Its name is an acronym for the Society for
Motion Picture T?? Engineering, and timecode is just one of the standards
they defined, but its the most well known. Because of its origins in the
film/video world, SMPTE is very centered on the time units that matter to
film/video editors. The base unit is called a "frame" and corresponds to a
single still image in a film or video. There are typically on the order of
20-30 frames per second, so the actual resolution of SMPTE timecode is not
very good compared to audio-based units where there are tens of thousands
of "frames" per second.
</para>
</section>
<section id="SMPTE">
<title>SMPTE</title>
<para>
SMPTE defines time using a combinations of hours, minutes, seconds, frames
and subframes, combined with the frame rate. In a film/video environment,
SMPTE is typically stored on the film/video media, and sent from the device
used to play it. There are different ways of storing it on the media - you
may come across terms like LTR and VTC - but the crucial idea to grasp is
that the film/video has a timecode signal "stamped" into it, so that it is
always possible to determine "what time it is" when any given image is
visible.
</para>
<para>
SMPTE timecode is sent from one system to another as an analog audio
signal. You could listen to it if you wanted to, though it sounds like a
generally screeching and unpleasant noise. What the SMPTE standard defines
is a way to encode and decode the hrs:mins:secs:frames:subframes time into
or from this audio signal.
</para>
</section>
<section id="mtc">
<title>MTC</title>
<para>
The other very common form of timecode is known as "MTC" (MIDI Time Code).
However, MTC is actually nothing more than a different way to transmit
SMPTE timecode. It uses the exact same units as SMPTE timecode, but rather
than send the signal as audio MTC defines a transmission method that uses a
MIDI cabable and a data protocol. MTC consumes a measurable, but small,
percentage of the available bandwidth on a MIDI cable (on the order of
2-3%). Most of the time, it is wise to use a single cable for MTC and MMC
(MIDI Machine Control) and not share it with "musical" MIDI data (the kind
that an instrument would send while being played).
</para>
</section>
<section id="jack-transport">
<title>JACK Transport</title>
<para>
For Ardour and other programs that use <emphasis>JACK</emphasis>, there is
another method of doing timeline synchronization that is not based on SMPTE
or MTC.
</para>
</section>
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