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Synchronization
Background
What is synchronization?
Synchronization is the process of getting two or more de-
vices to play back together at the same exact speed and
position. These devices can range from audio and video
tape machines to digital audio workstations, MIDI sequenc-
ers, synchronization controllers, and digital video devices.
Synchronization basics
There are three basic components of audio/visual synchro-
nization: position, speed, and phase. If these parameters
are known for a particular device (the master), then a sec
-
ond device (the slave) can have its speed and position “re-
solved” to the first in order to have the two devices play in
perfect sync with one another.
Position
The position of a device is represented by either samples
(audio word clock), video frames (timecode), or musical
bars and beats (MIDI clock).
Speed
The speed of a device is measured either by the frame rate
of the timecode, the sample rate (audio word clock) or by
the tempo of the MIDI clock (bars and beats).
Phase
Phase is the alignment of the position and speed compo-
nents to each other. In other words, each pulse of the
speed component should be aligned with each measure
-
ment of the position for the most accuracy. Each frame of
timecode should be perfectly lined up with the correct
sample of audio. Put simply, phase is the very precise po
-
sition of a synchronized device relative to the master
(sample accuracy).
Master and slave
In this document, the following terms are used:
• The “timecode master” is the device generating position infor-
mation or timecode.
• The “timecode slave” is any device receiving the timecode and
synchronizing or “locking” to it.
Timecode (positional references)
The position of any device is most often described using
timecode. Timecode represents time using hours, min-
utes, seconds, and frames to provide a location for each
device. Each frame represents a visual film or video frame.
Timecode can be communicated in several ways:
• LTC (Longitudinal Timecode) is an analog signal that can be
recorded on tape. It should be used for positional information
primarily. It can also be used for speed and phase information
as a last resort if no other clock source is available.
• VITC (Vertical Interval Timecode) is contained within a compos-
ite video signal. It is recorded onto video tape and is physically
tied to each video frame.
• MTC (MIDI Timecode) is identical to LTC except that it is a
digital signal transmitted via MIDI.
Timecode standards
Timecode has several standards. The subject of the various
timecode formats can be very confusing due to the use and
misuse of the shorthand names for specific timecode stan
-
dards and frame rates. The reasons for this confusion are
described in detail below. The timecode format can be di-
vided into two variables: frame count and frame rate.
Frame count (frames per second)
The frame count of timecode defines the standard with
which it is labeled. There are four timecode standards:
• 24 fps Film (F)
This frame count is the traditional count for film. It is also used for HD video
formats and commonly referred to as “24
p”. However, with HD video, the
actual frame rate or speed of the video sync reference is slower, 23.976
frames per second, so timecode does not reflect the actual realtime on the
clock for 24p HD video.
• 25 fps PAL (P)
This is the broadcast video standard frame count for European (and other
PAL countries) television broadcast.
• 30 fps non-drop SMPTE (N)
This is the frame count of NTSC broadcast video. However, the actual
frame rate or speed of the video format runs at 29.97
fps. This timecode
clock does not run in realtime. It is slightly slower by 0.1
%.