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Synchronization
• 30 fps drop-frame SMPTE (D)
The 30 fps drop-frame count is an adaptation that allows a timecode dis-
play running at 29.97 fps to actually show the clock-on-the-wall-time of
the timeline by “dropping” or skipping specific frame numbers in order to
“catch the clock up” to realtime.
Confused? Just remember to keep the timecode standard
(or frame count) and frame rate (or speed) separate.
Frame rate (speed)
Regardless of the frame counting system, the actual
speed at which frames of video go by in realtime is the
true frame rate.
In Cubase the following frame rates are available:
• 24 fps
This is the true speed of standard film cameras.
• 25 fps
This is the frame rate of PAL video.
• 29.97 fps
This is the frame rate of NTSC video. The count can be either non-drop
or drop-frame.
• 30 fps
This frame rate is not a video standard anymore but has been commonly
used in music recording. Many years ago it was the black and white
NTSC broadcast standard. It is equal to NTSC video being pulled up to
film speed after a 2-3 telecine transfer.
Frame count vs. frame rate
Part of the confusion in timecode stems from the use of
“frames per second” in both the timecode standard and
the actual frame rate. When used to describe a timecode
standard, frames per second defines how many frames of
timecode are counted before one second on the counter
increments. When describing frame rates, frames per sec
-
ond define how many frames are played back during the
span of one second of realtime. In other words: Regard-
less of how many frames of video there are per second of
timecode (frame count), those frames can be moving at
different rates depending on the speed (frame rate) of the
video format. For example, NTSC timecode (SMPTE) has
a frame count of 30
fps. However, NTSC video runs at a
rate of 29.97
fps. So the NTSC timecode standard known
as SMPTE is a 30
fps standard that runs at 29.97 realtime.
Clock sources (speed references)
Once the position is established, the next essential factor
for synchronization is the playback speed. Once two de-
vices start playing from the same position, they must run at
exactly the same speed in order to remain in sync. There
-
fore, a single speed reference must be used and all de-
vices in the system must follow that reference. With digital
audio, the speed is determined by the audio clock rate.
With video, the speed is determined by the video sync
signal.
Audio clock
Audio clock signals run at the speed of the sample rate
used by a digital audio device and are transmitted in sev-
eral ways:
Word clock
Word clock is a dedicated signal running at the current
sample rate that is fed over BNC coaxial cables between
devices. It is the most reliable form of audio clock and is
relatively easy to connect and use.
AES/SPDIF Digital Audio
An audio clock source is embedded within AES and SPDIF
digital audio signals. This clock source can be used as a
speed reference. Preferably, the signal itself does not con
-
tain any actual audio (digital black), but any digital audio
source can be used if necessary.
ADAT Lightpipe
ADAT Lightpipe, the 8-channel digital audio protocol de-
veloped by Alesis, also contains audio clock and can be
used as a speed reference. It is transmitted via optical ca
-
bles between devices.