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Synchronization
MIDI Clock
MIDI Clock is a tempo-based type of synchronization sig-
nals, i.e. it is related to the number of “beats per minute”.
MIDI Clock signals are suitable for synchronizing two de-
vices that agree on tempo, such as Cubase AI and a drum
machine.
Word Clock
Word clock is basically a replacement for the sample rate
clock (e.g. in an audio card). Word clock hence runs at
the same rate as the sample rate in the audio, 44.1kHz,
48kHz, etc.
Word clock does not contain any position information, it is
only a “simple” signal for clocking the audio at its sample
rate.
Word clock comes in many formats, analog on coaxial ca-
ble, digital as part of an S/PDIF, AES/EBU or ADAT audio
signal, etc.
Synchronizing the transport vs.
synchronizing audio
How timing is handled in a non-synchronized
system
Let’s first look at the situation where Cubase AI is not syn-
chronized to any external source:
Any digital playback system has an internal clock that ulti-
mately affects the playback speed and stability, and PC au-
dio hardware is no exception. This clock is extremely stable.
When Cubase AI is playing back with no external synchro-
nization, all playback is internally synchronized to the inter-
nal digital audio clock.
Synchronizing Cubase AI’s playback
Let’s assume now that we use external timecode synchro-
nization with Cubase AI. For example, we might synchro-
nize playback to a tape recorder.
Timecode coming from an analog tape recorder will al-
ways vary slightly in speed. Different timecode generators
and different tape recorders will also supply timecode with
slight differences in speed. In addition, the shuttling of
tape mechanisms due to overdubs and re-recordings can
cause the physical tape to wear and stretch, which affects
the speed of the timecode.
If you use a synchronizer that generates word clock and set
up Cubase AI to sync to incoming timecode, it will vary its
overall playback speed to compensate for such fluctuations
in the speed of the timecode – that’s the whole purpose of
synchronization.
What happens with the digital audio?
The fact that Cubase AI’s playback is synchronized to the
timecode does not affect the playback of the digital audio.
It still relies on the perfectly stable, built-in clock in the au-
dio hardware.
As might be expected, problems will appear when the per-
fectly stable digital audio gets related to the slightly vary-
ing speed of a system synchronized to timecode.
The playback timing of each event will not be in total ac-
cordance with the tape or the MIDI playback, since the
playback speed of the audio is determined by the digital
audio hardware’s built-in clock.
Resolving to word clock
The solution to this problem is to use one external clock for
all components in the system. One master clock is used to
derive whatever type of clock signal each component in
the system needs. For example, something called a house
clock can be used to generate sample rate clocks for the
digital audio hardware and timecode for Cubase AI. This
ensures that all components in the system use the same
reference source for their timing.
Synchronizing digital audio to external clocks running at
sample rate is often called “resolving” or “synchronizing to
word clock”.
!
MIDI Clock is not suitable as a master sync source
for an application like Cubase AI. Therefore Cubase
AI will transmit MIDI Clock signals to other devices,
but it will not receive MIDI Clock.