User manual

423
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 for example Cubase
and a drum machine.
Word Clock
Word clock is basically a replacement for the sample rate
clock in for example 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 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
audio hardware is no exception. This clock is extremely
stable.
When Cubase is playing back with no external synchroni-
zation, all playback is internally synchronized to the inter-
nal digital audio clock.
Synchronizing Cubase’s playback
Let’s assume now that we use external timecode synchro-
nization with Cubase. For example, we might synchronize
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 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’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 compo-
nent 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.
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. Therefore Cubase will
transmit MIDI Clock signals to other devices, but it
will not receive MIDI Clock.