Clock sources (speed references)
Once the position is established, the next essential factor for synchronization is the playback speed. Once 2 devices start playing from the same position, they must run at exactly the same speed in order to remain in sync. Therefore, a single speed reference must be used and all devices 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 several 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 contain 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 developed by Alesis, also contains audio clock and can be used as a speed reference. It is transmitted via optical cables between devices.
Do not confuse the audio clock embedded in the Lightpipe protocol with ADAT Sync, which has timecode and machine control running over a proprietary DIN plug connection.
Video sync
Video sync signals are transmitted over BNC coaxial connections between devices and run at the frame rate of the video device.
There are 2 types of video sync signals:
bi-level (also known as black burst)
tri-level (used for HD video)
Complications arise when a video device is used as a speed reference. The video sync signal must be converted into an audio clock signal in order for audio devices to synchronize at the correct speed. This can be accomplished as follows:
Using a dedicated synchronizer such as the Steinberg SyncStation.
A dedicated synchronizer can take a video sync signal (among others) and generate either word clock or AES/SPDIF signals that may be used as an audio clock source.
Using a house sync generator.
A master sync generator such as the Rosendahl Nanosync HD can simultaneously generate video sync and audio clock from the same source. This ensures that both video and audio devices fed by the sync generator will run at the same speed.
Some audio cards and interfaces can accept a video sync signal as an audio clock source, performing a similar function to a dedicated synchronizer.
Care must be taken to ensure that the incoming video sync frame rate matches that of the Nuendo project.
MIDI clock
MIDI clock is a signal that uses position and timing data based on musical bars and beats to determine location and speed (tempo). It can perform the same function as a positional reference and a speed reference for other MIDI devices. Nuendo supports sending MIDI clock to external devices but cannot slave to incoming MIDI clock.
MIDI clock cannot be used to synchronize digital audio. It is only used for MIDI devices to play in musical sync with one another. Nuendo does not support being a MIDI clock slave.