A digital audio device is always “clocked” by some signal running at the same frequency as the sample rate (most often 44.1 or 48kHz). This clock is often provided by a built-in, extremely accurate, crystal circuitry.
When you make digital audio connections between two devices, the clock signals must be synchronized, or you will get glitches in the audio. This is done by routing a sync signal from the master (the device “transmitting” the clock signal) to the slave (the device receiving the clock signal). The slave device then replaces its internal clock with the one provided by the master. In this way the two devices are in perfect sync.
In the most basic connection, like when transferring digital audio between two DAT type recorders, the synchronization signal is part of the audio signal itself. In this setup, the recording device is automatically “locked” to the clock of the incoming audio.
In more advanced setups, the synchronization signal – often referred to as “word clock” – can be carried either as part of some audio signal (in a number of formats – S/PDIF, AES/EBU or ADAT for example) or in a separate cable (again in a number of different formats).
When setting up a digital audio system, it is extremely important that all the devices are synchronized to the same clock source. That is, there can only be one master, but an infinite number of slaves.
P Failing to provide proper synchronization for digital audio will most likely lead in glitches or distortion in your audio recordings.Please consult the dealer providing the equipment for exact instructions on how to set things up.
The sample rate is the speed with which you are capturing the digital audio. CDs use a sample rate of 44.1kHz and DAT recorders most often use 48kHz. Some systems use a sample rate of 96 kHz. Most audio hardware can be switched between several sample rates.
To be able to digitally transfer audio between components in your system, all recordings must be done at the same sample rate, the sample rate that Cubase VST is set to. If you intend to include recordings made at other rates, you must first use some separate application to “sample rate convert” them to the rate used in your Cubase VST system.
The audio synchronization described above may not be the only type of sync required. If you are using Cubase VST in conjunction with some other type of recorder, (digital multitrack tape machine, analog tape recorder or other hard disk based recorder) you will probably need to set up time code synchronization so that the actual transports are synchronized. That is, you need to make all components that have playback facilities agree on time positions. This is done in a similar fashion, but by providing a time code signal (rather than a word clock signal) that can be read by all devices. See the chapter “Synchronization” for details.
Both audio and time code synchronization is sometimes required in a digital audio system (for syncing to video for example). Although they are related, neither is a replacement for the other. Furthermore, it is very important that the same master source is used for both type of sync. That is, one and the same unit in the system should be the master source for all time code and word clock signals in the system. This master can be a digital tape recorder, a special synchronization device or some other device with similar capabilities.
ASIO Positioning ProtocolThe ASIO Positioning Protocol (a part of the ASIO 2.0 feature specification) is a technology that ensures that audio in Cubase VST is in sample accurate sync with external devices. By combining word clock sync and time code (transport) sync, ASIO Positioning Protocol facilitates sample accurate positioning and synchronization, as described on page 613. If your audio hardware and its ASIO driver support ASIO Positioning Protocol, we recommend that you use this.
If you are using Cubase VST together with some other type of recorder, you might be able to use MMC (MIDI Machine Control) to control its transport from the Transport panel in Cubase VST. Combine this with Tape Tracks, and you can record, punch in and play back all tracks in the system, all from the Cubase Arrange window! It is also possible to combine MMC with ASIO Positioning Protocol.
MIDI Machine Control and Tape Tracks are described in a separate document.