The ASIO Positioning Protocol is a technology that ensures that audio in Cubase VST is in sample accurate sync with external devices. It is part of the ASIO 2.0 feature specification. Implementation of the ASIO 2.0 protocol can be found in various audio software and hardware. However, this text mainly describes its implementation in Cubase VST.
In order to take advantage of the ASIO Positioning Protocol, your audio hardware must be suitably equipped and the functionality must be included in the ASIO driver for the hardware.
P If in doubt, consult the documentation for the hardware and its ASIO driver.When transferring audio digitally between devices, it is important that synchronization is established on two levels:
• Level 1 : Sample Rate (word clock sync)
If this type of sync isn’t established, you may run into problems with for example clicks and pops or distortion.
• Level 2 : Sample Position (time code sync)
If the two devices do not agree on time positions, inaccuracies in positioning of the material will occur.
When performing audio transfers between devices, you need both types of sync (word clock and time code). Furthermore, the two need to be completely correlated. If not, the audio will not be recorded at the exact intended (sample accurate) position, which can cause various types of problems.
A typical situation is when transferring material from a digital multi-track tape recorder to Cubase VST (for editing) and then back again. If you do not have sample accurate synchronization set up, you can not be sure that the material will appear in its exact original position, when transferred back to the tape recorder.
An expansion of the situation above is when an entire digital audio system is locked to an external clock (as in video post-production, for example). If that external clock fluctuates in speed (for example, if it is generated by an analog video deck), the entire system must follow those fluctuations to a sample accurate level.
While this can be achieved with a combination of various hardware (audio interfaces and synchronizers), the ASIO Positioning Protocol makes it possible to use one hardware system for everything, which facilitates set-up and provides a higher degree of accuracy.
About time code in this contextNormally, the term time code refers to SMPTE/EBU (audio format time code), MTC (MIDI format time code) and VITC (Video format time code). However, for sample accurate synchronization with ASIO 2.0, the concept must be interpreted slightly differently. Time code for ASIO 2.0 may be supplied in one of the following formats:
• ADAT sync (Alesis).
• LTC (often referred to as SMPTE).
• VITC.
Future high precision time code formats may also be supported. However, MTC (MIDI Time Code) is not recommended for sample accurate synchronization, since the resolution is too coarse.
This section provides an example of a system for doing sample accurate transfers, in this case transferring audio tracks from an Alesis ADAT to Cubase VST. Here the ADAT will be the sync master (though it doesn’t necessarily have to be). It provides both the digital audio (with an inherent word clock) and positional information (time code) via its ADAT sync protocol. The master clock is generated by the ADAT itself.
Hardware and Software Requirements• Your computer audio hardware (in our example this would be an ADAT card in your computer) must support all the functionality required for the ASIO Positioning Protocol. That is, it must be able to read the digital audio and the corresponding positional information from the external device.
• There must be an ASIO 2.0 driver for the audio hardware.
When you have set up sync to time code via ASIO Positioning Protocol as described on page 622, the external device will act as a master for your system. Audio that you transfer between the two devices will be sample accurately synchronized.
In this case, the idea is that the external device is in its turn synchronized to yet another device. An example would be a Lexicon Studio system synchronized via LTC to a video tape recorder. In this case, the incoming time code will vary with the speed of the video tape recorder. The Lexicon Studio will make sure the audio word clock is adapted to fluctuations in the LTC sync signal and will supply all this information to Cubase VST via its ASIO 2.0 driver.
Now, any fluctuation in speed of the incoming LTC will be accurately transferred to both the time code and word clock sync in Cubase VST. In other words, all devices are in complete, sample accurate sync with the master, a phenomenon referred to as continuous resynchronization or resolving.
Additional Hardware and Software RequirementsThis setup has the same requirements as the example above, plus the following:
• The audio hardware (in our example the Lexicon Studio system) must have a time code reader/writer on-board.
• The time code coming from the device must be completely hardware locked to the audio clock in the device.
P The ASIO Positioning Protocol exploits the inherent advantage of having an audio card that has a built in Timecode reader. With such a card and the ASIO Positioning Protocol, you can achieve sample accurate synchronization between the audio source and Cubase VST.