To avoid drift between audio and MIDI we recommend you to not synchronize Cubase VST externally at all when using audio, if possible. However, you might very well use Cubase VST as a “master” for other systems. Using MIDI Time Code or MIDI Clock generated by Cubase VST (for transmission to other devices) will ensure sync with the audio.
Let’s first look at the situation where Cubase VST is not synchronized to any external source.
Any digital playback system has an internal clock that ultimately affects the playback speed and stability, and PC audio hardware is no exception. This clock is extremely stable.
Normally, you will have the MIDI Sync Reference setting in the Audio System Setup dialog set to “Audio Clock”. This means that when Cubase VST plays back, the MIDI part of the program is internally synchronized to the digital audio to ensure synchronization between digital audio and MIDI.
Synchronizing Cubase VST’s playbackLet’s assume now that we use external synchronization, with Cubase VST, for example, we might synchronize playback to a tape recorder, using time code.
Time code coming from an analog tape recorder will always vary slightly in speed. Different time code generators and different tape recorders will also supply time code 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 time code.
If you now set the MIDI Sync Reference setting in the Audio System dialog to Time Code, and Cubase VST receives time code, it will vary its overall playback speed (the “song position speed”) to compensate for such fluctuations in the speed of the time code, that’s the whole purpose of synchronization.
What happens with the Digital Audio?The fact that Cubase VST’s playback is synchronized to the time code does not affect the playback of the digital audio. It still relies on the perfectly stable, built-in clock in the audio hardware.
As you may understand by now, problems will appear when the perfectly stable digital audio gets related to the slightly varying speed of a system synchronized to time code.
The playback timing of each segment will not be in total accordance 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.
So, what can I do about it?There are generally two methods that solve the problem, and a few others that minimize the problem to an acceptable level without actually solving it.
Resolving
The first – and best – solution 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 time code for Cubase VST. This ensures 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”.However this option is not available on standard PC audio cards. It might be possible to do on more advanced audio hardware, but even then it requires additional, fairly expensive synchronizing hardware.
Continuous ResynchronizationSome systems use digital signal processing techniques to simulate variations in playback speed. In a typical such situation you feed the digital audio hardware time code (or MIDI Time Code) and it uses this to calculate in what way it should adjust the playback speed to stay in sync with the time code (and hence with Cubase VST). This method is often called “continuous resync”.
Cubase VST SolutionsThe third solution is offered by Cubase VST, and comprises a set of tools and strategies to minimize the problems without the need of additional hardware, and without compromising audio quality. These solutions are described in the following paragraphs:
Choose your Audio Timing ReferenceIn the System dialog there are two settings that are related to sync (see page 538):
• The MIDI Sync Reference allows you to decide whether Cubase VST’s playback (and hence the MIDI) should slave to the external time code or to the audio hardware.
• The Audio Clock Source setting is for those with advanced audio hardware that supports resolving via external word clock signals. It allows you to clock the audio card from an external source.
Throughout a project, use the same synchronization source for all audio recording and editing. If you plan to sync Cubase VST to for example a tape recorder, make sure you are synchronized to that tape recorder at all times!
Use Generate SMPTEThe Generate SMPTE command on the Options menu allows Cubase VST to create a synthesized audio file containing ”perfect” SMPTE time code.
Once you have created the audio files that simulates a time code recording, you can stripe the tape you intend to synchronize your work to, with this code. The idea is that since this time code is generated at exactly the same speed as your digital audio hardware uses, it will make the MIDI playback play back in “perfect” sync with the digital audio.
We highly recommend using this feature to generate the time code before commencing any synchronized work with Cubase VST.The code is generated ”off-line” and is created very quickly. When finished, you can simply throw the file away or perhaps store it on a stable medium – such as DAT tape – for later use.