The introduction of the ASIO Guard technology now allows compensating for performance peaks and other system related issues much better than before. Especially on Mac OS X based systems, this helps to achieve a more solid performance with less audio drop outs. Depending on the hardware setup in use, PC systems can also benefit from this allowing for a better overall performance.

Background

The Cubase and Nuendo audio engine is designed to process and play back all audio signals in real time. Everything you hear has just been processed by the CPU. There is always a little ASIO related offset (the latency) but this latency can usually be reduced to an (almost) unnoticeable value by setting the buffer sizes accordingly.

The buffer size basically is the time frame in which the audio processing for each ASIO block has to be accomplished. The smaller the buffer size, the more important a properly configured computer system and well working drivers are. Only then the application is able to execute the processing reliably thus making optimal use of the available CPU resources.
If there is a delay, it can happen that there is not enough time left to process one audio block which leads to audio drop outs.

There are various reasons for such a delay. It always depends on the operating system, the audio hardware in use, drivers and the general system setup. However, the result is always the same: even though theoretically enough CPU power is available, audio drop outs occur.

ASIO Guard

This is where ASIO Guard sets in. The aim is to become as independent as possible from the strict ASIO real time requirements. Whenever there is CPU time left despite the required real time processing, the upcoming buffer blocks are pre-calculated to have them available for the next ASIO block cycle. Under certain circumstances, this method can stabilize or even increase the resources available for audio processes.

The effect on the overall performance not only depends on the hardware environment but also the specific use case. For instance, audio channels that are monitor enabled cannot be pre-calculated and have to be processed in the time-critical real time path leaving fewer resources for ASIO Guard. This increases the risk for audio drop outs again.

With ASIO Guard enabled, Cubase/Nuendo will try to utilize the less time-critical path as much as possible. The assignment is an automatic and dynamic process. A monitor enabled channel will be removed from the ASIO Guard mode automatically. Once the monitoring is disabled again, the channel will return into the ASIO Guard path.
Because of this dynamic reallocation, it is advisable to monitor the VST performance meter to be able to address potential overloads.

ASIO Guard Limitations

ASIO Guard cannot be used for

• real time dependent signals
• VST Instruments using specific controllers (z.B. Maschine by Native Instruments)
• VST editors/plug-ins used to control external sound modules and effects
• External FX and instruments
• Plug-ins using the VST Bridge

Additional limitations in Cubase 7 and Nuendo 6

• VST Instruments with more than one MIDI input
• VST Instruments using disk streaming

Further hints for ASIO-Guard in Cubase 7 and Nuendo 6

• As soon as an audio channel is monitor enabled or a VST Instrument track is record enabled that channel/track will be switched from ASIO Guard to real time (and back). In Cubase 7 and Nuendo 6, the resulting fade in (and fade out) of the corresponding audio signal may be perceived as a small interruption.
• In Cubase 7 and Nuendo 6, ASIO Guard is only active on audio channels by default. ASIO Guard for VST Instruments must be enabled individually under Devices > Plug-in Information > ASIO Guard. It is also possible, to disable ASIO Guard for individual VST effects. 

ASIO Guard in Cubase 8

ASIO-Guard and multi-timbral VST Instruments

In Cubase 8, multi-timbral VST Instruments are running in ASIO-Guard mode, too, which enhances performance additionally. However, multi-timbral VST Instruments will be switched to real-time usage as soon as a MIDI track is selected, that is assigned to the instrument. This may increase the real-time processing load significantly and cause drop outs. The performance drop when switching from ASIO-Guard mode to real-time can be softened by using multiple mono-timbral VST Instruments instead of a single multi-timbral VST Instrument.

Offline Audio Mixdown, Freezing und Render in Place

The optimised ASIO-Guard in Cubase 8 allows for even smaller latencies. In this regard, it should be noted that the time the calculation of offline Audio Mixdown, Freezing and Render in Place requires, depends on the buffer size and thus latency. The lower the latency the longer it takes to calculate these processes. In extreme cases, the offline Audio Mixdown can even take longer than exporting in real-time.

Audio Mixdown, Freezing and Render in Place can be accelerated by increasing latency. This might reduce the resolution of automation though. VST 2 plug-ins calculate one automation value per parameter for each ASIO block (which equals buffer size in samples), whereas VST 3 plug-ins decide independently if automation data is being rendered blockwise, sample-accurately or at a self-defined resolution.