Tutorials and Topics - Peabody Computer Music

Audio I/OAudio input andoutput with MSPThe Signal Vector Size is how many audio samples MSP calculates at a time. There aretwo vector sizes you can control. The I/O Vector Size (I/O stands for input/output)controls the number of samples that are transferred to and from the audio interface atone time. The Signal Vector Size sets the number of samples that are calculated by MSPobjects at one time. This can be less than or equal to the I/O Vector Size, but not more. Ifthe Signal Vector Size is less than the I/O Vector Size, MSP calculates two or more signalvectors in succession for each I/O vector that needs to be calculated.With an I/O vector size of 256, and a sampling rate of 44.1 kHz, MSP calculates about 5.8milliseconds of audio data at a time.The Max Scheduler in Overdrive option enables you to turn Max's Overdrive setting onand off from within the DSP Status window. When Overdrive is enabled, the Max eventscheduler runs at interrupt level. The event scheduler does things like trigger the bangfrom a repeating metro object, as well as send out any recently received MIDI data. Whenit is not enabled, overdrive runs the event scheduler inside a lower-priority eventhandling loop that can be interrupted by doing things like pulling down a menu. You canalso enable and disable Overdrive using the Options menu. Overdrive generally improvestiming accuracy, but there may be exceptions, and some third-party software may notwork properly when Overdrive is enabled.The Scheduler in Audio Interrupt feature is available when Overdrive is enabled. It runsthe Max event scheduler immediately before processing a signal vector’s worth of audio.Enabling Scheduler in Audio Interrupt can greatly improve the timing of audio eventsthat are triggered from control processes or external MIDI input. However, theimprovement in timing can be directly related to your choice of I/O Vector Size, sincethis determines the interval at which events outside the scheduler (such as MIDI inputand output) affect Max. When the Signal Vector Size is 512, the scheduler will run every512 samples. At 44.1 kHz, this is every 11.61 milliseconds, which is just at the outer limitsof timing acceptability. With smaller Signal Vector Sizes (256, 128, 64), the timing willsound “tighter.” Since you can change all of these parameters as the music is playing, youcan experiment to find acceptable combination of precision and performance.41