This document describes how clocks and time work in the audio subsystem.

Clocks and sample rate

Audio samples are send to the computer at a rate that is determined by the clock of the audio interface.

Professional audio interfaces may have the ability to synchronize their clock to an external clock. This allows multiple audio interfaces to share a clock, inside a clock-domain.

On MacOS these clocks are configurable through the API.

Example sample-clocks:

• Internal: This is the internal oscillator, usally these internal clocks are not very accurate and the sample rate could be different by a few Hz and the rate can change over time and temperature.
• Word Clock: Professional audio interfaces often have BNC connectors for input and output word-clock. A word clock is signal which goes high when a sample is taken.
• Digital Audio SPDIF, ADAT, MADI digital audio inputs can also be used to synchronize to.
• LTC SMPTE/EBU Some audio interfaces that have a buildin LTC SMPTE/EBU decoder it can use this signal as a clock source.
• Firewire Firewire audio interface can use the clock of the firewire interface, multiple audio interfaces connected on the same bus can synchronize through this.