This section seeks to clarify some potentially confusing issues to do with synchronization. See the Operating multiple units section for the current status of multiple unit operation.
Sync sources, masters and slaves
Atlas behaves as a class compliant UAC2 (USB Audio Class 2.0) device, and operates in asynchronous mode. This means that the sample clock is always controlled from within the Atlas unit, and is NEVER locked to the USB bus or the host computer.
Where Atlas is generating its sample clock from its internal reference clock (local mode) it is considered to be the clock master, and its front panel 'master' LED is lit. When it is locked to an external sync source (Wordclock, DI or ADAT) it is considered to be a clock slave, and its front panel 'master' LED is not lit. Where an external sync source has been selected but is either invalid or is operating at a different frequency from the nominated sample rate, the 'master' LED flashes and the internal (local) reference is used instead. An exception to this is that during stand-alone operation, the sample rate automatically follows the frequency of the selected sync source so long as it is valid. For more information see the Unit settings and Stand-alone operation sections.
Every input and output channel must operate at a common sample rate and is locked to a common synchronization source. An exception to this is when a sample rate converter (SRC) is configured in Atlas's digital input or output. In the former case, the SRC simply converts any incoming digital signal of whatever sample rate to the sample rate of the Atlas unit. If the SRC is in the digital output, it is necessary to specify what the output sample rate must be; furthermore, it may be necessary to lock the output rate to an arbitrary external reference. Atlas allows for this; as described in the Outputs tab section of the Atlas Control Panel app chapter.
As well as outputting a clock at the selected sample rate, Atlas's Wordclock output can be configured to produce a '256x clock' (a clock at 256x the selected sample rate, e.g. a 'superclock') or a 'Baseclock' (44.1kHz if the sample rate is a 44.1kHz multiple, or 48kHz if the sample rate is a 48kHz multiple).
It may not be possible to change the sample rate whilst any of Atlas's ports are in use by an audio application (e.g. a DAW). In this case it is necessary to disconnect the ports from the application before changing the sample rate, or (where possible) to change the rate from within the application.
If an external sync source such as DI or Wordclock is selected, but is either absent or at a different rate from the selected sample rate, the unit reverts to local (internal) sync at the selected sample rate. If the reference is later applied at the appropriate rate, audio is re-enabled. Note that during Stand-alone operation an external sync source is allowed to control the unit's sample rate.