Start-up time, A/V offset and end-to-end latency for each audio transport
The sync source flashes one of eight bars and plays a matching-pitch beep at the start of every
second. The page watches the canvas and the audio output and pairs each flash with its beep.
A/V offset is positive when audio is late. Latency compares against the server's clock, so it is only
meaningful when the server runs on this machine (or clocks are synced). Other sources report start-up only.