Determining Output Latency

Hi Chris,

It's become apparent that on some devices and Web Audio implementations, an AudioContext's currentTime reports a time that is somewhat ahead of the time of the actual audio signal emerging from the device, by a fixed amount.  To be more specific, if a sound is scheduled (even very far in advance) to be played at time T, the sound will actually be played when AudioContext.currentTime = T + L where L is a fixed number which for the purposes of this email I'll call "output latency".

I think the only reason this hasn't been noticed before is that until recently, the output latency on the implementations that I've been exposed to has been too small to notice. But in some implementations it can be substantial and noticeable.

When this occurs, is this 1) a problem with the implementation of the spec, or 2) an anticipated phenomenon that may vary from one implementation to another?

If the answer is 1), then at a minimum the spec needs to clarify the meaning of context.currentTime with respect to physical audio playback so that implementors realize they must add L back into the reported value of currentTime to make it correct.  But if the answer is 2), then we have a different problem: there doesn't appear to be any way to interrogate the API to determine the value of L on any particular platform.

Can you or others on the list provide any guidance on this point? Should I file a bug and, if so, what for?

Best,

... .  .    .       Joe

Joe Berkovitz
President

Noteflight LLC
Boston, Mass.
phone: +1 978 314 6271
www.noteflight.com
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