[pulseaudio-discuss] Best Case Latency

[Patrick Shirkey]

On Thu, September 19, 2013 6:42 pm, Tanu Kaskinen wrote:
> On Wed, 2013-09-18 at 04:28 +1000, Patrick Shirkey wrote:
>> Hi,
>>
>> Can someone shed some light on the best case and typical latency that
>> Pulse provides for standard operations?
>
> What do you mean by standard operations? I'd estimate volume changes
> etc. normally (on an ALSA sink) have a few millisecond latency (best
> case 0.5 ms audio buffer + scheduling latency).
>

Just a "normal" round trip signal flow, for example : PA (in) -> audacity
(in) -> audacity (out) -> pa (out)


>> Also how that is affected by using jack-sink assuming jack is set to the
>> lowest possible latency that a device can handle?
>
> When jackd asks for N frames from PulseAudio, pulseaudio will render N
> frames. There's no extra buffering done in the Jack sink, so volume
> changes etc. will have latency that is roughly the same as the normal
> Jack client latency.
>

Do you have an suggestions on how to get an accurate measurement?


> If by "standard operations" you meant the stream latency that PulseAudio
> clients see (in which case "standard operations" is a strange way to say
> it, IMO), then that depends on what the clients request. If they let
> PulseAudio choose, the latency is typically 2 seconds. If they want
> minimal latency, with infinite CPU power I think the lower bound is
> maybe a couple of milliseconds, but since computers don't have infinite
> CPU power, the practical limit is some tens of milliseconds based on
> what I've heard (the exact limit naturally depends on how beefy the CPU
> is).

I would like to accurately measure the lower bounds. Do you know of any
tools for that? I know PA provides latency measurements as part of the API
but is there an app the already provides this data in an accessible
format?




--
Patrick Shirkey
Boost Hardware Ltd