[LAU] Testing JACK and PA latency

[Patrick Shirkey]

On Thu, September 19, 2013 7:42 pm, Fons Adriaensen wrote:
> On Thu, Sep 19, 2013 at 06:46:11PM +1000, Patrick Shirkey wrote:
>
>> jack_system (in) -> pa_source (in) -> audacity (in) -> audacity (out) ->
>> pa sink (out) -> jack_system (out)
>>
>> Audacity is run in pass through mode with internal latency set to 0.
>>
>> I would like to measure the round trip latency from jack_system (in) to
>> jack_system (out)
>
> Easy using jack_delay.
>
> 1. Measure the round-trip latency of your sound card (with an
> external analog loop).
>

Can I use jack_delay running on a second computer connected to the
external i/o of the first computer to get this value?


> 2. Measure
>
>   jack_delay -> pa_source -> audacity -> pa_sink -> jack_delay.
>


Does this look reasonable?

  1023.978 frames     21.333 ms total roundtrip latency
	extra loopback latency: 1023 frames
	use 511 for the backend arguments -I and -O
  1023.976 frames     21.333 ms total roundtrip latency
	extra loopback latency: 1023 frames
	use 511 for the backend arguments -I and -O
  1023.977 frames     21.333 ms total roundtrip latency
	extra loopback latency: 1023 frames
	use 511 for the backend arguments -I and -O



> 3. If pa_source and pa_sink are a single Jack client (probably not),
>    subtract one period from the result of (2).
>

Can you explain that with the data above?


> 4. Add the two values.
>

I would like to provide an app for this task. Do you think it would be
worthwhile to extend jack_iodelay for this purpose?




--
Patrick Shirkey
Boost Hardware Ltd