[Jack-Devel] I'm confused about latency

[Alexandre BIQUE]

On 4/5/19 2:56 AM, Thomas Brand wrote:
> On Fri, April 5, 2019 00:28, Alexandre BIQUE wrote:
>> On 4/4/19 8:41 PM, Thomas Brand wrote:
>>
>>> Did I understand your use case correctly: you want to be able to tell
>>> (from a client's perspective) how long it will take from *now* (this
>>> cycle) until the start of the buffer is played out (eg. the first sample
>>>  of buffer hits the DAC) ~ ??
>>
>> Yes, something equivalent to snd_pcm_htimestamp().
>>
>>
> This page has a nice diagram:
> http://manual.ardour.org/synchronization/latency-and-latency-compensation/
> I don't know how accurate the delay from data leaving jack until it is
> heard at the analog output can be known. It's around 2.5 ms. But is it the
> same for every ALSA device and driver? jack_iodelay can measure round-trip
> latency and you could adjust your system based on jack_iodely output.

2.5 ms is quite a lot, and should be somehow reported by device API
(ALSA, ASIO, CoreAudio, ...):
 - the driver should have a rough idea of how much hardware latency is
introduced by the audio interface
 - the OS should have a rough idea of how much scheduling/internals
latency is introduced

Even if those values are not correct if they can reduce the "error" it
is worth it right?

I'm skeptical that people want to put a feedback cable into their audio
interface and measure exactly how much latency is introduced, and then
configure their tools with some mystic latency compensation. I really
think that if we can get some values from the OS which minimize the
error that would be better.

Cheers,
Alexandre