[Jack-Devel] questions about latency ranges

Xavier Mendez me at jmendeth.com
Sat Nov 18 21:55:14 CET 2017

El 18/11/17 a les 00:50, Markus Grabner ha escrit:
> Am Mittwoch, 15. November 2017, 15:33:29 CET schrieb Xavier Mendez:
>> (shameless self-citation) I made jack_lsync that does this, both
>> jack_lset and jack_lsync can be found in:
>> https://github.com/jmendeth/jack-tools/tree/latency-tools
> Excellent, I was just starting to implement a similar tool myself, you saved
> me a lot of work! Dropping in two instances of your client in the capture and
> playback path precisely aligns the round-trip pulses in my Ardour test setup
> (even in the pathological case that both signals are recorded into the same
> track).
> I vote for adding these tools to the Jack example clients. What do you think
> about this?

It'd be nice if they also supported synchronizing MIDI ports, but that's 
TODO for now. For the rest, IMO they're pretty much ready to go.

> BTW, one question regarding the "coefficient" parameter: I thought this should
> always be "1" (align to maximum latency), but didn't notice any difference
> when leaving its default value 0.5 (align to center), both variants gave
> correct results. How does this coefficient affect the behaviour of your client?

The coefficient has no effect when all ports' input latency has the same 
minimum and maximum (i.e. one port [ 337 337 ] and the other [ 867 867 
]). That's expected most of the time.

If one port had [ 523 1024 ] latency you'd probably be doing it wrong, 
but jack_lsync still has to deal with it, and that's what the parameter 
is for. If coefficient was 0, lsync would assume 523 latency when 
calculating the delays to apply, if it's 1 it'll assume 1024, and if 
it's the default it'll just take the mean of min & max, and assume 773.

So it's only there for those strange cases that should ideally never happen.

PS: If you want to use alsa_out or alsa_in, note this bug which may make 
them report wrong playback/capture latencies in some cases 


> 	Thanks & kind regards,
> 		Markus
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