[Jack-Devel] questions about latency ranges

[Xavier Mendez]

I still don't understand what's happening on the OP example. If both A 
and B are connected to C, you really should be getting capture latency 
of [ 337 867 ] there.

I created a quick test setup:

$ jackd -d dummy -r 48000 -p 2048 -C 0 -P 1 &
$ jack_lset -n source1 --capture 337 &
$ jack_lset -n source2 --capture 867 &
$ jack_connect source2:output_1 system:playback_1
$ jack_connect source1:output_1 system:playback_1
$ jack_lsp -l
system:playback_1
	port latency = 4096 frames
	port playback latency = [ 4096 4096 ] frames
	port capture latency = [ 337 867 ] frames
source1:output_1
	port latency = 0 frames
	port playback latency = [ 4096 4096 ] frames
	port capture latency = [ 337 337 ] frames
source2:output_1
	port latency = 0 frames
	port playback latency = [ 4096 4096 ] frames
	port capture latency = [ 867 867 ] frames

JACK should always set the minimum value of the pair to the minimum 
among its connected ports, not set both values to the maximum. Am I 
missing something here?

 > You could add a jack-client in-between that delays the signal and sets
 > port-latencies to include the delay.

(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

Xavi

El 15/11/17 a les 03:46, Robin Gareus ha escrit:
> On 11/14/2017 11:50 PM, Markus Grabner wrote:
>> 	Hi!
>>
>> I have some questions about the minimum and maximum values of a port's latency
>> (i.e., its latency range). The documentation states the following:
>>
>> "Both latencies might potentially have more than one value because there may
>> be multiple pathways to/from a given port and a terminal port."
>>
>> Does this also apply to a setup with two output ports "A" and "B"
>> (representing two distinct physical devices) with different capture latency,
>> which are both connected to the same input port "C"?
>>
>> Here are some details of an example setup (useful for demonstration purposes,
>> but probably not for doing any real recording work) as reported by jack_lsp:
>>
>> "A" (Focusrite Saffire PRO 14 Firewire audio interface):
>> firewire_pcm:00130e0402403491_1394/Out:01 (Mic/Lin/Inst:01)_in
>>          port latency = 337 frames
>>          port playback latency = [ 0 1792 ] frames
>>          port capture latency = [ 337 337 ] frames
>>          properties: output,physical,terminal,
>>
>> "B" (Line6 PODxt Live guitar effects processor synchronized by zita-ajbridge):
>> line6_in:capture_1
>>          port latency = 867 frames
>>          port playback latency = [ 1792 1792 ] frames
>>          port capture latency = [ 867 867 ] frames
>>          properties: output,physical,terminal,
>>
>> "C" (Ardour track):
>> ardour:track 1/audio_in 1
>>          port latency = 0 frames
>>          port playback latency = [ 1792 1792 ] frames
>>          port capture latency = [ 867 867 ] frames
>>          properties: input,
>>
>> Under the assumption that the physical inputs of "A" and "B" represent the
>> same time scale, the correct answer for the port capture latency at port "C"
>> would be [ 337 867 ] and not [ 867 867 ].
>>
>> So my questions are:
>> *) What is the rationale behind jack reporting the larger of the two latencies
>> of "A" and "B" both as the minimum and maximum latency of "C", and ignoring
>> the smaller one?
> 
> In this case it's Ardour (not JACK) that sets the port latencies. I
> guess you use Ardour5 which [wrongly] always reports the worst-case latency.
> 
>> *) Is it possible to provide additional information to jack to state that the
>> signals at "A" and "B" originate from the same physical source (or from
>> different physical sources sharing the same time scale), such that both
>> latencies of "A" and "B" are taken into account for the latency range of "C"?
> 
> I guess you use alsa_in or zita-aj2 for port "B", the latter tool has
> options to set additional systemic latencies.
> 
> Just forcing the values to some different number won't help you.
> You could add a jack-client in-between that delays the signal and sets
> port-latencies to include the delay.
> Or wait for Ardour6 which already does this, but but at this point in
> time is still far from a release, and the session format is not stable)
> 
> ciao,
> robin
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