Re: [LAU] jdelay - measured latency of a usb interface

On Tue, 24 Mar 2009 11:14:00 +0100
<hollunder@email-addr-hidden> wrote:

> Hi everyone.
> I was kind of curious about the real latency of my system and did a
> series of tests using jdelay (http://www.kokkinizita.net/linuxaudio/).
>
> I share my results so others can see what to expect from such a usb
> audio device and also because..
>
> ! Important !
> I ran in a number of oddities that I can't explain myself. This could
> mean that my results here are totally useless, so please don't take
> them for real until someone can hopefully clarify those matters.
>
>
> General Information:
> -------------------
>
> Audio Interface: Edirol UA-25
> http://www.rolandus.com/products/productdetails.aspx?ObjectId=704
> http://alsa.opensrc.org/index.php/Edirol_UA-25
>
> It can work in 44.1, 48kHz duplex and 96kHz either record or playback,
> so I couldn't (and wouldn't have anyway) tested 44.1 and 48kHz.
>
> There are Periods/Buffer from 2 to 42 possible, most common is I
> believe 2 and 3 Periods/Buffer, I only tested these.
>
> My pc is rather old, mainboard is a via km400 or km400a if that helps
> anything.
>
> On the software side there is Arch Linux with it's latest stock
> kernel, 2.6.28.8, jack 0.116.2 and qjackctl 0.3.4 (the patchbay made
> this a lot easier).
>
> Measurement Results:
> -------------------
>
> SR 44100
> -----
> Frames Periods F*P/SR Measured
>
> 64 2 2.9 8.2
> 128 2 5.8 11.1
> 256 2 11.6 15.9
> 512 2 23.2 28.5
> 1024 2 46.4 52.7
> 2048 2 92.9 102.1
> 4096 2 185.8 187.1
>
> 64 3 4.4 9.7
> 128 3 8.7 14.0
> 256 3 17.4 22.7
> 512 3 34.8 30.1
> 1024 3 69.7 65.0
> 2048 3 139.3 138.6
> 4096 3 278.6 279.9
>
>
> SR 48000
> -----
> Frames Periods F*P/SR Measured
>
> 64 2 2.7 7.9
> 128 2 5.3 10.5
> 256 2 10.7 15.9
> 512 2 21.3 26.5
> 1024 2 42.7 48.9
> 2048 2 85.3 94.5
> 4096 2 170.7 171.9
>
> 64 3 4 9.2
> 128 3 8 13.2
> 256 3 16 21.2
> 512 3 32 27.2
> 1024 3 64 60.2
> 2048 3 128 127.2
> 4096 3 256 257.2
>
>
> Questions to the wise guys:
> --------------------------
>
> 1)
> As probably everyone has noticed, with 3 periods/buffer and
> frames/period of 512, 1024 and 2048 the measured latency is smaller
> than the calculated latency. I don't see how this can be possible
> since, as far as I understand it, the measured latency should be the
> calculated + some.
>
>
> 2)
> When doing those measurements I noticed that with 44100 the Frames
> reported by jdelay always ended in .732 Frames and with 48000 always
> ended in .761 Frames.
>
> I found out that I can change that value by changing the sensitivity
> of the input or the amplification of the output. In the case the
> variance is below one frame, but I find it very strange that it's
> always the same at a given amplification/sensitivity.
>
>
> 3)
> Is it normal that the difference between calculated and measured
> latency varies so much? Shouldn't it just always be the same?
>
>
> 4)
> Should these results be similar with any usb interface?
> What does have most influence on such results?
>
>
> I hope this can be of use after the remaining questions have been
> answered.
>
> Best regards,
> Philipp

Don't rely on those results.
I just had very different results for the same setting.

48kHz, 256/3 had 21.2ms, now I also got 15.9ms and 20.2ms, and I have
no idea what caused that.
Note that it never occurred while jdelay was running but have a look at
this:

1017.757 frames
^C
hollunder ~ $ jdelay
761.754 frames

I absolutely don't know how this can be, what happened in these
1-2 seconds in between, what's at fault, and it looks like it could be a
serious issue.

Regards
Philipp
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Received on Tue Mar 24 16:15:03 2009