[LAU] jdelay - measured latency of a usb interface

[hollunder at gmx.at]

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