[LAD] Jitter analysis

Fons Adriaensen fons at linuxaudio.org
Sat Oct 7 21:37:02 UTC 2017

On Fri, Oct 06, 2017 at 11:03:01AM -0700, benravin wrote:
> Fons Adriaensen-3 wrote
> > It's impossible to say anything about this if you don't provide
> > numbers. How big is the resulting resampling ratio variation ?
> > 
> > If a few ms jitter leads to anything perceptible then your DLL
> > and/or resampling control loop are not dimensioned correctly,
> > or there is another basic problem with your design.
> > 
> Actually I'm working on Embedded Linux. I have a requirement to configure
> the SoC as audio output master or slave. If I use the SoC audio output as
> slave and DAC as master then zita-ajbridge will work as expected. 
> But if SoC audio output is configured as master and DAC as slave and use the
> SoC system clock for timestamping the write and read buffers, then any drift
> on SoC clock will not be detected. 

Apparently you try to implement something similar to zita-ajbridge
or zita-njbridge, or at least based on the algorithm described in
my LAC paper. Since at least November 2016 I've been trying to
help you, and I remember having spent hours writing emails to
explain things. But each time I've done so, you come back a week
or so later, telling me that you have tried something different,
and that is doesn't work. 

For example:

> At the write side, t_wA and t_wB are the timestamps of buffer1 and buffer2
>  and read side t_rA and t_rB
> t_w = t_wB - t_wA
> t_r = t_rB - t_rA 
> error = (t_r – t_w ),  
> This error is minimized by audio control loop. 
> And if SoC is configured as master then any drift on write side will be
> there on read side as well. How to detect and correct it ?

This is not how the algorithm of zita-[an]jbridge works. It doesn't
try to minimise t_r - t_w whatever those are, and it doesn't use
fixed-size reads and writes on the resampler side of the buffer.
I've told you already a number of times that it couldn't work that
way and explained why. Together with a lot of other things which
you consistently choose to ignore.

When I ask for more info on how your thing works you never provide
any useful information. All I get are some details which are pretty
useless without the full context. 

This has been going on for at least 10 months now, and it ends
here as far as I'm concerned.



A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

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