Re: [proaudio] Real-time for audio

[Carl <proaudio@xxxxxxxxxxxxxxxx>]

On Mon, 10 May 2010, Grant wrote:

> > Jitter and latency are completely unrelated generically. Latency is
> > just delay caused by any source. It might be Jack cycles, software
> > delays, etc. Latency is always a positive number and always delays the
> > arrival of your data at the destination.
> >
> > Jitter, on the other hand, is a _random_ change in the timing of each
> > clock cycle. It's caused by physical processes, such as variations in
> > how the crystal source and PLLs in your clock decide it's time to
> > switch between 1 and 0 or due to cabling in a noisy environment.
> > Typical consumer grade crystals might have a jitter or 200 PPM.
> > Industrial grade crystals might be spec'ed at 50 PPM and then further
> > screened to look for those with even less jitter. Still, if you work
> > out what 200PPM means in terms of data delivery is ain't much, but is
> > possibly audible.
> >
> > None the less and in general, for any system with a specific jitter I
> > can delay data and create more latency any time I want, therefore
> > jitter and latency are orthogonal.
> 
> The quote I posted before about rtirq references both latency and
> jitter with regard to the software.  That makes me think real-time
> software can reduce jitter.  Is that not true?  Or maybe I'm
> misunderstanding your comment above?
> 
> "With a measured worst case latency of five microseconds and with a
> typical jitter below one microsecond at an interrupt period of up to
> 100 kHz an rtirq-enhanced linux kernel may be usable for a broad range
> of hard real time control loop applications."
> 
> http://www.linuxfordevices.com/c/a/Linux-For-Devices-Articles/The-Linux-real-time-interrupt-patch/
> 
> > Yes, but IIRC the FLAC encoder has a flag to do lossy compression. I
> > could be wrong about that though and it's not important in this
> > conversation. My only point was to say I understand why a wave file
> > and an mp3 might be perceived as sounding different. I do not
> > understand why wave and FLAC should result in different bit streams
> > and I contend (clearly as this is maybe the 3rd time I've said it) ;-)
> > if the bit streams are identical and the equipment used to play them
> > is one system then the sound MUST be identical.
> 
> But the bit streams can be identical and the sound can be audibly
> different because of jitter, right?  Couldn't WAV vs. FLAC playback
> conceivably produce difference levels of jitter?
> 
> - Grant

Yes, the sound can be audibly different because of jitter in the clock driving the DAC, but in a properly designed system there shouldn't be any correlation between that jitter and if it is the wav- or flac-file that is beeing played. (Jitter should be RANDOM at the lowest possible level in both cases.)

In a poorly designed system on the other hand there could be disturbances caused by varying power levels or RF-signals emitted from other parts of the system, for example the CPU beeing more or less busy. Perhaps this kind of problems could cause the type of correlation you claim to see. I for sure wouldn't even recommend my worst enemy to try solving a playback problem by switching to a rt-kernel.

/Carl