[PlanetCCRMA] Quality of sound on RHEL 6.0 beta and Fedora 13

[Fernando Lopez-Lezcano]

On Mon, 2010-05-17 at 08:25 -0700, Niels Mayer wrote:
> On Mon, May 17, 2010 at 6:55 AM, Donald
> Steven <t6sn7gt at aim.com> wrote:
>         Is it my imagination, or is the basic sound quality of RHEL
>         6.0 beta and
>         Fedora 13 improved over previous versions.  To my ears, it has
>         a clarity
>         I haven't heard before.

Hi Donald, which sound card are you using? Are you doing a/b tests? How?

> A more stable and less jittery clocksource might be the reason for
> improved clarity. Jittery clocks are the main reason why ditgital
> sound can be so crappy, and the reason why studios aiming for the
> highest fidelity usually use external precision wordclocks
> ( http://www.apogeedigital.com/products/big-ben.php )and
> jitter-reducing external a/d&d/a's
> like http://www.apogeedigital.com/products/rosetta-series.php  .
>  
> Perhaps F13 implements the "audiophile tweaks" I've heard mentioned in
> chat:
> http://ccrma-mail.stanford.edu/pipermail/planetccrma/2010-March/016511.html

See also:
http://ccrma-mail.stanford.edu/pipermail/planetccrma/2010-March/016513.html

> Might be a good idea to see how your HPET timers are configured... A
> stock F12 x86_64 gives:
> 
> 
>         gnulem-163-~>
>         cat /sys/devices/system/clocksource/clocksource0/available_clocksource
>         tsc hpet acpi_pm 
>         gnulem-164-~>
>         cat /sys/devices/system/clocksource/clocksource0/
>         available_clocksource  current_clocksource    
>         gnulem-164-~>
>         cat /sys/devices/system/clocksource/clocksource0/current_clocksource 
>         tsc
> 
> 
> What are the results for F13? 

This is IMHO irrelevant. 

The HPET or other clock sources that are used for timing in the kernel
are _not_ physically connected (to my knowledge) to the clock source
that ships the samples through the sound card. That is a separate clock,
either a crystal in the sound card hardware itself or a clock signal
derived from the master crystal that drives the cpu clock. It does not
go through the kernel, it is a hardware feature. The kernel will
(should?) not affect where and how the hardware (a/d:d/a) gets the clock
from. 

So, an improvement in HPET or other timing sources will __not__ affect
the jitter of the digital audio stream. 

If there is a measurable improvement in sound quality, it is more likely
to come from a change in default settings in the alsa driver mixer,
perhaps bypass digital processing that was happening in the signal path
before (for example, a simple change in a _volume_ setting might account
for better perceived sound, _all_ other things being equal). 

-- Fernando