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Subject: Rockbox Audio clipping problem
From: Gonz (gonzini_at_freenet.co.uk)
Date: 2003-05-21


Hi,
Heres one for you!
I noticed this a while back but didnt post my findings.
When i first brought my FM20 i did some tests with my scope to see how the
hardware performed.I did the same with rockbox and i have found that under
some circumstance that the audio output will clip early and
un-symmetrically.Noticeably on the minus rail first at about 1vp/p.I have
not found reason why this might happen whilst using rockbox.
If i then boot up using the archos firmware the problem disappears, and i
will get symmetrical clipping at 3.2v p/p.
Now if i reboot using rockbox again the problem will disappear and again
symmetrical clipping at 3.2 v p/p, all good!

For you info:- This experiment was carried out with ALL sound parameters set
FLAT, with a 0db 44.1khz 1khz sine wave CBR 320kbps mp3.This should in
*theory..!* use all the available bit depth of the archos, D/A included!
Also no load was placed on the output.

Rockbox with strange behaviour:-
Vol @ 75 just before clipping
Vol @ 76 onset of clipping
Vol @ 77 minus waveform clipping early approx 1v p/p *no load condition*

Rockbox Ok:-
Vol @ 93 onset of clipping
Vol @ 94 symmetrical clipping @ 3.2v p/p *no load condition*

The archos firmware performed as above, one notch back = onset of clipping /
full volume symmetrical clipping.
I took some crude photos of the scope to show the problem (sorry about the
quality, really cheap camera..)

*I tryed to quickly post them on my personal webspace, but the links dont
work for some reason.I dont have the time now but if anyone whats to see
them i can post them to your email address... *

l'll post somemore tests ie, frequency response noise figures when i get a
chance.There are differences between the frequency response of archos
firmware and rockbox though this i would imagine to be down to different
centre *neutral* sound settings,
Though both are reasonably flat.
Cheers
Gonz



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