#11170 LOW 1.75-ha: XO-1.75 sound output quality regression over XO-1.5

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#11170: XO-1.75 sound output quality regression over XO-1.5
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           Reporter:  Quozl                |       Owner:  wmb at firmworks.com
               Type:  defect               |      Status:  new              
           Priority:  low                  |   Milestone:  1.75-hardware    
          Component:  ofw - open firmware  |     Version:  1.75-B1          
         Resolution:                       |    Keywords:                   
        Next_action:  diagnose             |    Verified:  0                
Deployment_affected:                       |   Blockedby:                   
           Blocking:                       |  
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Changes (by wmb at firmworks.com):

  * owner:  wad => wmb at firmworks.com
  * component:  hardware => ofw - open firmware


Comment:

 The problem is aliasing.

 The ALC5631Q CODEC ostensibly supports low sample rates like 8kHz in
 hardware, but it doesn't do digital anti-aliasing (interpolation), so when
 you play an 8kHz sound, you get stair-stepping.  I verified this by
 looking at that headphone output with a scope using this command sequence:

 {{{
 ok dev /audio
 ok decimal  patch 8000 48000 init-codec
 ok select /audio
 ok d# 1000 tone
 }}}

 The stair-stepping was quite evident.  If you do the same thing without
 the patch, the signal looks much better, as the steps are much smaller, so
 the aliasing will be at higher frequencies and mostly inaudible.

 The solution is to change OFW to play the sound at 48 kHz instead of 8
 kHz.  The sound file itself is at 8kHz, so it will need to be interpolated
 in software to 48kHz, which is how it works on XO-1.5, whose codec does
 not support non-48kHz sample rates.

 There is filtering code in cpu/x86/firfilter.fth .  It contains some x86
 assembly language that needs to be ported to ARM.  Extra credit for doing
 the interpolation in real-time, interleaved with the audio DMA buffer
 management.  The timing of that may be tight, but probably do-able,
 especially if one uses the wMMX instructions.

-- 
Ticket URL: <http://dev.laptop.org/ticket/11170#comment:3>
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