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Report of DSP audio quality problem

morgaine

This is the first I've heard about this DSP issue, but it appears to be researched technically and evidence is presented:

 

"Can't get good sound on RPi. Power problems" -- http://slashdot.org/comments.pl?sid=3070945&cid=41116889

 

"I bought two Raspberry Pi(es) to use as audio servers and have been disappointed by the sound quality.  The on-board audio out's DSP has limited bandwidth so sound is down-sampled to 11 bits.  Scratchy.  It's not advertised so that was a let-down.

 

Using a USB AUDIO dongle is no-go either, because of the crappy USB drivers. Stutters non-stop. Here are oscilloscope grabs of two music samples and a 1Khz tone: http://imgur.com/a/rVR99 . The flat parts shouldn't be there. The only way to get good sound now is to use rather expensive USB soundboards or the HDMI output, but extracting line-level audio signals from that isn't a simple or cheap proposition."

 

 

I find it hard to believe that it's appeared first on Slashdot and not on the RPF forums.  Have any of you come across it before?

 

Morgaine.

Parents Reply
  • in reply to Former Member

    Wow SCO the Linux that would not die. Or should that be Unix. I can't seriously believe that anyone would risk even a single $ in that company unless they have a huge "idden patent portfolio. image

     

    My sound is fine through the HDMI. But I have yet to try any other methods. Are we saying that ALL Pi's have these problems or that some have a manufacturing issue that make the sound design probs worse? image

     

    Ray

     

    Perhaps I should state that I am almost totally deaf so Pi-Fi sound is not that important to me. But I can hear some audio with hearing aid in one ear. (I usually aim the other ear at the wife) image

Children
  • in reply to Former Member

    It's a problem with all the Pi's. They simply use a pwm output to create the sound. The signal runs on a 100 Mhz clock. So, at 48 kHz this gives you roughly the equivalent of an 11 bit DA converter. Someone on the Rpi forum also discovered that the clamping diodes in the audio output are placed behind the dc decoupling capacitors instead of before them. As long as the signal stays below 0.5V, that shouldn't be responsable for the distortions.