KyleFromBullEx
Posts: 3
Joined: Mon Jul 07, 2014 5:48 pm

Raspberry Pi B+ Audio Hissing

Thu Mar 12, 2015 4:50 pm

Hello all,

I work for a company that makes products based on Raspberry Pi's. We ship approximately 1,000 Pi's per year. We've been trying to migrate to the new B+ model, but issues with the audio are holding us up. When we try to use the latest firmware, there is audible white noise on the audio output (both on the B and B+.) We've been shipping with old B-only firmware for a long time because of this, and haven't been able to fully migrate to the B+.

One of my coworkers already tried to resolve this issue in this thread:
http://www.raspberrypi.org/forums/viewt ... se#p596302

I would suggest reading the above thread for information on this issue.

It looks like the Raspberry Pi dithers the audio before sending it out the PWM output. Is there a way to disable this dithering, or otherwise disable the white noise?

Kyle

jdb
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 2202
Joined: Thu Jul 11, 2013 2:37 pm

Re: Raspberry Pi B+ Audio Hissing

Thu Mar 12, 2015 4:53 pm

https://github.com/raspberrypi/firmware/issues/380

It can be disabled with a config.txt setting.
Rockets are loud.
https://astro-pi.org

KyleFromBullEx
Posts: 3
Joined: Mon Jul 07, 2014 5:48 pm

Re: Raspberry Pi B+ Audio Hissing

Thu Mar 12, 2015 6:43 pm

I just tried adding this option, and it worked. The hissing is no longer there. Thank you!

However, this raises several questions for me.
  1. Why does the firmware ship out of the box with dithering enabled, if it causes this hissing? I have seen other people with the same issue, and the solution was to turn on disable_audio_dither=1.
  2. We sometimes ship products to Europe, which requires the CE mark. When we first started shipping to Europe, we had to do lots of testing and adjusting to cut down on noise levels. One of the things we had to do was bypass the PWM entirely and use an external USB audio dongle. Is the new B+ audio more or less CE compliant than the old B audio?
Again, thanks for your help.

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