Audio output "hiss" when should be silent

[PeterOGB]

The on board audio (PWM to 3.5mm Jack) has started to produce a very noticeable level of "hiss" when it is open but being fed silence. Mostly reported on the forums as a ALSA problem but I have openMAX IL code that bypasses ALSA and has the same problem. I've only noticed it since upgrading to 2015-02-16 Raspbian but it may have been there longer. Douglas6 on the forums says "An easy way to produce the symptoms seems to be to install and run fluidsynth."

[P33M]

Is the hiss still present if you add

disable_audio_dither=1 to config.txt?

[PeterOGB]

Yes that seems to have fixed it :-) Should I recommend that as the fix on the forum or is there more to be done to find a better fix ? I assume there is a down side to disabling the dither ?

[P33M]

Disabling it means that the noise floor becomes a function of the input signal. In effect you get slightly more distortion without dither than with.

[PeterOGB]

Ok, but is it the right solution in situations where silence is going to be produced by the application ?

[popcornmix]

The dither is added as a fractional bit (out of the 11 bits of PWM) in range [0..1).
As long as source is outputting silence as zero (or zeros outsize of most significant 11 bits) then the behaviour with or without dither should be the same.
If silence is constant, but non zero, you will hear the dither.

Are you sure the silence being output is zero?

[PeterOGB]

I'm going to say "yes" as the hiss only starts up after the recorded samples have stoppped. (My app has short sound effects for button presses etc). When no effects are playing the output buffer is all zeros. And to be 100% sure I just changed my code to write all zeros into the sample buffer just before the ALSA write is called.... And HISS is still present when dither is enabled.

PS: Problem is not present using HDMI or external USB sound card.
PPS: If you are coming to the Party this weekend I can show you the problem,

[P33M]

You say you already have code that exhibits this - can you post (or gist) this?

[PeterOGB]

I do, but it is part of a large application. I'll chop out the sound stuff for you.

[PeterOGB]

I realised there is no need to write any code.... Just run this...
aplay -t raw -r 48000 -c 2 -f S16_LE /dev/zero

PeterO

[pelwell]

Neat, but I'm surprised it works - I thought /dev/null returned EOF, whereas /dev/zero returns zeroes.

[PeterOGB]

I typed /dev/null by mistake and I guess you got a notification before I changed it to /dev/zeros :-) Grrrr.... /dev/zero

[P33M]

I can replicate the effect.

[Clay-Tron]

Adding the line to disable audio dither seems to work great at silencing the hiss between programs, and fixes an issue with emulation station. However I'm continuing to find is that the hiss remains whilst music is playing. This can be noticed when running any of the python games provided with Raspbian.

[Clay-Tron]

To be more specific there is no hiss on the 3.5mm audio line whilst sound is playing in emulators on Retro-Pie distribution, but there is whist sounds are played on Raspbian. There must be a configuration setting that differs between the two that breaks the sound quality in Raspbian. I see that retroarch manages the sound on the retro-pie distribution. Any ideas? Whilst I'm here a big, big thank you for solving the hiss during zero sound output!