Pi B+ background hiss?

Has anyone else found that RISC OS on the Pi B+ (haven’t tried a Pi2) has a constant background hiss from the audio socket? As soon as RISC OS starts up, the speakers start hissing. Starting in Raspbian is silent. Swapping back to a Model B and the speakers are again silent. This is with a standard RC14 disc image.

Yes I’ve certainly found this too and distinctly annoying it can be. It seems to be related to the OS version as it occurs on the B+ with 5.21 (4 June 14) but not with 5.21 (31 Jan 14).
The same behaviour occurs on the B but the hiss level with 5.21 (4 June 14) is very nuch lower and could pass unnoticed

Interesting. I noticed a hiss on my B+ too but didn’t realise that it was a RISC OS issue as it was the only time that I’d plugged anything into the audio socket. It’s good to know that it doesn’t affect earlier releases; it should be fairly easy to figure out what changed between them.

I suspect I had a quick look at this at the time, but since it’s now been mentioned in another thread I should probably post something about it:

Nothing changed in BCMSound between the two dates mentioned (indeed, the two closest changes are in 2013 and 2015). So if there is a problem related to the ROM version then it must be down to a change in another component.

I don’t have a B+ to test with, but I’ll try to remember to check my early model B against my Pi 2 tonight. If I can’t hear anything unusual there then I think one of you guys will have to help with the testing.

Just a thought – when switching between old and new ROM versions, was the firmware the same?

Not sure if I unterstand the problem right.
The B+ has a audio/video socket in one. I mean if I use a normal audio cable I have some noice in the background, if I use a audio/video cable not.

Testing with a B and a Pi 2, both with firmware from July 15th, they both have quite a lot of background hiss. This is with a standard pair of headphones – no special audio/video cable for the Pi 2.

If I add disable_audio_dither=1 to the config.txt then the background noise goes away completely (but it does result in slightly “rougher” audio). Looking back at the firmware history, it looks like PWM dithering was added in February 2014, which places it nicely between the two dates that are mentioned here. Also, it looks like it’s a known bug that the dithering produces hiss when the output should be silent: https://github.com/raspberrypi/firmware/issues/380

I’d guess that Raspbian doesn’t show the effect because it only enables the audio output when something is actually using it – unlike on RISC OS where it’s generally enabled all the time. (*audio off did stop the hiss)

I’d guess that Raspbian doesn’t show the effect because it only enables the audio output when something is actually using it.

Hmm, that could explain why it (not RISC OS) clicks in my headphones before playing audio (movie, mp3s…).

If ‘PWM’ dither produces so much audible hiss then either the audio hardware is crap, or the dither process, or both! Dither is supposed to improve audible performance. Not produce annoying audible noise as the price of reducing distortion.

I don’t have an RPi. But from what has been said it looks like people should avoid its analogue output, at least for anything like serious listening. I assume the HDMI will be better. But can only guess.

Maybe time to add the USB Audio modules… Still awaiting their arrival in the ROMsets on the other hardware!

Jim

Has anyone else found that RISC OS on the Pi B+ (haven’t tried a Pi2) has a constant background hiss from the audio socket?

Unfortunately I can only confirm this issue. While it’s not really noticeable on monitor’s speakers it’s clearly audible on headphones.

Maybe time to add the USB Audio modules…

I’d love to see some audiophile grade DAC support :-)

I assume the HDMI will be better.

Not everyone owns/wants HDMI display (I’m perfectly happy with my array of 15" LCDs)