Why is HDMI audio working differently for Sony and Samsung?

Question

I have a strange different behaviour in raspbian showing up between two TVs. A Sony Bravia 32W670A and a Samsung LA32A650A1F

The simplest demonstration for this problem is to build the hello_audio.bin in /opt/vc/src/hello_pi/hello_audio/

I run hello_audio.bin over and over in one console with this command

$ while date; do ./hello_audio.bin 1; done

That part works the same - the annoying sound will come from whichever TV is plugged into the HDMI at the time.

The difference is when I open Python in another console

Python 2.7.3 (default, Jan 13 2013, 11:20:46) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from pygame import mixer
>>> mixer.init()
>>> mixer.music.load("foo.mp3")
>>> mixer.music.play()

The Samsung continues to play the annoying sound from the first console in addition to "foo.mp3"

The Sony goes silent as soon as mixer.init() is executed, and remains silent until Python exits (ie. you don't get to hear "foo.mp3" either!)

I only have the two TVs to test on at this moment, but I hope to add the results for a Panasonic to the list in the next couple of days.

Is it possible that the TV is changing the output of the PI? Or is something just confuddling the Sony?

Edit: I found another way to reproduce. Instead of using pygame, simply

$ mpg123 foo.mp3

Again - either hello_audio.bin or mpg123 work fine on their own, but playing both at once gives silence from the Sony. The Samsung plays both sources simultaneously.

Answer 1

You don't say how sound is being sent to the two TVs. It sounds a bit like for one you're getting audio from the Pi (the Samsung) so the Pi can do the mixing. On the other (the Sony) you're sending digital data so the Pi has no opportunity to check.

You can force the issue by setting some options in config.txt and rebooting so you can control how audio is sent. Look at the hdmi_force_edid_audio option.

This is pure speculation, but it does fit the facts as you describe them.