Re: Fwd: Odd issue with ALSA connecting directly to HDMI audio output

[Geraldo Nascimento <geraldogabriel@gmail.com>]

On Tue, Jul 18, 2023 at 12:22:35PM -0400, Ico Bukvic wrote:
> Hi all,

Hi Ico and friends,

> 
> I have a consumer-level setup audio question. My research project uses a
> Raspberry Pi in conjunction with a small portable monitor that has built-in
> loudspeakers that are to be used via HDMI connection. The goal here is
> portability and minimal cost.
> 
> Curiously, when trying to connect to monitor's loudspeakers (via HDMI)
> using ALSA, no matter what settings I pick, at worst I cannot get
> audio out, and at best, get a buzzy sound whose waveform is in the attached
> image. The sound is clearly periodic and impulse-like. It happens every 59
> samples. Changing sample rate increases pitch, suggesting 59 samples remain
> constant regardless of the sampling rate. When playing the actual audio,
> you can hear it in the background, but this buzz is easily overpowering it.
> 

Ico, just a wild guess but are you sure you are not capturing the raw SPDIF
signal going through HDMI instead of Linear PCM audio signal?

> Changing sample rate or bit depth, or buffering, makes no difference.
> Selecting 8 channels for output (one of HDMI's standards, as far as I can
> tell), sometimes manages to output a tiny chunk of a good audio sample, and
> then goes silent.
> 
> Now, even more curiously, when using the portaudio backend and connecting
> to the default pulseaudio sink, works just fine. What could be causing this
> discrepancy?

Perhaps PortAudio is forcing the Linear PCM bits on the underlying ALSA
backend somehow, see for example:
https://fossies.org/linux/alsa-lib/src/conf/pcm/iec958.conf
But this is just wild speculation, it may or may not be the case with
your report. It's best if you ask Takashi Iwai and Jaroslav Kysela
directly through the list with Cc: in case nobody else has any more
ideas of what could be the root cause of your report.

Good luck,
Geraldo Nascimento

> 
> Best,
> 
> Ico
> 
> -- 
> Ivica Ico Bukvic, D.M.A.
> Director, Creativity + Innovation
> Director, Human-Centered Design iPhD
> Institute for Creativity, Arts, and Technology
> 
> Virginia Tech
> Creative Technologies in Music
> School of Performing Arts – 0141
> Blacksburg, VA 24061
> (540) 231-6139
> ico@vt.edu
> 
> ci.icat.vt.edu
> l2ork.icat.vt.edu
> ico.bukvic.net