[pulseaudio-tickets] [Bug 99607] Lack of hardware volume no longer supported with HifimeDIY Tiny

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=99607

--- Comment #6 from Jerome <an.inbox at free.fr> ---
Sure, this policy makes sense. After digging more, the issue is mostly on the
hardware side. There may be a way to be more user friendly however, TBC.

There are two levels of oddities for the hardware, at the device architecture
and USB levels. The device architecture is:

          ----------------------              -------------
  USB --->| PCM2706 Burr Brown |-- IEC958 --->| Sabre DAC |---> Analog out
          ----------------------              -------------     Headset
                    |
                    V
                Analog out
                 UNUSED!

The PCM2706 is only used for its USB interface. It has an internal DAC with an
analog output, but this output is not used. Instead, the PCM2706 has a digital
output, which is used to feed another better DAC from Sabre. This other DAC
output is the device output (headset 3.5 mm jack).

But at the USB level, only the PCM2706 is seen with a completely stock
configuration. Even the USB id is generic for the PCM2706, so there's no way to
identify this specific set-up.

And the USB audio class descriptors of the PCM2706 do not even reflect its
internal set-up. It would have been possible to describe at the USB level both
analog and digital output, but that's not done. From the USB descriptors, the
PCM2706 is a simple pipe with just the analog output:

   --------------------     ---------------     -----------------
   | USB input stream |---->| Mute+Volume |---->| Analog output |
   --------------------     ---------------     -----------------

There is indeed hardware volume support, but from the PCM2706 datasheet it
applies only to the (unused) analog output, not to the digital IEC958
interface.

I saw two changes when upgrading from Jessie to Stretch:
  1) selecting the analog profile used to give no sound, now it does
     with a step volume effect;
  2) the IEC95 profile had disappeared.

The first change (1) I now understand: it's just that the IEC958 output is
enabled too when the analog output is enabled. The driver level correctly
report a hardware volume control for the analog output. But it doesn't apply to
the actually used IEC958 output (per the PCM2706 datasheet). And when the
volume is set to 0, the mute is likely enabled and this does apply to the
IEC958 output too. So the observed behavior is OK. Enabling the IEC958 output
too confused me, but why not.
(I still don't understand why using the keyboard volume control switches to
software volume for a split second before going back to hardware, but it's a
minor point)

The second change (2) was the real problem in my case. But after the suggested
work-around the IEC958 profile is back, and when using it all is ok again. I
set it as default profile, and it's sticky across reboots / plug and unplug
now. I didn't see it disappear again since.

Because the IEC958 output is not part of the USB description, this profile must
be added by a driver handling the PCM2706. I tried to grep around for the Burr
Brown id and this device family (270[4567]) in the kernel source and ALSA data,
but couldn't find where it is managed. The datasheet is not clear either on how
the digital output is controlled. So I don't understand how this interface is
handled and why it disappeared.

Anyway, considering the situation and with the IEC958 profile back, both
pulseaudio and the driver level both reflect the reality of the device
hardware.

The only improvement (maybe?) could be to prefer the IEC958 profile to the
analog output of the PCM2706, if this digital port is connected. From what I
see the PCM2706 is now mostly used for its USB interface, and the HifimeDIY
type of design is common. There are other better cheap DAC for the analog
ouput, and even a cheap USB DAC dongle like the HifimeDIY can afford one. This
would avoid the strange experience with the default analog output (higher
priority for now). 
However this would require reliably detecting that the PCM2706 digital output
is used, to avoid a regression for old PCM2706 only device using its built-in
DAC and where the IEC958 output is not used. I don't know if this is possible.
Even if it's not, this comment can at least help clarify what's happening for
users of this USB DAC or similar designs ;)

Thanks

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