[Brainstorming] Bit-perfect with PipeWire

[Rollbacke]

Hey @vicrodh

I’m using PipeWire w/ WirePlumber.

• PipeWire + WirePlumber
• USB DAC: my famous SMSL AD18
• QBZ “bit-perfect” wizard created user drop-ins for:
  • ~/.config/pipewire/pipewire.conf.d/99-qbz-dac-amp.conf
  • ~/.config/pipewire/pipewire-pulse.conf.d/99-qbz-bitperfect-amp.conf
  • ~/.config/wireplumber/wireplumber.conf.d/99-qbz-dac-amp.conf

The wizard basically disables resampling/channelmix for QBZ and pins allowed rates => OK.

What I see

With a 96 kHz track playing, pw-top shows matching rates (good), but the stream format is float:

S   ID  QUANT   RATE    WAIT    BUSY   W/Q   B/Q  ERR FORMAT           NAME
R   56   1024  96000  52,7us  11,4us  0,00  0,00    0    S24LE 2 96000 alsa_output.usb-SMSL_AUDIO_SMSL_AD18_AMP-00.analog-stereo
R  100   9600  96000   8,5us   8,9us  0,00  0,00    0    F32LE 2 96000  + alsa_playback.qbz

And wpctl inspect on the QBZ stream shows it’s going through audioconvert:

$ wpctl inspect 100
id 100, type PipeWire:Interface:Node
    adapt.follower.spa-node = ""
  * application.name = "PipeWire ALSA [qbz]"
  * client.id = "111"
    clock.quantum-limit = "8192"
  * factory.id = "9"
    library.name = "audioconvert/libspa-audioconvert"
  * media.category = "Playback"
  * media.class = "Stream/Output/Audio"
    media.name = "ALSA Playback"
  * media.type = "Audio"
    node.autoconnect = "true"
  * node.description = "ALSA Playback [qbz]"
    node.driver-id = "59"
    node.latency = "9600/96000"
    node.loop.name = "data-loop.0"
  * node.name = "alsa_playback.qbz"
    node.rate = "1/96000"
    node.want-driver = "true"
    object.register = "false"
  * object.serial = "185"
    port.group = "stream.0"
    remote.name = "pipewire-0"
    stream.is-live = "true"

First observation: the wizard configures pipewire-pulseaudio, whereas apparently by default (at least on my system), it uses pipewire-alsa.
second: no resampling (rate matches), but there is at least a format conversion (F32LE > S24LE) (float 32 bits > 24 bits int).

Brainstorming

Is it possible for QBZ (or its PipeWire-ALSA output path) to request/use an integer ALSA format directly (e.g. S24_LE), so the stream is not float32?

GPT purprose:

mkdir -p ~/.config/pipewire/client-rt.conf.d
cat > ~/.config/pipewire/client-rt.conf.d/99-qbz-s24.conf << 'EOF'
# QBZ via PipeWire-ALSA: force format ALSA
alsa.rules = [
  {
    matches = [
      { application.name = "PipeWire ALSA [qbz]" }
      # (optionnal) if the bin name can match:
      # { application.process.binary = "qbz" }
    ]
    actions = {
      update-props = {
        alsa.format = "S24_LE"
      }
    }
  }
]
EOF

systemctl --user restart pipewire wireplumber

But it doesn't work directly.
alsa.rules + alsa.format = "S24_LE" is indeed a valid ALSA client-side option (so for applications that use PipeWire ALSA).

However, the PipeWire graph processes audio as a 32-bit float. Therefore, seeing F32LE in pw-top is normal and can persist even if everything is "clean" on the hardware side, because PipeWire does its internal processing as a float (YEAH).

And anyway, the file was ultimately useless for me (I hadn't checked before), because the format was already correct before.

On the other hand, apparently, it can be useful (the ~/.config/pipewire/client-rt.conf.d/99-qbz-s24.conf file) when QBZ and something else (Firefox/Discord/YouTube) are running simultaneously. I say "maybe" because, on my system, QBZ apparently already had priority.
Should do some tests :/

So, in this case, it's important to note that to have a real bit-perfect, we need:

• volume at 1.00 / 0 dB everywhere
• no normalization/EQ/crossfade/replaygain applied
• no other applications playing simultaneously (Firefox at 48 kHz typically forces resampling somewhere if it outputs sound).

My 2 cents.

[Rollbacke]

Issue about this #82