shairport-sync and UxPlay have different sound in airplay1

[24fpsDaVinci]

I've been a/b testing shairport and uxplay for airplay1 and uxplay seem to produce a slightly different sound, is this due to different decoders being used or something else? (I tested with the default config for both)

should all airplay1 sound identical?

[mikebrady]

Interesting post. A good number of years ago, it was established that Shairport Sync was bit-perfect in classic AirPlay (AirPlay 1). That is, the bit stream coming through AirPlay and through the decoder was identical to the bit stream coming from the CD. I don't think that has changed. AirPlay 2, of course, is different, as the encoding can be lossy AAC or lossless 24-bit 48K ALAC, for which we don't have ready access to the source.

But there is more to it than that -- the audio may be processed afterwards by attenuators, mixers, ALSA plug-ins, sound servers like PipeWire, and maybe more besides, before the audio gets to the output.

If you don't use a hardware mixer / attenuator to control output volume, Shairport Sync will use its built-in software attenuator, which must alter the audio by the nature of its operation. If it actually does attenuate, it also adds dither. (The attenuator effectively uses 64-bit integer arithmetic to do attenuation and dither, BTW.) A hardware attenuator will also, obviously, modify the audio, and should also add dither. ALSA plug-ins also modify the audio.

To get a clean, unmodified stream of audio from Shairport Sync, set ignore_volume_control to "yes". This will bypass the built-in software attenuator and give you the original audio. To avoid any synchronisation-related interpolation, send the audio to the pipe or STDOUT backends.

[24fpsDaVinci]

i think what i was hearing was a difference between builds, I tested three different builds:

1.shairport-sync 4.3.7-1 package from the apt repository on raspbian (maintained by Chris Boot)
2.shairportsync-classic docker container
3.build from git (airplay 1)

all three sounds different from each other, maybe it's a user error on my part,
but 1. shairport-sync 4.3.7 package sounds the best and most correct to me with default config, and sounds more correct than raspbian's uxplay package

[mikebrady]

Thanks.

In principle, (1) and (2) above should be identical (Chris Boot's 4.3.7 package is Classic AirPlay), unless something different happens to the audio on the way out of both, or different mixers, etc.