Hi,

For Airplay, it is the client player that typically enforces resample to 16/44100, but also an Airplay receiver (Apple TV, RaspyFi) may perform a second resampling to 16/48000

For example, lets use a 24/96 music file as our source and iTunes as our player. iTunes will first resample the file to 16/44100 prior to streaming to the Airplay receiver. Then here is what happens at the receiver end:

1. Apple Airport Express 16/44100 output (no resample)
2. Apple TV resample to 16/48000 output
3. RaspyFi resample to 16/48000 output

The worst cases are 2 and 3 where 24/96 resample to 16/44 in the player then resampled again to 16/48 in the receiver. This is not good! Thankfully there are some simple changes you can make in RaspyFi that prevent resampling when using Airplay.

Disabling alsa automatic sample rate conversion i.e., use hw instead of dmix

- sudo nano /etc/asound.conf
- pcm.!default {
type hw
card 0
device 0
}

- OR -

Changing the rate if using dmix automatic sample rate conversion

- sudo nano /usr/share/alsa/alsa.conf
- defaults.pcm.dmix.rate 44100 (default is 48000)

And for those who might want to change the Airplay name that shows up in your player.

- sudo nano /etc/init.d/shairport
- NAME="Airfi – Test"

The Airplay name change was just an extra I threw in. It has no effect whatsoever on audio

What model DAC are you using?

Possibly when disabling dmix the sound was distorted because no conversion to S32_LE was happening. Maybe its dmix that does the conversion to the byte format your DAC needs?

I think in my case, since HRT MSII supports both S16_LE and S24_3LE I can safely bypass dmix for 16/44 and 24/96 files. Just guessing.

Dump for HRT MSII DAC.
[email protected]:~$ cat /proc/asound/card0/stream0
http://www.hirestech.com 2013 REV 2.3 Music Streamer II at usb-bcm2708_usb-1.3, full s : USB Audio

Playback:
Status: Stop
Interface 1
Altset 1
Format: S24_3LE
Channels: 2
Endpoint: 1 OUT (ASYNC)
Rates: 96000, 88200, 48000, 44100, 32000
Interface 1
Altset 2
Format: S16_LE
Channels: 2
Endpoint: 1 OUT (ASYNC)
Rates: 96000, 88200, 48000, 44100, 32000

If dmix and alsa are like Windows audio engine, then as long as all DSP effects (bass boost, eq, preamp, etc) are off and source sample rate = mix output sample rate, then no DSP will occur on the samples other than volume and a final dithering that the mixer will perform.

Low volume when playing through Shairport may be due to how the player app volume and system (Master) volume are, or are not working together. What player app and OS are you using?