Determine ALAC or AAC used for stream

[moodeaudio]

Is there a way to determine whether ALAC lossless or AAC 256K is being used to encode the stream?
For example a tag in the metadata feed that indicates which codec is used.

[mikebrady]

Thanks for the question. Yes indeed, a metadata sdsc ("source description") message with a text payload is sent. The payload is one of the following:

ALAC/44100/S16_LE/2
ALAC/48000/S24_LE/2
AAC/44100/F24/2
AAC/48000/F24/2
AAC/48000/F24/5.1
AAC/48000/F24/7.1
None (0)

There is a similar odsc ("output description") code for the output format. This outputs the same kind of thing, but without the encoding, as it's always Linear integer PCM. Thus, for example, an odsc code could be 48000/S32_LE/2.

This is somewhat from memory, so you might need to check. (I must update the metadata reader example...)

[moodeaudio]

Fantastic :-)

I had a quick look at the reader source and there is a case statement for `sdsc'

          case 'sdsc':
            printf("Source Format \"%s\".\n", payload);
            break;

But it does not appear in the metadata pipe. Does it need to be added upstream in shairport-sync?

Here's some sample output.

pi@moode9:~ cat /tmp/shairport-sync-metadata | shairport-sync-metadata-reader

Metadata bundle "1102275542" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Metadata bundle "1102275542" end.
Pause. (AirPlay 2 only.)
Metadata bundle "1102243121" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Metadata bundle "1102243121" end.
Resume. (AirPlay 2 only.)
Play -- first frame received/time in ns: "".
Progress String "1094703693/1102247379/1112783414".
Resume.
Metadata bundle "1102247673" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "astc": 0x0001
"core" "caps": 0x02
Metadata bundle "1102247673" end.
Picture "1102247789" start.
Picture received, length 0 bytes.
Picture "1102247789" end.
Metadata bundle "1102248034" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Metadata bundle "1102248034" end.
Metadata bundle "1102248143" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Metadata bundle "1102248143" end.
Metadata bundle "1102248287" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Metadata bundle "1102248287" end.
Progress String "1094705144/1102248450/1112784865".
Metadata bundle "1102248569" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Metadata bundle "1102248569" end.
Metadata bundle "1102248696" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Metadata bundle "1102248696" end.
Picture "1102248840" start.
Picture received, length 83563 bytes.
Picture "1102248840" end.
Picture "1102250642" start.
Picture received, length 83563 bytes.
Picture "1102250642" end.
Progress String "1094705144/1102252544/1112784865".
Metadata bundle "1102252669" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "astn": 0x0001
"core" "astc": 0x0001
Song Data Kind ("asdk"): (possibly 0 == timed track, 1 == untimed stream): "0".
"core" "caps": 0x01
Track length: 409971 milliseconds.
Metadata bundle "1102252669" end.
Metadata bundle "1102532627" start.
Persistent ID: 0x5f775061ce2fd71c.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Winos In the Midst".
"core" "caps": 0x01
Track length: 409907 milliseconds.
Metadata bundle "1102532627" end.
Progress String "1094736573/1102532918/1112813471".
Pause. (AirPlay 2 only.)
Resume. (AirPlay 2 only.)
Play -- first frame received/time in ns: "".
Resume.
Picture "1837926327" start.
Picture received, length 83563 bytes.
Picture "1837926327" end.
Progress String "1837901411/1837929557/1850233402".
Metadata bundle "1837929683" start.
Persistent ID: 0x805f516427c215a3.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Innards".
"core" "astn": 0x0001
"core" "astc": 0x0000
Song Data Kind ("asdk"): (possibly 0 == timed track, 1 == untimed stream): "0".
"core" "caps": 0x01
Track length: 279637 milliseconds.
Metadata bundle "1837929683" end.
Metadata bundle "1837951390" start.
Persistent ID: 0x805f516427c215a3.
Album Name: "A Different Ball Game".
Artist: "Bullitnuts".
Composer: "".
Genre: "Dance".
Title: "Innards".
"core" "caps": 0x01
Metadata bundle "1837951390" end.
Progress String "1837948351/1837952365/1850280342".

[mikebrady]

Thanks. Are you using Shairport Sync 5?

[moodeaudio]

Using 4.3.6.

pi@moode:~ $ shairport-sync -V
4.3.6-AirPlay2-smi10-libdaemon-OpenSSL-Avahi-ALSA-jack-pa-dummy-stdout-pipe-soxr-convolution-metadata-mqtt-dbus-mpris-sysconfdir:/etc

[mikebrady]

Thanks. These metadata tokens are a new feature of version 5.

[moodeaudio]

Got it.

Thx and looking forward to v5 :-)

[moodeaudio]

Hi,

Starting to test v5 :-)

Using the Apple Music app on an iMac (Ventura 13.7.8) I'm playing a track listed as "Lossless 24/96K ALAC" with Audio qual stream settings "HiRes ALAC up to 24/192" but seeing Source Format "AAC/44100/F24/2". in the metadata feed

I'm not sure what process is used by the Apple Music app to determine the format for an AirPlay stream but it looks like some transcoding might be happening.

Metadata feed (some parts removed for brevity)

pi@test:~ $ sudo cat /tmp/shairport-sync-metadata | shairport-sync-metadata-reader

Enter Active State.
Play Session Begin.
Volume: "-13.66,-25.01,-96.30,0.00".
Resume. (AirPlay 2 only.)
Picture "2621507220" start.
Picture received, length 91226 bytes.
Picture "2621507220" end.
Progress String "2621469331/2621510158/2639639280".

Metadata bundle "2621510251" start.
Persistent ID: 0x807e4e1b54da6ec6.
Album Name: "BaRcoDe".
Artist: "Ben Wendel".
Composer: "Ben Wendel".
Genre: "Contemporary Jazz".
Title: "Clouds".
"core" "astn": 0x0001
"core" "astc": 0x0001
Song Data Kind ("asdk"): (possibly 0 == timed track, 1 == untimed stream): "0".
"core" "caps": 0x02
Track length: 412017 milliseconds.
Metadata bundle "2621510251" end.

Resume.
Play -- first frame received/time in ns: "2621529209/3084004666942".
Stream type: "Buffered".
Source Format "AAC/44100/F24/2".
Output Format "44100/S16_LE/2".

Metadata bundle "2621531455" start.
Persistent ID: 0x807e4e1b54da6ec6.
Album Name: "BaRcoDe".
Artist: "Ben Wendel".
Composer: "Ben Wendel".
Genre: "Contemporary Jazz".
Title: "Clouds".
"core" "caps": 0x01
Metadata bundle "2621531455" end.
Progress String "2621529211/2621534601/2639699160".
"ssnc" "dapo": 0x0000C48D
"ssnc" "dapo": 0x0000C48D
Pause. (AirPlay 2 only.)
Play Session End.
The AirPlay client at "fdc7:b725:29a3:40d9:4d3:adcc:88a3:4626" has disconnected from this player. (AirPlay 2 only.)
Exit Active State.

Some screen shots

[mikebrady]

Hi Tim. Thanks for the post.

If the source device is an iPhone, iPad or macOS, then Buffered Audio is 48K on recent OSes, 44.1k on older OSes, 24-bit floating point ("F24"), stereo, AAC, irrespective of what Apple Music says.

If it's Realtime Audio (e.g. from free-tier Spotify and from macOS "System Audio"), it's 44.1K, 16-bit signed integer ("S16") stereo ALAC, as before. (BTW, this means that Shairport Sync will switch output rates going between Buffered Audio and Realtime Audio if allowed.)

If the source is a HomePod mini or an Apple TV4k (and possibly other similar devices) and if you have enabled lossless reception, then Shairport Sync will receive 48k 24-bit signed integer ("S24") ALAC, i.e. lossless audio. We don't know how to decipher higher resolutions. On the same devices, if audio is encoded in Dolby Atmos, Shairport Sync will decode it as F24 AAC Surround Sound 5.1 or 7.1, depending on what the source is. It does not see the Atmos additions. You can disable Surround Sound on Shairport Sync if you wish.

Encoding (AAC/ALAC/F24/S24/S16/Stereo/Surround Sound) is dynamic, in that the feed can switch from lossy to lossless dynamically.

Hope this helps!

[mikebrady]

Just noticing that your macOS is quite a bit back, and it's supplying Buffered Audio at 44.1k AAC rather than 48k AAC that more recent versions use...

[moodeaudio]

Yes the Mac is a bit dated but still working hassle-free, and its good to have a "legacy" machine for testing ;-)

Thats super interesting about how the different audio formats are handled. It's a lot of usage scenarios that are being considered. I'll need to create a help file for this!

I totally forgot about AirPlay within the Apple Music app vs MacOS "system" Airplay. Here's a test using system AirPlay. The screenshot is final moOde WebUI rendering.

Metadata pipe shows:

Stream type: "Realtime".
Source Format "ALAC/44100/S16_LE/2".