How are direct play audio streams handled on the Apple TV?

Plex Players Streaming Devices
1

I’ve been converting all my MKV’s from DTS, TrueHD and AAC to AC3 because AC3 is the only format my soundbar can play (Sonos Beam). I noticed however then I forgot to convert a MKV to AC3 that is played just fine. The Plex Media Server transcodes the audio to FLAC. So, I just wanted to know how that works since I checked the Sonos app and saw that my soundbar receives a Dolby Digital 5.1 signal. I can imagine one of the following reasons this works:

  1. The Plex client transcodes FLAC to AC3.
  2. The Plex client passes FLAC to the Apple TV and the Apple TV transcodes it to AC3.
  3. (Unlikely) The Plex client passes FLAC to the Apple TV and the Apple TV passes it through to my TV, which then transcodes it to AC3.

Which of these options is the correct one? And to my final question. Is the on device transcoding better then converting non-AC3 formats to AC3 using ffmpeg beforehand like I’ve been doing?

2

You might want to check the audio settings of the Apple TV itself.
The Apple TV usually has 2 options to deal with audio – it’ll pass on the audio as PCM stream (simplified: decoded audio format) or convert it to AC3.

So my guess would be it’s #2 but I cannot tell for sure.

3

Thanks for you tip. I check the settings and saw that the Apple TV was set to PCM, which meant that my TV converted PCM to AC3.
I trust that my Apple TV will do a better job of transcoding audio than my TV does, so I set the TV to passthrough. This my soundbar only supports PCM Stereo and not multi-channel I changed the Apple TV to change to audio format to Dolby Digital 5.1.
Now Stereo content is send to the soundbar in PCM Stereo, which multi-channel is transcoded and send to the soundbar in Dolby Digital 5.1.

I will stop converting audio formats to AC3 and play them as is and let the Apple TV handle the conversion.

4

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