Apple TV 4K has 4 audio formats available:
- Dolby Atmos (in Dolby MAT), when available in the source (requires ‘Auto’ PCM Multichannel setting)
- PCM Multichannel (‘Auto’), decodes to uncompressed 7.1/5.1/2.0/1.0; same number of channels as source.
- Dolby Digital 5.1, fallback (conversion) option, for systems that can’t handle PCM Multichannel.
- PCM Stereo 2.0, fallback (conversion) option, for 2-speaker systems.
Apple TV should have the Audio Format setting set to ‘Auto’ (PCM Multichannel), when connected to a great receiver like yours. Apple TV is designed to default decompress Dolby Digital/Dolby Digital Plus to PCM 5.1/7.1. There is no passthrough option for Apple TV. You will not see a DD logo light up on the audio system.
PCM does not denote Stereo.
(‘Dolby Surround’ is an ambiguous term, and has seen a 2014 re-definition.)
What sources cause this Dolby Surround to 2.1 conversion? Does this happen for any particular app or service?