PCM vs Bitstream Atmos
Bitstream vs PCM For Dolby Atmos
The success of Dolby Atmos adds an important nuance to our PCM vs Bitstream discussion. Ordinary channel-based PCM does not contain Atmos object metadata, but Dolby MAT can carry Atmos metadata together with lossless PCM over HDMI. Therefore, the correct setting depends on how the source device outputs Atmos.
For a Blu-ray player sending Dolby TrueHD with Atmos, Bitstream or “Bitstream Unprocessed” is normally the correct choice so the receiver or soundbar can decode it. For a console, PC or streamer that uses Dolby MAT, Atmos may arrive as lossless PCM plus metadata instead.
For Blu-ray and Ultra HD Blu-ray, use Bitstream Unprocessed and disable secondary-audio mixing when you want the original Dolby TrueHD/Atmos stream decoded downstream. For streaming through a compatible TV, Auto or Pass-Through commonly preserves Dolby Digital Plus with Atmos.
Do not force a re-encoded Dolby or DTS mode unless compatibility requires it. Verify that the source, television, HDMI connection and receiver or soundbar all support the Atmos carrier being used.
PCM is not limited to only 2.0, 5.1 and 7.1 as a technology, and channel count alone does not determine whether Atmos survives. HDMI eARC can carry uncompressed multichannel PCM, but ordinary LPCM contains channels rather than Atmos object metadata. Dolby MAT is the exception because it adds the Atmos metadata required by the renderer.
A Dolby Surround upmixer can create height effects from channel-based PCM, but that is not the same as reproducing the original Atmos metadata. Native Atmos metadata can reach the decoder through a compatible Dolby Digital Plus or Dolby TrueHD bitstream, or through Dolby MAT.
Dolby Atmos combines audio objects, channel beds and positional metadata. For consumer delivery it can be carried by Dolby Digital Plus, Dolby TrueHD or Dolby MAT.
Atmos can add height and object placement when the content and playback system support it, but the label does not guarantee that one master or presentation will sound better than another.
PCM and Dolby Atmos are not mutually exclusive. PCM is a way of representing sampled digital audio, while Atmos is an immersive audio system that also requires positional metadata. Dolby MAT brings both together.
Dolby Atmos is not inherently a lossy codec. Atmos in Dolby Digital Plus is lossy; Atmos in Dolby TrueHD is lossless; and Dolby MAT carries Atmos metadata with lossless PCM audio over HDMI.