Bitstream vs PCM For Dolby Atmos
The success of Dolby Atmos forces us to deviate from my orthodox position in the PCM vs Bitstream discussion. One factor is that PCM cannot actually handle Atmos. The other factor is that almost all current consumer audio home systems, have a function “Auto”, so they will select automatically PCM or Bitstream according to the corresponding landscape architecture that the audiophile has at home.
Bitstream should be preferred over PCM for Dolby Atmos through the option “Bitstream” or “Auto” in modern consumer home audio systems. The raw, unprocessed Bitstream will be sent by the player, and the receiver will decode it.
In the selection Bitstream vs PCM for Dolby Atmos, Bitstream can play spatial sound like Atmos and PCM cannot. The raw unprocessed Bitstream will be sent by the player and the receiver will decode it.
For Dolby Atmos to be decoded by a TV, sound bar, or receiver, the source device (streaming device, Blu-ray player, or UHD player) must be set to send the bitstream output. When a device converts the audio to PCM, it must down mix it into channels.
If you have several versions of Bitstream (“Bitstream reencoded for Dolby” or for DTS) select the raw Bitstream, which will be “Bitstream” plain or “Bitstream Unprocessed”. Your receiver would be Dolby Atmos enabled and will do the decoding.
The only standards for PCM are 2.0, 5.1, or 7.1 channels. So if you only have a 7.1 or less system with only speakers around you, and no overheads, then PCM will probably sound the same. But anything more than 7.1 speakers around you, you will not get all of the benefits of Dolby Atmos through PCM. PCM will decode the 7.1 or 5.1 core only and bypass the Atmos encoding.
Some systems can activate the “Dolby Surround Upmixer” to then create overhead sounds, but these will not be the same as the content creator had intended. All of the extra data needed to produce the real-time panning and overhead sounds can only get to the decoder by sending the original bitstream
Dolby Atmos is metadata that rides on top of Dolby Digital Plus or Dolby TrueHD which provides the codes for height channels in a Dolby Atmos-compatible surround sound setup.
Dolby Digital provides sound from your existing speaker setup while Dolby Atmos uses the software as well as compatible hardware. This means that Dolby Atmos creates a superior sound experience than Dolby Digital because of the required hardware.
Just let me clarify that PCM and Dolby Atmos are two unrelated entities.
PCM is one way of digitally representing sampled analog signals. It is the decoded uncompressed form of most digital audio chains and systems, right before the DAC. Whatever processing has happened ultimately results in a PCM stream. A PCM stream can have as many discrete channels in it as required (typically from 1 to 10).
Dolby Atmos is a lossy audio codec, one of many there are. After the signal has been demuxed (so processed through a demultiplexer), after coming off the source, it is decoded, processed, and uncompressed and the result is a PCM stream that has the requisite number of channels in it, ready to be fed to the DAC.
I am Bob. I work as an audio engineer and audio technician. I work in mastering and arranging bridges in existing songs and the arrangement and orchestration of the chorus. In Planet HiFi I test gear for a couple of days and write a review. I also write about AV topics, amplifiers, speakers, and headphones.
