Think of it as a container with 16 slots. Any MLP soundtrack with 9-16 waveforms will be carried in the 16-channel container. The empty slots waste no data space due to lossless compression.Does "16-channel presentation" mean phisically 16 discrete waveforms? 7.1+8=16 MLP waveforms.
That's due to the statistics of sound allocation within the soundfield. The LCR channels are most heavily used, then the surrounds, and then the heights. It is often the case that sound exists in the base channels for long durations with no sound in the overheads. But rarely does the opposite happen. This means that the amount of information needed to carry a consumer Atmos mix is not going to double what it takes for a conventional 7.1 mix.The Atmos TrueHD should be double-sized comparing to "classic" 7.1 TrueHD. But it is not.
The 4th substream obviously carries audio. If not, then the following from the Dolby Atmos Production Suite Guide would not be true.Also I forget that mmfpeg ignores the 4th substream due to "4th substream does not include audio data". I suppose that mmfpeg team knows more about TrueHD structure. Something is wrong for me here.
Perhaps you are referring to this statement from ffmpeg:Dolby TrueHD: In this case, the spatially coded objects are losslessly delivered to
consumer playback devices. Typically, the Dolby TrueHD encoder creates a bitstream
containing the spatially coded objects, a 7.1-ch render of the objects, and 5.1-ch and 2-ch
downmixes. The 7.1, 5.1 and 2-ch presentations are backward-compatible with legacy
Dolby TrueHD decoders. A Dolby Atmos-capable Dolby TrueHD decoder losslessly reverses
the downmixes and render to recreate the original spatially coded objects. Dolby TrueHD
also supports independent 7.1, 5.1, and 2-channel presentations of 7.1.
That does not mean there's no audio present. It means they cannot decode it.The fourth substream is being discarded, since its not raw audio data,
but an encoded Atmos stream which needs a specialized decoder.