Atmos vs 5.1

[Owen Smith]

From what I understand the substreams build off of each other. They are not individual mixes, but each substream is correction data to morph the previous substream into a new channel layout.

That is how DTS works, it is core plus extensions. So you always get a DTS 5.1 core at 1.5 megabits, and then extensions for 24/96, or ES 6.1 discrete, or HD MA lossless at either 5.1 or 7.1.

However that is not how True HD works. The AC3 5.1 or stereo substream is completely independent of the MLP 5.1 or 7.1 or whatever it is in. MLP is a totally standalone codec developed for DVD-Audio and has no relationship to AC3 and does not build on it. For example, with DTS HD MA to go from 7.1 to 5.1 the rear channels are discarded because everything has to be in the 5.1 core which includes the surrounds. Whereas on True HD to go from 7.1 to 5.1 the rears and surrounds are summed because True HD 7.1 is genuinely 7.1 independent channels.

Now it is true that with Atmos, as with DTS:X, the 5.1 or 7.1 bed contains all the audio and when played on a player that decodes the height channels they are subtracted from the 5.1 or 7.1 bed.

 

[SeeMoreDigital]

I don't. This all started for me trying to understand when the AC3 would be used.

The lossy 5.1 Dolby Digital (or lossy DTS) audio stream is included for people who don't have AVR's that can decode lossless audio streams. It forms part of the original Blu-ray disc specification that dates back 20 years!

 

[Owen Smith]

Okay I've found a paper where Dolby is explicitly describing TrueHD in words and not code or specifications. Looks like my understanding was mostly correct.

Which says:

We chose to handle the lossy and lossless codecs independently

ie. the 5.1 AC3 substream is completely independent and the MLP substreams do not rely on it. They're taking a pointless dig at DTS here, which has a 1.5megabits DTS core which DTS HD MA uses to good effect. AC3 is so much crapper and lower bit rate there is no point trying to use it as a core, and MLP was developed independently anyway.

 

[Mr. Afternoon]

That is how DTS works, it is core plus extensions. So you always get a DTS 5.1 core at 1.5 megabits, and then extensions for 24/96, or ES 6.1 discrete, or HD MA lossless at either 5.1 or 7.1.

However that is not how True HD works. The AC3 5.1 or stereo substream is completely independent of the MLP 5.1 or 7.1 or whatever it is in. MLP is a totally standalone codec developed for DVD-Audio and has no relationship to AC3 and does not build on it. For example, with DTS HD MA to go from 7.1 to 5.1 the rear channels are discarded because everything has to be in the 5.1 core which includes the surrounds. Whereas on True HD to go from 7.1 to 5.1 the rears and surrounds are summed because True HD 7.1 is genuinely 7.1 independent channels.

Now it is true that with Atmos, as with DTS:X, the 5.1 or 7.1 bed contains all the audio and when played on a player that decodes the height channels they are subtracted from the 5.1 or 7.1 bed.

I am not saying TrueHD builds on AC3. I have already stated the AC3 stream and the TrueHD stream are separate.

I am saying that the way TrueHD stores its lossless channels is similar to the "extension" concept that DTS uses. A stereo downmix is stored in the first substream and corrective data to unfold that downmix to a larger amount of channels is stored in each subsequent substream. Just as Atmos is decoded with the height channels/objects subtracted from the 7.1 bed, the 7.1 bed itself is decoded by subtracting the rears from the 5.1 bed, which is decoded by subtracting the rears and the center from the stereo downmix.

 

[Owen Smith]

I am saying that the way TrueHD stores its lossless channels is similar to the "extension" concept that DTS uses. A downmix is stored in the first substream and corrective data to unfold that downmix to a larger amount of channels is stored in each subsequent substream. Just as Atmos is decoded with the height channels/objects subtracted from the 7.1 bed, the 7.1 bed itself is decoded by subtracting the rears from the 5.1 bed, which is decoded by subtracting the rears and the center from the stereo downmix.

I agree but I'm not sure it's stored in the substreams as there aren't enough of them. One substream for AC3 and one for Atmos metadata leaves two at most, and for stereo plus upgrade to 5.1 plus upgrade to 7.1 we'd need three. I think the heirachy being described here lives entirely in the 7.1 True HD substream.

 

[Mr. Afternoon]

I agree but I'm not sure it's stored in the substreams as there aren't enough of them. One substream for AC3 and one for Atmos metadata leaves two at most, and for stereo plus upgrade to 5.1 plus upgrade to 7.1 we'd need three. I think the heirachy being described here lives entirely in the 7.1 True HD substream.

Actually the AC3 core is not stored in the substreams at all. The substreams I refer to are stored inside the TrueHD data, which as far as I'm aware is tacked on as extension data to an AC3 core. TrueHD itself has 4 substreams as of now, one for stereo, one for 5.1 unfolding, one for 7.1 unfolding, and one for Atmos unfolding.

This is how a player can choose to discard TrueHD and play only the AC3 core if it doesn't understand it.

 

[Owen Smith]

Actually the AC3 core is not stored in the substreams at all. The substreams I refer to are stored inside the TrueHD data, which as far as I'm aware is tacked on as extension data to an AC3 core. TrueHD itself has 4 substreams as of now, one for stereo, one for 5.1 unfolding, one for 7.1 unfolding, and one for Atmos unfolding.

This is how a player can choose to discard TrueHD and play only the AC3 core if it doesn't understand it.

That's also possible since the documents don't either explicitly include or exclude it. I assumed, possibly incorrectly, that the substreams were part of the AC3 format.