True-HD/DTS-MA - Does Handbrake Transcode the Lossless Audio or the Lossy Core?

[jaloo^&7n]

For movies with a True-HD or DTS-MA track I usually transcode from the THD/DTS-MA track to AC-3 448kbps in order to save space and still preserve reasonable quality.

My question is, is Handbrake actually transcoding from the lossless output of the DTS-MA/THD track or is it transcoding from the lossy core of the track? I always select the full THD/DTS-MA track to transcode from, not the second track, which shows as 5.1. I presume that track is the core lossy track so I do not select it.


I'm using the latest Nightly build, v20170311203819-3abeb1-master(2017031201)-64bit

[Deleted User 11865]

TrueHD is decoded losslessly (except the Dolby Atmos extension, if present). For DTS-HD, the core is decoded -- HandBrake 1.0.x has partial support for the XLL HD extension, but I don't think it gets decoded 100% losslessly.

When using DTS-HD Passthru, the full track is passed through without modification (so, it's lossless).

[jaloo^&7n]

I see. So, when going from Dolby TrueHD to AC-3 @ 448kbps I'm starting with the lossless audio data. So, I'm not technically "transcoding" in that case as there would be no generational loss by reencoding a lossy track to another lossy codec.

However, when going from DTS-MA, I may actually be transcoding the lossy 1536kbps track to lossy AC-3 @ 448kbps. Does that sound correct?

Thanks for the quick response, by the way.

[Deleted User 11865]

I see. So, when going from Dolby TrueHD to AC-3 @ 448kbps I'm starting with the lossless audio data. So, I'm not technically "transcoding" in that case as there would be no generational loss by reencoding a lossy track to another lossy codec.

Erm, AC-3 is lossy, so there will be a loss. But you're starting with a lossless track, so that's only 1 "generation" of loss.

However, when going from DTS-MA, I may actually be transcoding the lossy 1536kbps track to lossy AC-3 @ 448kbps. Does that sound correct?

Yes, so in this case there are 2 "generations" of loss: studio master to lossy DTS, and then lossy DTS to AC-3.

[jaloo^&7n]

Quote: "Erm, AC-3 is lossy, so there will be a loss. But you're starting with a lossless track, so that's only 1 "generation" of loss."

Right, that's what I meant. My concern was doing two generations of lossy coding when I start with DTS-MA versus only one when starting with TrueHD. Given the space savings I think I can live with it with DTS source material.

Thanks, this was very helpful.

[rollin_eng]

Does TrueHD have an AC3 core? If so wouldn't it be better to just extract that?

[jaloo^&7n]

Does TrueHD have an AC3 core? If so wouldn't it be better to just extract that?

TrueHD stores two data streams. One is the TrueHD lossless data in full and the other is a "regular" AC-3 track, usually at 640kbps. The tracks are completely separate and usable on their own. So, when starting with a TrueHD track you should select the TrueHD track to transcode from not the lossy AC-3 track.

You could pass-thru the 640kbps lossy track but, if you're looking to reduce storage requirements by having a soundtrack at 448kbps or even 384kbps, it would be best from a sound quality perspective to start with the TrueHD track.

[rollin_eng]

If you are trying to save space AAC might give you better results if your player supports it.

[JohnAStebbins]

TrueHD stores two data streams. One is the TrueHD lossless data in full and the other is a "regular" AC-3 track, usually at 640kbps. The tracks are completely separate and usable on their own.

It's a little more convoluted than that. But you're approximately right. In BD transport streams (the most abundant source of TrueHD streams), the TrueHD and it's associated AC-3 core are identified together by a single PID. After collecting transport stream packets and assembling PES packets, you can extract AC-3 packets from the full TrueHD lossless stream by inspecting a stream id extension in the PES header. So they are fairly easily distinguishable, but not completely separate either. HandBrake presents 2 tracks whenever we see TrueHD in transport streams because we know how to separate them.

But TrueHD streams contained in matroska have had the TS and PES layers stripped. So the AC-3 core is no longer easily distinguishable in this case and HandBrake does not present a separate AC-3 track in this case.