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Is Anything Above DTS 768 Useful?

To this point A/V audio formats haven't really mattered to me (put the receiver on direct, tell the HTPC it supports everything, ignore). However, I have started ripping discs and I am staggered by the size of the audio track on a Blu Ray! Doing the math from red book bit rates, it seems that properly encoded 7.1 should never require more than 617 kbps (assuming that the .1 channel is sampled at 200hz and hence virtually irrelevant). I have at least one file approaching 10 mbps just for audio! That is a huge amount of storage being consumed by nonsense. There doesn't seem to be any rhyme or reason to it, since in the case above the sample rate is only 48khz and I don't think that 24-bit audio explains such a high rate

My question is twofold: Ignoring object-based audio (which my receiver doesn't support), is there any argument for these higher bit rate tracks? If not, is there any way to bring these back down to the real world? I would like to keep them in a form that my receiver can decode so that my whiddle passively cooled HTPC has one less thing to worry about.

Thanks!

Re: Is Anything Above DTS 768 Useful?

Reply #1
My suggestion to you: extract the core DTS (it's at 1510 kbps) from DTS-HD Master Audio. It's quite likely excessive but it saves you from re-encoding. And your receiver can decode it with no problem. I'm not sure that for Dolby TrueHD discs always include a regular Dolby Digital track but they usually do. In that case go with that.

If you wish to re-encode, I remember reading that AC3 at 640kbps was considered transparent (maybe it's in the Wiki here, I'm sure I read it somewhere).
Edit: by further searching, my memory might be serving me badly. Check this: https://tech.ebu.ch/docs/tech/tech3324.pdf .

It was linked from this discussion: https://hydrogenaud.io/index.php/topic,65936.0.html

Re: Is Anything Above DTS 768 Useful?

Reply #2
That thread and document are interesting, I want to take some time to delve into them further. Anyway, thank you for confirming my thoughts. I was hoping to get away without needing to re-rip the discs I had already done, but it's only around a dozen. That's hardly the end of the world.

Re: Is Anything Above DTS 768 Useful?

Reply #3
Pretty sure on discs with Dolby TrueHD that they have to include a standard DD track even if it's not a core track like they have with DTS-MA HD

Re: Is Anything Above DTS 768 Useful?

Reply #4
Quote
(assuming that the .1 channel is sampled at 200hz and hence virtually irrelevant).
All channels share the same sample rate (48kHz).   And  uncompressed bass-only file (or a silent file) takes the same amount of space as a full-range file.

Quote
I have at least one file approaching 10 mbps just for audio!
That's about right for an uncompressed 7.1 channel 24-bit file...  
24 bits x 8 channels x 48k samples-per-second = 9216 kbps = 9.2 mbps.


Re: Is Anything Above DTS 768 Useful?

Reply #5
Quote
(assuming that the .1 channel is sampled at 200hz and hence virtually irrelevant).
All channels share the same sample rate (48kHz).   And  uncompressed bass-only file (or a silent file) takes the same amount of space as a full-range file.

That seems very wasteful, given that the .1 channel shouldn't be handling anything over 120 hz.

Quote
I have at least one file approaching 10 mbps just for audio!
That's about right for an uncompressed 7.1 channel 24-bit file...  
24 bits x 8 channels x 48k samples-per-second = 9216 kbps = 9.2 mbps.
24 bits is 3 bytes, so it should be:

     3 x 8 x 48 = 1152 Kbps = 1.15 Mbps

If the math worked as you describe a CD would be:

     16 x 2 x 44.1 = 1411Kbps = 1.4Mbps

A CD would therefore only be able to hold about 9 minutes of music. If these audio files are genuinely sampling 24 bytes then their dynamic range probably exceeds that of any physical material that could ever exist.

EDIT: I must eat some crow here. I just went back to the file in question and I had falsely presumed that the values were in bytes not bits. It is indeed 9216 kbits p/s as you say, which is not nearly as disastrous.

Re: Is Anything Above DTS 768 Useful?

Reply #6
To update the practical side of this, turning off DTS-HD MA seems to be worth 2-3 GB depending on the movie. That's pretty significant. It does sometimes mean losing 7.1 (and of course DTS:X) but if I ever put in a 7.1 system I am happy to rerip that handful of discs.

Re: Is Anything Above DTS 768 Useful?

Reply #7
On the production end there is no set in stone standard for channel assignment so the LFE might end up on any of the channels. So while it seems wasteful, HD video alone chews through 1.5 gigabits/second so 1.1 megabits/second for an audio channel is a drop in the bucket.


Re: Is Anything Above DTS 768 Useful?

Reply #8
Quote
(assuming that the .1 channel is sampled at 200hz and hence virtually irrelevant).
All channels share the same sample rate (48kHz).   And  uncompressed bass-only file (or a silent file) takes the same amount of space as a full-range file.

That seems very wasteful, given that the .1 channel shouldn't be handling anything over 120 hz.

Quote
I have at least one file approaching 10 mbps just for audio!
That's about right for an uncompressed 7.1 channel 24-bit file...  
24 bits x 8 channels x 48k samples-per-second = 9216 kbps = 9.2 mbps.
24 bits is 3 bytes, so it should be:

     3 x 8 x 48 = 1152 Kbps = 1.15 Mbps

If the math worked as you describe a CD would be:

     16 x 2 x 44.1 = 1411Kbps = 1.4Mbps

A CD would therefore only be able to hold about 9 minutes of music. If these audio files are genuinely sampling 24 bytes then their dynamic range probably exceeds that of any physical material that could ever exist.

EDIT: I must eat some crow here. I just went back to the file in question and I had falsely presumed that the values were in bytes not bits. It is indeed 9216 kbits p/s as you say, which is not nearly as disastrous.

For future reference, lower-case b usually means bits and capital "B" means bytes. Bitrate in A/V is almost always given and measured in bits per seconds.

But also, the bitrates you quoted are for uncompressed audio. Lossless bluray audio formats are variable bitrate compressed, so the average bitrate should be lower than that. Where did you get that number? The program showing it might be showing the decoded audio bitrate, which is different from the size of what's encoded in the file.

To your general point, I agree though. The bluray lossless audio formats are a bit of a waste of space, and it's a shame that they became the standard for bluray, and now UHD. The "high res" lossy formats like DTS-HD HR and DD+ also aren't ideal, cause they're constant bitrate, so not the most efficient out there. Dunno if there's a technical reason or if it's just more audiophoolia fueled marketing. In any case, very few blurays use them. DTS-HD HR seems to be a non-starter for streaming since apparently its minimum bitrate is 3Mbps, but DD+ is widely used by Netflix and other streaming/downloading services, at a lower bitrate (I think 640kbps maybe), and Netflix is even piggybacking Dolby Atmos on top of it.

Also, bear in mind that legacy DTS can only do up to 6.1 discrete channels (DTS-ES), and I think the majority of 7.1 DTS-HD have a core of 5.1 only. I've seen one or two DTS-ES 6.1 cores, but IIRC the DTS-HD MA track was also 6.1. That said I don't think the HD has to be 6.1 for the core to also be 6.1, I don't see why that would be the case.

Re: Is Anything Above DTS 768 Useful?

Reply #9
But also, the bitrates you quoted are for uncompressed audio. Lossless bluray audio formats are variable bitrate compressed, so the average bitrate should be lower than that.
Audio tracks on BD (Blu-ray Disc) can also be PCM. Of course that is very space consuming, especially with 24bits and/or 96kHz.
In theory, there is no difference between theory and practice. In practice there is.

Re: Is Anything Above DTS 768 Useful?

Reply #10
That number comes from the details as listed in MPC.

Audio: DTS 48000Hz 8ch 9216kbps [A: BRrip(dts-hd ma, 48000 Hz, 7.1, s24) [default]]

That said, if you subtract the listed video bitrate (10.8 Mb/s) from the listed overall bitrate (18.1 Mb/s) you only get 7.3 Mb/s so what those numbers mean does get a bit hazy.

So far every movie I have ripped has had a core DTS of 5.1. A few have had DTS-HD MA tracks of 7.1.

Putting Atmos into a streaming service before getting full quality into the service seems like...exactly what they would do. It seems odd though, how many people have Dolby Atmos setups? How many of them are going to want to watch those movies over Netflix?

Re: Is Anything Above DTS 768 Useful?

Reply #11
It seems that MPC is listing the uncompressed audio bitrate. I have seen that before in ffdshow as well for ordinary formats. 7.3/9.2 Mbit is rather low compression ratio. I suspect that DTS-MA might be less efficient because of the lossy core + correction approach. You might get better compression with a FLAC or WavPack, which can compress the bass channel well if it only contains low frequencies or silence, but, as said above, regular DTS core is probably the best choice for its compatibility and relatively small size.

Re: Is Anything Above DTS 768 Useful?

Reply #12
It seems that MPC is listing the uncompressed audio bitrate. I have seen that before in ffdshow as well for ordinary formats. 7.3/9.2 Mbit is rather low compression ratio. I suspect that DTS-MA might be less efficient because of the lossy core + correction approach. You might get better compression with a FLAC or WavPack, which can compress the bass channel well if it only contains low frequencies or silence, but, as said above, regular DTS core is probably the best choice for its compatibility and relatively small size.

I just wrote something silly, nvm. Deleted >_>

Re: Is Anything Above DTS 768 Useful?

Reply #13
We know that a DTS-HD MA channel is built by upsampling the core channel, nulling it with the master and storing the result in a lossless file. The core track is 1536 kbps lossy meaning that each channel is:

1536 kbps / 6 channels = 256 kbps

On top of that there are two new lossless channels. We don't know exactly how these things are encoded, but maybe we can take a stab at it.

Presumably the extra 7.1 channels will never be  more than 256 kbps larger on average than the corrected channels (then it would be more efficient to do the lossy malarkey). So to start with lets tack on another 512 to our starting point to get 2048 kbps. That means we need to account for 5.2 mbps remaining in that file. The makes each lossless track:

5.2 mbps / 8 channels = 650 kbps

The standalone tracks by my ballpark logic would be 906 kbps (adding that 256 I removed earlier), compared to 1152 kbps for uncompressed 24 bit 48k audio. FLAC is not shaking in its boots. I don't know how much of that is consumed by DTS-X data and PGS subtitles (or even if the subtitles are part of that count), but even if we lop off an entire mbps for those things we still get 781 kbps for a file which I would expect to contain very little dynamic range on average.

These are all armchair ballpark guesstimates, but that bitrate has to be going SOMEWHERE...I'm going to leave my files in DTS, but I want to try converting one to FLAC. The size difference would be interesting reading.

EDIT: Handbrake won't do FLAC above 5.1, I will have to rip something else with 5.1 DTS-HD MA. That is for tomorrow.

 

Re: Is Anything Above DTS 768 Useful?

Reply #14
Can't you just divide the audio track size by its length to get a fairly accurate average bitrate?

Also, I don't know if tools like mediainfo give you the real bitrate of the track, but I do know LAV filters does list the uncompressed audio bitrate for the audio track, which is probably what you're using with MPC-HC?

 
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