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Discussion Starter · #1 ·
I'm just trying to understand the technical difference between uncompressed PCM and a Dolby TrueHD (and Master Audio) stream. TrueHD is a lossless compression meaning it bit-for-bit identical to how it was before compression. But if the signal is not being compressed in the first place (like a 5.1 or 7.1 PCM stream on HDMI output from a Bray or HDD source to a pre1.3 HDMI receiver), wouldn't it also be "bit-for-bit" identical? I think I'm missing something here. I haven't found anything that has a clear explanation on the differences. Can someone clear this up for me?
 

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Both will be identical to the master, however the TrueHD audio would take up less room on the HD-DVD/BluRay disc, leaving more room for picture quality, more soundtracks, or extra features.


The easiest analogy is to a Zip file on a PC. The files in it are exactly the same as they were before you added them to the Zip file, they just take up less space.
 

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TrueHD (I am guessing a bit on this number) : Maximum bit rate is 96khz/24 bits (8 channels = 18432kbits/s, or 18Mb/s)


PCM can support bitrates of up to 192khz and 24 bits of resolution. I am not sure whether it supports 192/24*, but it should certainly be able to support 96/24. So there should be no loss of resolution unless the player is downsampling to a lower rate/resolution.


* Quoting my HDMI document: "...HDMI can carry a single such stream at sample rates up to 192KHz or from two to four such streams (3 to 8 audio channels) at sample rates up to 96KHz."
 

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The packing formats pack up a PCM sound track into less space on disc. The encoded format needs to be unpacked back into PCM before you can do anything with it, and the PCM that comes out of the decoder is IDENTICAL to what went in.


Now PCM sound tracks can be produced at different quality levels. One aspect of quality is the "bandwidth" of the PCM itself -- sample rate and sample size. Higher bandwidth PCM takes more space and requires a higher bit transfer rate when reading the disc.


The packing formats give the studios the OPTION of shipping a higher bandwidth PCM track because the packed format will still fit on the disc (both in actual size and in bit rate when reading data off the disc).


However there is nothing magical about these packing formats which insures higher quality. And in fact, on many discs currently shipping, the PCM packed up inside TrueHD (or whatever) is NO DIFFERENT from what would have otherwise shipped as unencoded PCM. This is particularly obvious on Blue Ray discs which today ship with unencoded PCM tracks as well as whatever packed up format(s) they have.


If the studio makes a separate, higher bandwidth PCM sound track, and packs that up in TrueHD (or whatever), then you MIGHT hear an improvement. There are practical limits here, however. Beyond some level of sample rate and sample size the ear just can't hear the difference any more.


So you have to know what went into the TrueHD encoder in the first place to know if there is even a possibility that track might be better. And you have to realize that these tracks are pushing the limit on what you can do with audio, so the potential increase in bandwidth may not produce audible improvement.


Finally, if the TrueHD sound track originated as a new mix with better production values, THAT may be what you are actually hearing, rather than some technological improvement. I.e., you could get the same quality improvement if they shipped the same production quality of track mix as unpacked PCM.


TrueHD is not new. It has been around for years. It is what makes DVD-Audio work. They just came up with a marketing name for it.

--Bob
 

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Plus there's the issue of bandwidth; this was a primary driver for TrueHD when used on DVD-Audio (under the name MLP - Meridian Lossless Packing); the bandwidth had to stay below 9.6Mb/s. This could still be a problem on HD DVD - you don't want the audio eating into your video bandwidth.


Also, the compressed systems also have some extra features. For example:


1) TrueHD uses matrixed storage to allow 2.0, 5.1 and 7.1 mixes to be extracted without any need to process more channels than you require. A 2.0-channel downmix from 7.1 PCM would require all 8 channels to be processed and downmixed in the player, but with TrueHD you just decode the 2 already-downmixed channels.


2) TrueHD can use different sample rates and bitdepths for different channels, to save space; this isn't true of the PCM implementations on most formats (but I'm not totally sure if this is still true for HD DVD and BD).


3) TrueHD can store dynamic range control and dialogue normalisation parameters, as well as flags for Dolby Surround, Dolby EX and suchlike.


4) DTS HD Master Audio is based around a lossy DTS core; this means that simple players that don't have Master Audio decoders can extract that part of the HD-MA track to play the lossy sound, so there's no need for discs to provide separate lossy and lossless soundtracks.


There's probably more, but that's all I can think of.


However, the most important thing about these formats is that it gives Dolby and DTS an income stream. If everyone used PCM, like on CD, then there wouldn't be any money to be made
 

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Quote:
Originally Posted by KMO /forum/post/0


Plus there's the issue of bandwidth; this was a primary driver for TrueHD when used on DVD-Audio (under the name MLP - Meridian Lossless Packing); the bandwidth had to stay below 9.6Mb/s.

I think it might be the other way around. The maximum audio rate is determined by the video timings. Audio is allowed a higher bandwidth when video requires a higher bandwidth. I believe 1080p allows for an audio bandwidth of 196/24 or 36Mbits/s. I got this from reading the spec, it's possible I misunderstood it though.
 

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I think you're talking about HDMI there, unless I'm mistaken. HDMI does have that weird "higher video resolution allows higher audio resolution" feature; although it's not a hard limit - you can double, triple or quadruple the pixel clock to fit more, so you can have maximum res audio with 480i video if the equipment's properly designed.


I'm talking data read rate off the disc - there's a hard limit there because of the drive speed.
 

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Discussion Starter · #9 ·
I think it's starting to make sense but I still have a few questions (actually some new ones). So right now, current gen high def disc carry a "TrueHD" compressed codec along with uncompressed PCM codecs? Also what is the current requirement of HDD and Bray players in terms of carrying uncompressed PCM? I've heard that they have to at least carry 2.1 and some carry up to 5.1 (please correct me if I'm wrong). Lastly, when video bandwith usage starts becoming larger (deep color...etc) are they going to do away with the uncompressed PCM due to space constraints?
 

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HD DVD and Blu-ray don't do deep color. Maybe some future format will.


HD DVD and Blu-ray discs can carry a number of different audio soundtrack types. The choice will partly be determined by what players are required to support.


HD DVD players have to be able to decode TrueHD; Blu-ray players don't. So if TrueHD is used on a Blu-ray disc, some other alternative has to be provided for Blu-ray players that don't handle TrueHD. At the moment, as I understand it, some Blu-ray discs offer both TrueHD and PCM - this is almost totally pointless; they should have just stuck with PCM.


Audio tracks can be anything from 1.0 to 7.1 channels; the players have to be able to downmix where necessary.
 

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I saw some articles used the term "Lossless Uncompressed PCM".


Is this correct? Since there's no compression involved why use the word "Lossless"?


Also, what's the difference between lossy & lossless?
 

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lossy = some information is thrown away during compression. MP3 is lossy, FLAC is lossless. DD/DTS is lossy, TrueHD and MLP is lossless
 

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Ok noted. Thanks SiriuslyCold
 

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Quote:
Originally Posted by SiriuslyCold /forum/post/0


lossy = some information is thrown away during compression. MP3 is lossy, FLAC is lossless. DD/DTS is lossy, TrueHD and MLP is lossless

FINALLY someone is making the right comparison; FLAC and ALAC are PERFECT examples of what TrueHD should be viewed as, whereas WAVE and AIFF are more like uncompressed PCM. Then DD, DD+, DTS, etc, are more like mp3 and AAC. The principle difference being that these "movie audio" formats have a (potentially) much wider frequency response. Whether that's at all relevant is under debate, but I'd say most people do believe that inaudible frequencies are still important.
 

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Hi,


I'm also new to this sort of things and this thread was a great help.


I want to add some questions to better understand myself. Eventually, It might be safe I think to say that one format will go down, PCM vs TrueHD. I mean, they do take a lot of space on the medium and both distribute the same uncompress sound.


So my questions :


- If I buy a receiver, I can benefit right now from uncompress PCM going through an digital input without going through HDMI. But if a BD or HDDVD come up with no PCM track, but only TrueHD, I understarnd that those format will only pass in an HDMI right?


- Can a BD or HDDVD can treat internally an TrueHD to convert it to PCM so it can be output else than via HDMI?


- And for the newb questions, what's Pre and Post HDMI?


Thx!
 

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Without an HDMI 1.1 or greater path between player and receiver, you'll have to use the player's 5.1 analog outputs, or maybe the SPDIF digitial output (the kind we've been using for years) if the player will decode and then reencode using DTS or such. It's a lossy algorithm and only does 5.1 channels, but it might or might not work out better for your gear than the analog outs (and I doubt the players will have 7.1 analog outputs). Best you read the players' manuals in detail slowly and make sure you understand what they support and will actually do before buying.


If only getting PCM out an SPDIF output, it will be two channel only. Either downmixed from 5.1 to 2.0 using Pro Logic II or simply dropping all channels but FL/FR. Has to be compressed/encoded with DD or DTS to get multichannel digital over an SPDIF output.
 

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Quote:
Originally Posted by SMac770 /forum/post/0


Without an HDMI 1.1 or greater path between player and receiver, you'll have to use the player's 5.1 analog outputs, or maybe the SPDIF digitial output (the kind we've been using for years) if the player will decode and then reencode using DTS or such. It's a lossy algorithm and only does 5.1 channels, but it might or might not work out better for your gear than the analog outs (and I doubt the players will have 7.1 analog outputs). Best you read the players' manuals in detail slowly and make sure you understand what they support and will actually do before buying.


If only getting PCM out an SPDIF output, it will be two channel only. Either downmixed from 5.1 to 2.0 using Pro Logic II or simply dropping all channels but FL/FR. Has to be compressed/encoded with DD or DTS to get multichannel digital over an SPDIF output.

Nice answer. One other quick question. Will BD and HD DVD's continue to add a DD or DTS stream or eventually eliminate them all together. I am very new to the BD and HD DVD camps and just trying to learn as much as possible concerning the new audio codecs.
 

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Well, DD+ and DTS-HD HRA are the new gen lossy formats. It's almost entirely unlikely a HD-DVD or BD wouldn't have one of the two, and either are easily downconverted in the player to create a proper DD or DTS stream. There's really no reason DD or DTS should be on any next gen discs.


This is almost a year old, so not sure how correct it is today, nor how relevant it is to what publishers are actually sticking on next gen discs (I own no player of either disc format), but it's a start for you:

http://www.twice.com/article/CA6323699.html
 
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