HIFI4ALL Forum: Bitstream, TrueHD/DTS-HD, PCM

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DVD, HD-DVD, Blu-ray
HIFI4ALL Forum : DVD, HD-DVD, Blu-ray

Emne: Bitstream, TrueHD/DTS-HD, PCM (

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Bruce
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Bruger siden: 29 November 2003
Lokalitet: København
Status: Offline
Indlæg: 370

Sendt: 03 Juli 2007 kl. 12:50 \| IP-adresse registreret

I alle de foraer jeg kigger forbi, ser der ud til at herske tvivl om, hvorledes de nye lydformater TrueHD, DTS-HD, DD+ og DTS-HD MA bliver håndteret af henholdsvis afspillere og receivere. Jeg har fundet en tråd fra det amerikanske AVSForum, ved navn HDMI 1.1 X 1.3 for new HD AUdio Formats, omhandlende al forklaring om dette også i relation til HDMI standarderne.

Dette er den absolut bedste og enkleste forklaring af emnet jeg indtil videre er stødt på, som konkluderer, at INGEN afspillere, blu-ray eller HD-DVD, i dag er i stand til at sende et ukomprimeret TrueHD eller DTS-HD lydspor videre i bitstream format til en receiver. De nye Onkyo og Denon receivere, har altså endnu ikke mulighed for at bruge deres indbyggede dekodere, da ingen kan levere dem det nødvendige signal.

For de som ikke vil se i tråden gengives indlægget her i sin fulde ordlyd:

First some background. LPCM, often just referred to as PCM, can best be thought of as the simplest form of digital audio. There is one PCM stream for each intended speaker. The result of a studio mix for a movie is a set of PCM streams. Processing of digital audio in devices like receivers also involves manipulation of PCM.

Now PCM is not particularly compact, and there are details involved in keeping the separate PCM streams in sync, so packing formats were created. The packing formats combine the set of PCM streams together and "compress" them to produce a single, smaller, "bitstream".

Traditional Dolby Digital and DTS, as found on standard DVDs and in some HDTV broadcasts are "lossy" packing formats. Some audio quality is deliberately discarded in the packing process to get more compression. The PCM that comes out of decoding those is not the same as the PCM that went into the encoder in the studio.

The new Dolby TrueHD and DTS-HD MA formats, as found on HD-DVD and Blu-Ray discs, are "lossless" packing formats. The PCM that comes out of decoding them is bit for bit identical to the PCM that went into the encoder in the studio. As a result, these new formats can not produce as much compression. They take up more space on disc and a higher bit rate when reading the disc. Although that wouldn't work for standard DVDs, the new HD-DVD and Blu-Ray formats have enough capacity and bit rate to allow that.

But the packed audio formats have to be DECODED back into PCM before you can really do anything with the audio. For Dolby TrueHD and DTS-HD MA, the decoder could be in the player or it could be in the receiver. In either case, what comes out of the decoder is a set of PCM streams -- the IDENTICAL PCM that went into the encoder in the studio. The technology licensing from Dolby Labs and DTS insure this.

Now if the decoding is done in the player, you need to get the resulting set of high bandwidth, multi-channel PCM streams over to the receiver. To do that (digitally) requires HDMI V1.1 (or higher) at each end. It doesn't matter that both ends are at the same level of HDMI so long as each is at least HDMI V1.1. Now passing PCM this way is an *OPTIONAL* feature of HDMI V1.1 or higher. As it turns out, all the players do this, so the only thing you need to worry about is whether the RECEIVER you are looking at is also engineered to accept this style of PCM as digital audio input over HDMI, and whether the receiver "does the right thing" with that PCM once it arrives. Doing the right thing, here, means such stuff as doing speaker configuration management (steering bass from small speakers to your subwoofer for example), proper handling of the LFE channel, and any value added processing you would like such as THX post processing or the ability to take 2.0 or 5.1 channel input and manipulate it to produce 7.1 speaker output. See the "future proof" receiver sticky thread at the top of this forum for details.

It is just plain wrong to think of this as not being proper playback of the TrueHD or DTS-HD MA track. It is perfectly proper, and in fact it is how people are enjoying TrueHD today. [See below regarding DTS-HD MA.]

In the alternative, if the decoding is to be done in the RECEIVER, then you need to get the still-encoded "bitstream" -- the original, packed TrueHD or DTS-HD MA data -- over into the receiver. And THAT requires HDMI V1.3 or higher on both ends. But beware, that, once again, the ability to pass these new, "lossless" bitstreams is an *OPTIONAL* feature of HDMI V1.3. As it turns out, there are only a scant few players out there now that implement HDMI V1.3, and, as of today, *NONE OF THEM* actually implement passing these bitstreams to HDMI V1.3 receivers. This, of course, will change over time.

Once the bitstream gets into the receiver, and presuming the receiver has the right decoder, the bitstream will be decoded into PCM. This is THE SAME PCM as the player might produce. So there's really no difference. Either the player produces the PCM and transfers it to the receiver over HDMI (V1.1 or higher) or the player passes the bitstream to the receiver over HDMI (V1.3 of higher) and the receiver produces the IDENTICALLY SAME PCM.

Now there's one other gotcha here. HD-DVD today, and Blu-Ray later this year (player profile 1.1) support "in-player audio mixing". This is the ability of the player to read more than one audio stream off the disc and mix them together for playback. The studios making each disc get to decide whether or not to take advantage of this feature. Typically the feature would be used for sound effects in menus, overlayed producer commentary tracks, switching of languages by just swapping out the center speaker channel, and such like. Pretty much ALL HD-DVD discs are authored this way now and the assumption is that Blu-Ray discs will start being authored this way later this year when the new players are shipped that implement player profile 1.1.

And the gotcha is that in-player audio mixing can't happen unless THE PLAYER decodes the packed, lossless audio formats. Remember? Processing is done in PCM. So the player has to turn TrueHD or DTS-HD MA bitstreams into PCM before it can do the audio mixing. And then the result of that mixing, still as PCM, gets sent over HDMI to the receiver. So if you want these features of these discs to play correctly you HAVE TO get A PLAYER that has the audio decoders of interest to you.

Finally there is the issue of DTS-HD MA. No product has DTS-HD MA decoding today. No player, and no receiver. DTS-HD MA decoding will probably start showing up in products this Fall. It may show up in HDMI V1.3 receivers first, but players with the decoder will follow quite soon after. However, it is not clear whether any of the currently shipping players will ever be upgraded to ADD the missing DTS-HD MA decoder. They may expect you to buy a new player.

Dolby TrueHD decoding is standard in HD-DVD players. DTS-HD MA decoding is optional.

In Blu-Ray players, both TrueHD and DTS-HD MA decoding is optional. Some Blu-Ray discs include raw PCM tracks that are also high quality audio tracks and can best be thought of as "pre-decoded". You may be wondering why so many Blu-Ray discs include DTS-HD MA tracks if nobody can decode them yet. The answer is that there's a "core" subset of those tracks which can be easily extracted and passed along to receivers as a traditional, lossy, DTS bitstream. And that's how these players provide their "compatibility" digital audio track intended for use with older or less capable receivers -- e.g., over an optical digital audio cable.

Finally, all of the above was with respect to DIGITAL playback of these tracks. If the player happens to have a decoder for TrueHD for example, and ALSO has the ability to convert digital audio (PCM) to multi-channel analog audio, then the player can send multi-channel analog output to a receiver that has multi-channel analog inputs. But once again, notice that it is THE PLAYER doing the decoding that makes this work.

If all of this sounds confusing, well it is. These products are still in the "early adopter" stage, which means if you decide to buy now you will likely pay a premium price and will likely ALSO feel a strong need to replace your purchase with a new device over the next year. Blu-Ray customers, in particular, have the player profile 1.1 stuff coming up later this summer.

If you find a receiver you like and it happens to come with HDMI V1.3, then fine, but make sure it also accepts high bandwidth, multi-channel PCM over HDMI and does the right thing with it. If you find an HDMI V1.1 product you like and are worried that it is not HDMI V1.3, see the "why you don't need HDMI V1.3" sticky thread at the top of this forum.

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