Dolby Digital EX - a technical question

[deltacon]

Hi,

We're doing a research on Dolby Digital EX concerning the position of the flag, that indicates Dolby Digital EX.
I downloaded a documentation (Document 52 fromj ATSC) about the AC-3 Stream and analysed it.

Now I have a few questions:
I still wrote to Dolby and they told me, that the process is quite simple, but they didn't tell me any more!
Maybe anyone can help me!?

One frame in the AC-3 Bitstream consists of one Sync-Info-Header, one
Bit-Stream-Identification, 6 Audio-Blocks, one Auxilary Block and a
CRC Check. Normally, the flag is to be found in the
Bit-Stream-Identification and is set within 3 bits. My question is, if
the Information (the Flag) about Dolby Digital EX is positioned in
this Bit-Stream-Identification, or anywhere else!?

I still heard, that the Flag is set in the Timecode, is this true?
Many thanks for helping me!

Regards, help needed
B. Dengler

[1138]

Welcome to the forum deltacon!

Wow, that's a little more techincal than most.

I assume (from what you've listed here) that EX information would be present in the "Bit-Stream-Identification". Why esle call it "identification"? Could you provide a link to where you got this info from?

On a side note: one sync and crc for every frame? Makes one think that "jitter" ain't much of a problem these days.

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1138 - DVD File Forum Greeter/Moderator
The defense department regrets to inform you that your sons are dead because they were stupid.

[deltacon]

well, I got this information directly from the Dolby Homepage, respective the ATSC Homepage (www.atsc.org). It's called "Document 52".

Any other suggestions, where I can find another contact?

regards
bernie

[storm117]

FYI, there are two CRC words/frame in the AC-3 frame structure. Beginning and end.

In the AC-3 block structure there are block switch and dither flags. As far as I know, the bit stream config. would show only the number of coded channels and would be located in the BSI field. Perhaps looking in the unpack data or pseudocode could reveal more information.

Hope that helps ya
Peace,
Nathan

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KSUPC: the sky is our playground
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[1138]

Impressive. Most impressive.

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1138 - DVD File Forum Greeter/Moderator
Someday, I will be the most powerful Jedi ever.

[deltacon]

Yes, storm117, you're right, but there are not only two words of error check, there are 16 words in the Header and 17 words in the CRC Block at the end. But that's not what I'm looking for!!

In the BSI there is normally the flag, that tells the decoder, whether it's a Dolby Digital, Dolby Pro Logic, 2.0 Stereo....but there is no place any more for another flag that consists information about Dolby Digital EX, so the ingenieurs of Dolby must have placed it anywhere else, but where is this??

regards
Bernie

[tcjennings]

Dolby Digital EX? Is that the 6th channel at center rear, or are you talking about something else?

If DDEX is the 6th channel, then there isn't going to be a flag in the bitstream, it's extrapolated by the receiver just like ProLogic.

[deltacon]

Yes, it's about the enhanced center rear channel.

I know, that the encoding is the same as Pro Logic, but a Receiver, that's able to decode DD EX must know, that there is an EX Signal coming and not a 5.1 source. For this, there are some flags in the bitstream, but where those flags are normally, there is no place anymore!

But the flag for DD EX exists AND is in the Bitstream (in the Dolby specification of EX), tcjennings, so your statement isn't true!
So there must be a new place, because if there were no Dolby EX Flags, you would have to switch your receiver manually to EX decoding every time you got 6.1 material.

Any experts here, or only "normal" customers?

regards
Bernie

[tcjennings]

Now, I admit I'm not an electrical engineer or qualified expert, but my understanding of DDEX is that the 6th channel is not a discrete channel, but a phantom channel matrixed into the 2 rear channels. Because of that, I don't believe that the bitstream itself has to describe the 6th channel. The stream is still a 5.1 stream because the 6th channel is not discrete.

1999 saw the introduction of Dolby Digital Surround EX® (known in the home as THX Surround EX® as it is licensed for domestic use by the THX division of Lucasfilm Inc.) THX Surround EX adds a monaural surround back channel to the usual complement of five full-range and one bandwidth limited channels of a 5.1 system. This is achieved by matrix encoding the surround back (SB) channel into the two existing discrete surround channels, surround left (SL) and surround right (SR). A decode matrix then extracts the channel upon playback via a SR + SL = SB sum (similar to the way in which a Pro-logic® system extracts a centre channel) and conveys it to two loudspeakers.

From http://www.smr-home-theatre.org/myths/surroundex.html

So, there's no flag in the bitstream because the bitstream is still a 5.1 bitstream. My question to you is where did you learn the opposite of what I'm reading -- that there IS a flag in the bitstream that distinguishes between DD and DDEX?

[This message has been edited by tcjennings (edited 11-22-2001).]

[deltacon]

Hi TC!

Your understanding of DD EX is quite right, but I never said, that the 6th channel is a discrete channel or has to be described as a discrete one.
I got my information from Dolby, ATSC(specifies AC3) and practical research (we still have the possibility here to encode DD EX).
The bitstream doesn't have to describe the enhanced channel, but it must tell the receiver to switch to 6.1 decoding. This is done by a flag in the bitstream. And this flag I'm talking about!
Because the receiver doesn't know from itself, if the rear channel is matrix encoded, anybody must tell him "Hey! You have to matrix-decode the rear channels!".

If this wouldn't happen, the customer would have to switch to 6.1 decoding every time he wants to listen to 6.1 material.

regards, Bernie

[Tyler Durden]

Hi Bernie

I should have a definitive answer for you on this by Monday. However, Dts is the only 6.1 format that features a discrete centre back channel - Dolby's is matrixed and so when a user wishes to listen to 6.1 they do indeed have to manually select this option on their processor - to get round this, may new amps/processors that decode DDEX simply default to this setting - as is the case with Lexicon.

Cheers

T

[deltacon]

Hi Tyler!

You mean, the new receivers are default set to 6.1 decoding, except they get another information from the source (I can't imagine that)?

But if you take a new receiver (who detects, or should detect, automatically DD EX decoding) and you give it a 6.1 input, why does it switch to 6.1 decoding?
In the AC3 stream there must be the information for switching to 6.1, but where is this information??? I guess in the BSI, but I'm not sure!

I know, that DTS ES Discrete 6.1 is the only 7 channel discrete format, but there are too some matrixed formats by DTS, e.g. DTS Neo:6 or DTS Compatible 6.1 (which is based on the same encoding as DD EX)

regards
Bernie

[This message has been edited by deltacon (edited 11-23-2001).]

[deltacon]

Hi!

Got an answer from Dolby today. As I thought, the information (the flag) is in the BSI and the AC-3 specification was enhanced with some facilities for DD EX.

see you
Bernie