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Channel 70: Rear channel issues


Guest barracuda

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Guest barracuda

I hooked up my Humax HD 7000 via my Yamaha AV receiver yesterday to listen to the wonder of Dolby Digital 5.1 surround sound during the Channel 90 and Channel 70 HD loops. Channel 90 was great across all speakers, but Channel 70 tended to be experiencing issues on the rear speaker channels. It's hard to describe, but it sounded like ot was "missing" (like a dodgy spark plug lead) on both the left and right rear channels.

Swapping back to channel 90, all was fine.

Has anybody else experienced this on Channel 7 HD?

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This has been discussed plenty of times before. It's because Seven transmit their Dolby stream at 384kbits which isn't enough for 5.1

http://www.dtvforum.info/index.php?showtopic=9263

Umm, then why are many DD 5.1 DVDs only encoded at 384lbits which function perfectly ??

Is this some sort of technical implementation issue with the STB ?

I must admit I've never seen a 348kbits (5.1) DVD. Can you Email (or post) a title which I can check with a professional Dolby decoder.

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gepm, you are correct 348kb/s is not common, infact nonexistant, however 384kb/s is. Most DVD's in R4 are 384kb/s, although in the last year more R4 discs have used 448kb/s, probably 70% of new releases.

Examples of 384kb/s abound as stated above, but try Toy Story 2, Moulin Rouqe, Fight Club for a few prominent ones.

And AusZues, the STB only transports the DD stream, doesnt normally touch it (unless you get your STB to decode 2Ch of it), your 5.1 Amp decodes it. So answer is no the STB does not limit DD operation.

Regards

Cyril

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gepm, you are correct 348kb/s is not common, infact nonexistant, however 384kb/s is.  Most DVD's in R4 are 384kb/s, although in the last year more R4 discs have used 448kb/s, probably 70% of new releases.

Examples of 384kb/s abound as stated above, but try Toy Story 2, Moulin Rouqe, Fight Club for a few prominent ones.

And AusZues, the STB only transports the DD stream, doesnt normally touch it (unless you get your STB to decode 2Ch of it), your 5.1 Amp decodes it. So answer is no the STB does not limit DD operation.

Regards

Cyril

I hate it when the DVD makers drop the audio to 384kbits to add more special features :blink: I hate special features, I can't say I've ever watched them. Give me higher bitrates anyday.

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gepm, you are correct 348kb/s is not common, infact nonexistant, however 384kb/s is.  Most DVD's in R4 are 384kb/s, although in the last year more R4 discs have used 448kb/s, probably 70% of new releases.

Examples of 384kb/s abound as stated above, but try Toy Story 2, Moulin Rouqe, Fight Club for a few prominent ones.

And AusZues, the STB only transports the DD stream, doesnt normally touch it (unless you get your STB to decode 2Ch of it), your 5.1 Amp decodes it. So answer is no the STB does not limit DD operation.

Regards

Cyril

Too early in the morning. Sorry.

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And AusZues, the STB only transports the DD stream, doesnt normally touch it (unless you get your STB to decode 2Ch of it), your 5.1 Amp decodes it. So answer is no the STB does not limit DD operation.

So we are all in agreement that 384 as opposed to 448 is more than adequate for DD 5.1.

Thus, previous posters and the other referenced thread are incorrect when they say that 384 is not enough.

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And AusZues, the STB only transports the DD stream, doesnt normally touch it (unless you get your STB to decode 2Ch of it), your 5.1 Amp decodes it. So answer is no the STB does not limit DD operation.

So we are all in agreement that 384 as opposed to 448 is more than adequate for DD 5.1.

Thus, previous posters and the other referenced thread are incorrect when they say that 384 is not enough.

384kbits isn't enough. You lose frequency response and a lot of it.

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And AusZues, the STB only transports the DD stream, doesnt normally touch it (unless you get your STB to decode 2Ch of it), your 5.1 Amp decodes it. So answer is no the STB does not limit DD operation.

So we are all in agreement that 384 as opposed to 448 is more than adequate for DD 5.1.

Thus, previous posters and the other referenced thread are incorrect when they say that 384 is not enough.

No 384 is not enough. If you do have a DVD with 384 (5.1), I'll bet you the rear channels are not used much or at certain points/player combos, the audio will sound like Ch. 70. 384 is only used to get more space for extras.

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384 is only used to get more space for extras.

No, I do not accept that argument - there is more to it than that.

The size of many DVD's including extras come in well under the size available on a dual layer DVD. It is common for a DVD to be in the 6 - 7 Gig range with extras and a 384 DD audio - obviously a 448 DD soundtrack would fit with room to spare.

The choice for 384 instead of 448 is not made on size or capacity constraints.

With regards to comments regarding certain player combos I find it hard to understand your point. Are you referring to a DVD players ability to down convert the 5.1 DD to its analogue audio outputs ? If not, you do realise that in most applications the DD stream is output unchanged from tghe disc to the receiver and the sound decoding is a function of your receiver.

The 384 discs that I play sound perfectly fine. There is real no noticeable difference between 384 and 448 - I am feeding the digitial stream out to my receiver to decode.

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No 384 is not enough. If you do have a DVD with 384 (5.1), I'll bet you the rear channels are not used much or at certain points/player combos, the audio will sound like Ch. 70. 384 is only used to get more space for extras.

I think this will sum it up. Information provided by my AC3 decoder:

ABC 2/0 Rage

AC3
speakers:  2/0 (stereo)
sample rate: 48000Hz
bitrate: 448kbps
stream: 8 bit
frame size: 1792 bytes
nsamples: 1536
bsid: 8
clev: 0.0dB (1.0000)
slev: 0.0dB (1.0000)
dialnorm: -27dB
bandwidth: 20kHz/20kHz

Seven ED 3/2 + LFE

AC3
speakers:  3/2+LFE (5.1)
sample rate: 48000Hz
bitrate: 384kbps
stream: 8 bit
frame size: 1536 bytes
nsamples: 1536
bsid: 8
clev: -3.0dB (0.7071)
slev: -3.0dB (0.7071)
dialnorm: -27dB
bandwidth: 10kHz/18kHz

Nine HD Loop 3/2 + LFE

AC3
speakers:  3/2+LFE (5.1)
sample rate: 48000Hz
bitrate: 448kbps
stream: 8 bit
frame size: 1792 bytes
nsamples: 1536
bsid: 8
clev: -3.0dB (0.7071)
slev: -3.0dB (0.7071)
dialnorm: -27dB
bandwidth: 14kHz/20kHz

If I'm reading it properly, it looks like Seven ED only has upto 10khz for rear channels and 18khz for fronts. Humans can hear upto 20khz. Hope GEPM can explain it better than I can.

EDIT: Added more channels.

Nine Digital AC3 from the Gold Coast 2/0

AC3
speakers:  2/0 (stereo)
sample rate: 48000Hz
bitrate: 192kbps
stream: 8 bit
frame size: 768 bytes
nsamples: 1536
bsid: 8
clev: 0.0dB (1.0000)
slev: 0.0dB (1.0000)
dialnorm: -27dB
bandwidth: 10kHz/20kHz

Nine Digital AC3 Perth 2/0

AC3
speakers:  2/0 (stereo)
sample rate: 48000Hz
bitrate: 256kbps
stream: 8 bit
frame size: 1024 bytes
nsamples: 1536
bsid: 8
clev: 0.0dB (1.0000)
slev: 0.0dB (1.0000)
dialnorm: -27dB
bandwidth: 20kHz/20kHz

Nine HD showing regular programming Lt/Rt

AC3
speakers:  Dolby Surround
sample rate: 48000Hz
bitrate: 448kbps
stream: 8 bit
frame size: 1792 bytes
nsamples: 1536
bsid: 8
clev: 0.0dB (1.0000)
slev: 0.0dB (1.0000)
dialnorm: -27dB
bandwidth: 20kHz/20kHz

Edited by Champion_R
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No, I do not accept that argument - there is more to it than that. 

The size of many DVD's including extras come in well under the size available on a dual layer DVD.  It is common for a DVD to be in the 6 - 7 Gig range with extras and a 384 DD audio - obviously a 448 DD soundtrack would fit with room to spare.

The choice for 384 instead of 448 is not made on size or capacity constraints.

With regards to comments regarding certain player combos I find it hard to understand your point.  Are you referring to a DVD players ability to down convert the 5.1 DD to its analogue audio outputs ?  If not, you do realise that in most applications the DD stream is output unchanged from tghe disc to the receiver and the sound decoding is a function of your receiver.

The 384 discs that I play sound perfectly fine.  There is real no noticeable difference between 384 and 448 - I am feeding the digitial stream out to my receiver to decode.

I see your point for DVDs. Ch. 4+3 may be a different story. Also as Champion_R points out Ch. 70 band limits their audio to 15K front and 10K rear. No wonder it sounds crap in the rears.

As for DVD players they do have some control over the digital out. Pro-disc (local in Sydney) made a small test/sample DVD to give a way with their Pro-disc V-link 104 player.

Some of the AC3 5.1 audio was set to 640kbits. On certain DVD players the chip set will cut this out of the digital output. Also acording to Prod-disc the chip set has to have control of the digital audio stream as well. This is due to the de-coding and anti coping codes etc.

So you cannot to a 100% say any DVD palyer passes the AC3 to an external amp without some control. To that extant if they (the manufacture) has cheap chips or "what ever" in their circuts, who knows.

I can say I have a V-link 103 at home and a V-link 104 at work, and I have not found any DVD disc that will play up image or audio on these players. On other players you get what you get. The V-links play all AC3 audio streams perfectly.

I do have one set of DVDs that have slight pauses/interuptions at menu cahnge points but they play like that on most palyers. These are the full box set of Alien movies. When playing a movie from one of these DVDs, at the point where the original changes (compared) to the "directors cut", the player outputs pauses for about a second etc. This is the same wether playing the orig version or the directors cut. This is a result of poor authouring, and tis a pity for such a fine set of DVDs.

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I see your point for DVDs. Ch. 4+3 may be a different story. Also as Champion_R points out Ch. 70 band limits their audio to 15K front and 10K rear. No wonder it sounds crap in the rears.

I think the Dolby encoder limits the bandwidth because there isn't enough space to encode the full spectrum. I see the 18/10 on 384kbit DVDs. I did read somewhere that AC3 will drop higher frequencies first when there's a lack of data bandwidth which is what happens when 384kbits is used.

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Champion R,

Noticed ur bitrate info based on the Gold Coast, so there is actually "proper" 2ch AC3 on the HD channel there unlike Brisbane? (ie its alway retarded 5.1, thats "IF" ones receiver can even work out their mess)

384 seems fairly common on DVD's

Gladiator is another one from memory.

On the issue of frequency response loss etc, would not 384 only be a problem during more complicated sections where all channels are running with more complex information?

I'd image music to be the most complex type of sound to be encoded? Also music perhaps easier to pickup encoding issues (ie low bitrate MP3's)?

But on normal movies it is rare that music is from all channels, usually the mains. Effects etc, and often more suttle "indivdual" sounds are heard from the surrounds, id imagine such sounds could be encoded at quite low bit rate incomparison without issue?

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  • 2 weeks later...
Some of the AC3 5.1 audio was set to 640kbits. On certain DVD players the chip set will cut this out of the digital output. Also acording to Prod-disc the chip set has to have control of the digital audio stream as well. This is due to the de-coding and anti coping codes etc.

Just for your info. I have recently completed a chipset evaluation for an overseas company looking at six different DVB STB chipsets. After seeing the above comment I went back and looked in detail at the way these chip sets handle the DD digital output.

Essentially without exception none of the chip sets actually tamper with any of the data, they simply extract it from the stream and pipe it to the digital output. All had slide buffers to assist with resync to the PCR, and that was it. As gepm indicated, four of the chip sets would detect data rates above 488kb/s and mute data flow, but that's all. This would seem to be to protect older downstream decoders that may become confused by the higher rate, although the exact reason for this was not indicated by most of the data sheets/app notes provided.

I would expect DVD chip sets to behave in the same manner.

Regards

Cyril

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Essentially without exception none of the chip sets actually tamper with any of the data, they simply extract it from the stream and pipe it to the digital output. All had slide buffers to assist with resync to the PCR, and that was it. As gepm indicated, four of the chip sets would detect data rates above 488kb/s and mute data flow, but that's all. This would seem to be to protect older downstream decoders that may become confused by the higher rate, although the exact reason for this was not indicated by most of the data sheets/app notes provided.

I would expect DVD chip sets to behave in the same manner.

For DVD players would this only be relevant for AC3, as many DTS recordings have bit rates in excess of 640Kbits.

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AusZeus you are correct, the documentation seems to imply that the action is taken to protect the downstream decoder that may have limited ability. Obviously in the case of DTS a different set of rules apply, and DTS is allowed 1.5Mb/s.

Cyril

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