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Interesting superbit question.

Discussion in 'Movie Forum' started by mjn, Mar 19, 2002.

  1. mjn

    mjn
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    OK....follow me on this this.

    A superbit title has no extras on the disc which contains the film. All the space is used for the video and audio.

    If you go by the information in the case of a superbit title, around 66% of the disc is used by video and 33% is used by the DD5.1/DTS5.1. And there is no space free on the disc...

    Ok, all agreed so far.....

    So...say they give 90 minute film, and a 180 minute film the superbit treatment.

    Say both films had a 2.35:1 anamorphic picture, with the usual DD5.1 and DTS5.1.

    Will the film that is 90 mins, be encoded with a lesser compression ratio, to the 180 mins film??

    As if both films where encoded with the same compression ratio, either the 90 mins film would have free space on the disc, or the 180mins film would need to discs.

    Opinions please!!
     
  2. Jase

    Jase
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    I reckon the 180mins film would need the two discs. If they are being compressed the same way that is.

    They may be using different compression rates?? Dont know to be honest??

    Or its the high bit / low bit debate all over again!!
     
  3. Confucius

    Confucius
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    The max transfer rate of 10.08 Mbps for a DVD includes both DD (448 Kbps) and DTS (754.5 Kbps) in that figure, so in any eventuality the maximum picture transfer rate is 10322 - (448 + 754) = 9.57 Mbps.

    Now the capacity (7.95gig) of a RSDL disc is enough for 113 minutes at the max transfer rate, so any movie shorter than 113 minutes could in theory have a picture bit rate of 9.57 Mbps - the maximum possible allowing for the carriage of both DD & DTS sound. (I say in theory because we are ignoring the space taken up by menus etc).

    For films longer than 113 minutes the picture bit rate must be reduced and the sound bit rates stay the same. For a 180 minute movie the transfer rate would be 6.333 Mbps, taking away DD & DTS leaves us with a 5.16 Mbps transfer rate.

    As to how much of the potential bit rate is actually used on Superbit titles, I have no idea!
     
  4. slingshot

    slingshot
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    Interesting question, my Tosh DVD player has an option to display the transfer rate and while many DVD's appear to be around 6 or 7 for much of the film, the few extracts I watched of the superbit 5th element seemed to be pretty much at 10 meg all the time. I assume from the previous my player is giving me the overall transfer rate including sound.

    So I assume if I used this while watching some of my films I'd get some idea of how compressed the film is. Interesting thought are all my naff looking films due to compression ? After to try it out one night.

    Slingshot
     
  5. lmccauley

    lmccauley
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    I think you'll find that the original, R1 Fifth Element averages only about 5Mb/s - and that certainly doesn't have a naff picture. I think the compression rate is just one part of a chain of things that can affect the overall picture quality: the condition of the original source, the transfer process, the encoding process (not just the compression rate used), plus lots more that other, more knowledgable members of this forum could tell us about.

    If superbit is meant to maximise the video bitrate, why do they have 2 soundtracks using 1.2Mbps? Why not just use one or the other (with a low rate 2-channel DD if using DTS, to conform to the DVD standard) so that more bitrate is available for video?

    Cheers,
    Liam
     
  6. mjn

    mjn
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    its not just video bitrate they maximize, its the audio and video!!
     
  7. lechacal

    lechacal
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    I think the answer to that is that they want to include the DTS because of its perceived higher quality being in line with the values they want to project for the Superbit range. OTOH though, the DVD Video standard says that a disc MUST have a DD or a linear PCM (I think) track on it to conform.
    That's probably why there are two soundtracks.
     
  8. lmccauley

    lmccauley
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    mjn - but you only ever listen to one track (except for people like us on this forum that will probably do a comparison of the DD & DTS tracks). Go for one or the other, but not both.

    lechacal - I agree with what you say about the perceived higher quality (nicely phrased - I glad we haven't got into the DD/DTS quality debate ;)) being in line with the values they want to project for the Superbit range, but the DD track can just be low-rate 2 channel to conform to the DVD standard.

    Cheers,
    Liam
     
  9. mjn

    mjn
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    i'm sorry, i don't get your point?!!?
     
  10. JohnAd

    JohnAd
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    Any random noise be it film grain, specks on the film, brightness changes frame to frame is very hard to compress will take a lot of the availiable bits. Noise of any type is the enemy of compression as truely random noise is impossible to compress.

    John
     
  11. lmccauley

    lmccauley
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    My basic point is that if you want the highest bitrate for audio and video then you only want to have 1 audio track.

    From Confucius figures:
    The max bitrate for video + all audio tracks on a DVD is 10.08 Mbps
    DD5.1 audio track uses 448 Kbps
    DTS uses 754.5 Kbps

    If you have both DD5.1 and DTS, then you are left with 10322 - (448 + 754) = 8.91 Mbps for the video.

    If you have only DD5.1 (no DTS), then you are left with 10322 - 448 = 9.64 Mbps for the video.

    Similar calculation for DTS (& 2-channel DD) only.

    Now, maybe the answer is that the perceived difference between video rates of 8.91 & 9.64 Mbps is virtually undetectable.

    Which then begs the question, if you added a commentary track, using up another couple of hundred Kbps, would the drop from 8.91 to around 8.7 Mbps also be indistinguishable?

    I hope this clarifies my point. If not, please let me know.

    Cheers,
    Liam
     
  12. lechacal

    lechacal
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    You're forgetting that the bit-rate for a DD track is the same regardless of how many channels it carries (three hundred and some odd, or five hundred and some odd - I won't admit to being anal enough to recall exact figures!).
     
  13. mjn

    mjn
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    i get your point now, i just didn't 100% see what you where getting at.......

    but if's thats the case, is the maximum compression used, so that the film fits on the disc, and thus no free space is remaining?

    so for each film, the compression rate is adjusted?
     
  14. lmccauley

    lmccauley
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    I wasn't forgetting - I didn't know :rolleyes:.

    So, if you're going to have a DTS track, then you're stuck with at least another three hundred and odd DD track as well, whether it's the minimum number of channels required for the DVD standard or a full 5.1.

    Thanks for pointing that out - I now understand why the superbit titles have both DTS & DD 5.1.

    Cheers,
    Liam
     
  15. Confucius

    Confucius
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    Not so I'm afraid,

    From DVD Demystified:

    "Dolby Digital is multi-channel digital audio, using lossy AC-3 coding technology from original PCM with a sample rate of 48 kHz at up to 24 bits. The bitrate is 64 kbps to 448 kbps, with 384 or 448 being the normal rate for 5.1 channels and 192 being the typical rate for stereo (with or without surround encoding). (Most Dolby Digital decoders support up to 640 kbps.) The channel combinations are (front/surround): 1/0, 1+1/0 (dual mono), 2/0, 3/0, 2/1, 3/1, 2/2, and 3/2. The LFE channel is optional with all 8 combinations."
     
  16. Zacabeb

    Zacabeb
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    All DVD Video discs must contain either a PCM or DD soundtrack according to the specification, so there goes that space anyway :)

    Edit: On closer inspection of the thread I see that had already been mentioned and debated... my bad :cool:
     
  17. Confucius

    Confucius
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    Of course, it would be possible for a company to release a disc with only a DTS soundtrack on it, that was otherwise fully compliant with DVD specs; but it would not be allowed to carry any DVD logos.

    DTVD (Digital Theater Video Disc) perhaps?
     
  18. lechacal

    lechacal
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    <Sigh>
    It's a fair cop.
     

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