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SKY + Optical output!!!

Discussion in 'Sky Digital TV Forum' started by Jacob, Jul 31, 2005.

  1. Jacob

    Jacob
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    I've just had Sky + installed (£49 refurb deal!!) and foolishly thought that all the box office films would be in shown in 5.1 so I went and bought an optical cable to hook it to my amp.

    I now gather that most are not but a lot are billed as dolby surround. My questions are:

    1) If I set my Optical output on the Sky box to Digital (as oppose to Stereo), shouldn't my Amp show a digital signal as being received even if it's just in dolby surround. As it is the signal is PCM (athough i'm not quite sure what that means!)

    2) It also doesn't seem to make a difference whether the sky box optical out is set as stereo or digital! Why?

    Maybe i'm missing something or just don't understand but could someone please explain?

    cheers

    Jacob
     
  2. eviljohn2

    eviljohn2
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    You need to set the box output to Dolby Digital and your amp should indeed indicate that it's receiving a digital signal. Are you sure you have set the inputs correctly on it?

    The plain stereo output is delivered as PCM but there are several additional formats available for different channels. There are very few DD5.1 programs but many of the "DS" tagged films are transmitted as DD3.0 which your amp can decode. :)
     
  3. Nick_UK

    Nick_UK
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    No Box Office movies are broadcast in DD 5.1

    Only Sky Movies 1, 2 and 3, and The Disney Channel are in Dolby 5.1, and only on some programmes/movies. The "Digital" indicator on my Yamaha amp only comes on when a movie broadcast in DD 5.1 starts. In the sound menu of the Sky+ box, you have the option of stereo or DD output. When it's set to stereo, the output will always be in PCM (Pulse Code Modulation), but when it's set to Dolby, it will output a Dolby Digital bitstream when one is being broadcast, and a PCM stream at other times.

    Dolby Surround is not digital, and will not show up as a digital stream, only as PCM stereo.
     
  4. eviljohn2

    eviljohn2
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    Nick, some Dolby Surround tracks are encoded discretely (with a single rear channel) and broadcast over Sky Digital. There are a few DVDs like this too - the old R2 version of Scarface springs to mind.

    Helpfully, my amp indicates exactly which speaker channels are encoded within a digital stream and shows it on the front panel and I frequently see the front pair and a single rear channel. :)
     
  5. Nick_UK

    Nick_UK
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    Yes, I'm not disputing that Dolby Surround is broadcast, but it is not a digital bitstream as such. It is a stereo PCM feed which carries the phasing information for Dolby Surround. Dolby Surround can equally be carried over a pair of analogue audio feeds. Dolby Digital is a digital bitstream which can carry up to 7 separate digital audio feeds, and can only be sent over a digital optical or co-axial feed.
     
  6. eviljohn2

    eviljohn2
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    DS can be transmitted over the analogue pair but some films tagged as DS are transmitted through the digital feed with 3 discrete digital channels rather than having to compare the analogue signal. :)
     
  7. Nick_UK

    Nick_UK
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    If you look at the Dolby Site, you will see that Dolby Surround is always matrixed from a two channel source, which can be either analogue or digital in origin.

    Only Dolby Digital carries discrete 5.1 sound channels.
     
  8. Jacob

    Jacob
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    Thanks both,

    I got it slightly wrong I can actually see it is in digital even though it also says PCM. My amp still shows a digital signal even when the optical output is set as normal (not stereo as I previously said!) How is that?

    cheers
    Jacob
     
  9. chrise

    chrise
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    Jacob any signal coming down the optical SPDIF output is a "digital" signal whether coded as PCM or Dolby Digital (or indeed theoretically DTS but Sky does not use that technology).

    Almost all the TV channels on Sky+ are sent in PCM mode with a stereo signal that home cinema amps can apply processing using Pro Logic or similar systems to get surround effects.

    Sky movies 123 and the two main Disney channels transmit a Dolby Digital signal all the time. Only a limited amount of programming has true 5.1 soundtracks the balance is 2 channel but some films may have 3 channels as previously said. It is false to assume that DD is always multi-channel. Pro Logic type processing works here as well - PLIIx on my amp gives a good 7.1 effect on 5.1 software so I leave it switched in.

    Most Home Cinema amps are smart enough to sort this out on their own using the information in the signal. Relax using the optical output will maximise your sound enjoyment but its a shame the programmes are not better.
     
  10. Nick_UK

    Nick_UK
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    PCM is arguably a digital signal. The only options on a Sky+ box are Stereo out, or Dolby Digital out. When there is no DD stuff being transmitted, the output is PCM stereo (which may or may not contain Dolby Surround encoded material). When you press Select during normal viewing, the letters in the top right-hand corner of the Search & Scan banner will indicate the sound source (DS = Dolby Surround, DD = Dolby Digital). When you don't see either of these, assume a stereo sound source.
     
  11. Nick_UK

    Nick_UK
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    Sorry, don't want to appear pedantic, but the Dolby Digital signal is only activated during actual DD 5.1 movies on Sky Movies 1, 2 and 3. It's only switched on for the duration of the movie - at all other times the sound is PCM stereo. I don't want to confuse anyone who is setting up an amplifier, and make them think that the DD indicator should be continuously lit.
     
  12. chrise

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    Sorry Nick but you are making a classic mistake of confusing the encoding method Dolby Digital/PCM etc with the amount of channels perhaps the following from the Dolby website might help.

    Dolby Surround
    The original Dolby multichannel film sound format that revolutionized movie soundtracks. This technology encodes four channels of audio (Left/Center/Right/Surround) onto just two audio tracks for media such as TV broadcasts and feature films on VHS. Material encoded in Dolby Surround is fully compatible for playback on mono and stereo systems: upon playback on systems containing Dolby Pro Logic® technology, the audio is reconstructed to its original four-channel surround sound. Found in: Videocassettes; DVDs; Broadcast programming, film.

    Dolby Digital (AC-3)
    Surround sound technology that delivers high-quality digital audio for up to 5.1 discrete channels (Left/Center/Right/Left surround/Right surround/Low-Frequency Effects). The five speaker channels produce a directional and more realistic effect and the Low- Frequency Effects (LFE) channel (usually reproduced through a subwoofer) can often be felt as well as heard. Found in: DVDs; DVD players; broadcast programming in HDTV, digital cable, and (DBS, DTH) direct broadcast satellite (DirecTV/Dish/Sky TV); PCs; Microsoft Xbox game consoles; digital TVs; automotive DVD-Audio/Video players; automotive video, networking and home audio/video receivers; film soundtracks encoded in Dolby Digital.

    Note the up to 5.1 channel bit as indeed DD signals can in theory be mono and are frequently only stereo. If you look at the indicators on a reasonable Home Cinema amp you will see that the 5 DD channels show a DD flag even when note transmitting two channel material that may or indeed may not have Dolby Surround material encoded.

    These terms are frequently confused so you are not alone
     
  13. Nick_UK

    Nick_UK
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    I'm quite aware of all that, thanks, which is why (in an earlier post in the thread) I said:
    "Dolby Digital is a digital bitstream which can carry up to 7 separate digital audio feeds"

    I'm also very aware that the "DOLBY DIGITAL" indicator on my Yamaha Amp only comes on at the very start of a Dolby Digital movie (after Sky's blurb at the beginning), and reverts to "DOLBY SURROUND" at the moment the movie ends.

    I am also quite aware of the differences between DD and DS, which is what the original post was about.
     
  14. chrise

    chrise
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    Nick do go and look at

    http://www.dolby.com/consumer/technology/tech_overview.html

    you will find that several of your points are indeed inaccurate. Not least the point about 7 separate channels (hint look at Dolby EX and see what it actually says). This is a complex subject where terms are used innacurately.

    Sky+ can only output a digital audio stream from the optical output. This can be encoded as PCM (an uncompressed signal potentially multichannel but limited to 2 channels in practice) or DD (a lossy compression standard which can carry up to 6 discrete channels) Sky use PCM on all channels but add a second audio stream to the 3 Sky Movie channels and the Disney channel that can be selected in software on Sky+. The output remains set to PCM or DD as selected (sometimes detectable by a difference in the sound level between channels a common problem when Sky first brought in DD). DS is an analogue format.

    Where software transmitted has a soundtrack with DD 5.1 or similar a home cinema amp will detect the flag and decode the signal. Amps displays are something for the manufacturer but should not be relied on for accurate analysis of the signal.
     
  15. Nick_UK

    Nick_UK
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    Several, eh ? Didn't know I'd made that many ? :rolleyes:

    Depends on whether you class the LFE as a separate channel. How many channels in the left hand diagram below ?

    [​IMG]

    Six...... plus the LFE which = seven.

    Chris..... just 2 questions.....

    1. Have you got a granny ?

    2. Does she have an egg collection ? :D :hiya: ;)
     
  16. Jacob

    Jacob
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    And I thought I was asking a relatively simple question!! Anyway guys, you're getting a bit too techinical for me but I think I understand what I need to know

    So I know that I'm thinking along the right lines and just to clarify

    1) If I'm listening to Sky via the optical out, my amp will always show a digital source
    2) Optical out will give the best sound quality
    3) A digital source could be mono, Stereo, DS or 5.1

    I still don't understand why some programmes are shown as being DS. Surely all prgrammes in stereo can be split into 4 channels. If so, why are all programmes tranmitted in stereo not shown with the DS flag. (or maybe they are but there are still a lot of programmes shown in mono!)

    Finally, it was mentioned in one of the earlier posts that some DS films are actually tranmitted as DD3.0 which the amp can decode. I understand that stereo can be split into 4 channels so what is a 3 channel brodcast split into?!

    I hope I don't appear to dull!!!

    Cheers

    Jacob
     
  17. chedmaster

    chedmaster
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    nick the 7th (or 6th) channel in dd ex is never carried discretely, but matrixed, thats only dts-es discrete.
     
  18. chrise

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    Precisely
     
  19. chrise

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    Jacob the answer to all three questions is yes.

    Surround sound can be transmitted as discrete signals as in DD 5.1 or be extracted from a two channel signal such as DS. The chips in a home cinema amp sort out what signal to feed to each speaker. There are a number of ways this is done but Dolby codecs are the best known. The signal should contain a flag to tell the amp what to do automatically. I have no experience of 3 channel software.

    Dolby invented Pro Logic decoders (currently PLIIx is the most advanced) which will produce surround effects from a minimum 2 channel signals. Leave these on for TV watching and you will find reasonable effects on most programmes speech being mainly on the centre channel and effecs like crowd noise behind you music however can sound a bit wierd. Some 2 channel programmes (e.g. The Simpsons)have effects channels inserted into the stereo soundtrack, the Dolby trademark is shown in the credits, which do need a PL decoder to extract them but do give quite a convincing 4 channel effect effect(the surround signal is mono IIRC). Interestingly PLIIx produces excellent centre stereo rear channels from 5.1 material as well.

    Relax and enjoy the best sound a TV programme can currently produce.
     
  20. Nick_UK

    Nick_UK
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    Yes, well done, chaps. You've now proved how superior your knowledge of Dolby is, but the poor old OP still hasn't had his question answered. Still, as long as you've proved yourself right, that's the main thing, isn't it ? :suicide:
     
  21. ctcrm

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    its disapointing sky only transmit dd on movies 12 and 3 which are older films anyway arent they??

    whats the point of that ??
     
  22. chedmaster

    chedmaster
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    1 and 3 are the premier channels :confused:

    cinema are old ones
     

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