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CP750 and Faux Dolby Digital?

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  • CP750 and Faux Dolby Digital?

    When playing a Blu-ray in my screening room I used to use a player with built-in 5.1 decoding (Sony BDP-S550) and fed that to my CP500's 6-track input. I didn't care if the disk was Dolby Digital or DTS; the player handled it.

    But after going digital, I got a CP750 and would feed that via fiber. Of course the 750 knows how to do Dolby Digital but not DTS. Could not go back to the old way due to a significant ground loop hum which I was never able to pin down. But I have a freestanding decoder I can use if I want the best sound from DTS. If I just wanted basic sound (or if it was a vintage mono movie) I can tell the player to fold the 5.1 down to 2 channels and tell the 750 to do Pro-Logic decoding.

    My BDP-S550 developed a flaky HDMI port and has been relegated to home use with a projector that needs analog anyway. My alt player (a similar Sony, minus internal decoding) recently gave up the ghost on the blue laser (still plays DVDs). Still have a no-name player which used to be the emergency backup at the screening room.

    So I recently bought a super cheap Sony BDP-S3700 so I can have one at each possible playback location. This thing is so cheapo that the ONLY outputs are HDMI and a digital coax. No optical. But in playing with the settings found it offered what I thought would be a very useful feature. It can be told to convert DTS into Dolby Digital for output. I tested this feature on the decoder box as it will tell you what it's playing. I popped in a disk whose best track is DTS. Decoder box says DTS. Activated the feature and resumed playback. Sure enough, the decoder says it's playing Dolby Digital.

    BUT when I tried this at the screening room, feeding the CP750, it totally ignored the signal, as though there was nothing there at all. (Playback of a disk that is actually Dolby Digital worked fine.)

    So what do you suppose the difference is between "real" DD and this sort of faux DD the player creates?

  • #2
    You need more information on what the EDID it is broadcasting. The CP750 can't handle ALL Dolby formats but it can handle Dolby Digital (AC3). I don't think it does Dolby Digital Plus, for instance.

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    • #3
      Probably no easy way to find that out. Not that I could change it anyway. I would presume it would be the simplest form of DD but I have no idea. Perhaps it varies with the sort of DTS it is.

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      • #4
        The CP750 only supports plain old DD / AC3, no DD+ or other Dolby codecs... Keep in mind that almost all newer codex use more bandwidth than you can fit on an S/PDIF port, so those won't work anyway. S/PDIF is essentially just poor-mans AES/EBU.

        Are you really sure the BluRay player re-encodes the DTS stream to actual Dolby Digital-marked AC3? I'm asking this, because you need an extra license to do this. At least, 15 years or so ago, when I was actively involved in building Set Top Boxes, there were specific licenses for decoding DD/AC3 and encoding DD/AC3, where the latter one was quite a bit more expensive and also required a considerable amount of processing. Now, the latest part may not be such a big deal anymore, but the licensing fee sure is, especially for this low-end stuff that sells for a few bucks.

        I'd still spend those extra few bucks for a BluRay player with analog audio outputs, that's something that always just works, at least as long as you don't end up with a CP950.

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        • #5
          Yeah, I have a feeling that it's some sort of unlicensed encode that the 3rd party decoder is fine with but not a genuine Dolby. Are they even still making players with multi-channel output? Didn't the BD people ban players with analog HD video output several years ago? Not that that's the same thing but you never know.

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          • #6
            Tascam,Denon, Pioneer, and Panasonic have analog outputs (and I'm sure there are others).

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            • #7
              Yup, there are still quite a few, and if the analog multichannel input on the CP750 is not used for film, it is still the best option to connect audio from a Bluray Player - as with the S550, the player will decode every imaginable codec to a signal the cp750 will always happily process. Tascam and Denon offer rack mountable players with network and serial remote control capability. They offer most other bells and whistles for a theatrical/screening room presentation. With the exception that you can not shift subtitles vertically. I'm hoping for a software update. We still use a Sony 550 for most Bluray showings we do, and we have a Backup Panasonic player with wireless remote control, analog multichannel out, OSD off, subtitle shift, etc. Near perfect - BUT, the userinterface is total crap...you can't adjust anything without the full menu coming up on screen. We will try both the Tascam and Denon soon.

              From the 3700 manual:

              '[Dolby D Compatible Output][On]: Converts DTS audio source to Dolby Digital audio by using EDID (Extended Display Identification Data). [Off]: Turns off the function.'

              That sounds a bit weird. At least ambiguous...
              Last edited by Carsten Kurz; 08-12-2020, 06:22 PM.

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              • #8
                Sounds like it breaks more stuff than it fixes, if it does what it says it does. At least I interpret it like it doesn't really convert the data-stream but only changes the EDID from DTS to DD.

                Both formats are inherently incompatible. Besides the difference in packaging of the compressed audio data, DTS uses ADPCM as its main compression technology, while Dolby uses MDCT, which is used in most modern lossy audio compression schemes, including MP3, in some form or another.

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                • #9
                  Maybe they assume that 'some' boxes will we able to decode a wrapped dts just as well as ac3. Maybe it's just nonsense in the manual. Maybe it get's them around paying for a license. Maybe it does create 'SOME' ac3, but not one that the 750 is happy with. I know a colleague using an older Samsung or LG - Player with a DTS->AC3 transcoding feature on his JSD-80. I think that works with DTS Master/HD discs as well, as they carry a standard dts backup stream with them.
                  Last edited by Carsten Kurz; 08-13-2020, 06:08 AM.

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