Questions about DTS-HD and Dolby TrueHD - AVS Forum
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post #1 of 5 Old 07-11-2013, 10:48 AM - Thread Starter
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I have an older dedicated audio system that includes a Technics SH-AC500D pre-amp. It's old so it does not decode DTS-HD Master Audio or Dolby TrueHD. And as I understand it from doing a little of my own research, the digital audio connection SPDIF over an RCA or Toslink optical connector (both available on my SH-AC500D) also does not support either of those formats; DTS-HD Master Audio or Dolby TrueHD anyway.

Both my Blu-ray players are Sonys (BDP-S480 and BDP-S390) and have an HDMI as well as a digital SPDIF output on an RCA connnector. But the menus give relatively few options in the Audio output. One player has simply one choice, Bitstream or PCM. The other lets me choose differently for DTS and Dolby with a menu choice for each. DTS or PCM and Dolby or PCM. Neither is a very expensive player, my main requirement besides playing blu-rays was that they also do DLNA from my audio library on a pc. That works fine.

But what's getting sent over that SPDIF connector when I play a blu-ray disk that has DTS-HD-Master Audio? Does the blu-ray player convert that to regular DTS? I'm guessing it must since my SH-AC500D recognizes the signal as digital and as DTS.

I'm asking about all this because I've been having issues with distortion on some blu-ray disks and random audio dropouts on some dvds. I've played around with the audio settings on the players and found that does affect it. The PCM output will cure distortion on blu-ray disks on the S390 but my SH-AC500D does not report the signal as dolby digital surround sound or as dts surround sound. And subjectively it does not sound like good surround. I don't know if that because the downmix to PCM is really now two channel or if the SH-AC500D doesn't handle more than two channels as PCM. The sony player documentation gives no info as to exactly what signal the player is outputting when the PCM choice is made. And I'm not sure if it is affected by other menu choices on the players.

I know the kneejerk answer is to just get a surround sound system that handles the newer hd formats. I'm a little bit of an audiophile and my main speakers are bi-amped. At the moment dedicated surround sound pre-amps that handle the new formats are a bit pricey for me and there's not much on the secondary market yet either.

Thanks all!
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post #2 of 5 Old 07-11-2013, 12:02 PM
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http://forum.blu-ray.com/showthread.php?t=41820 has the breakdown of various audio codecs and connections...

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post #3 of 5 Old 07-11-2013, 01:06 PM - Thread Starter
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Thanks, that was a pretty helpful thread. Looks like my theory on what I'm getting out of the digital out is true. I guess that means the player is converted dts-hd ma to regular dts. I wonder if there are issues in that conversion that might explain my problems with distortion on some blu-ray disks.

I used to have a ps3 as one of my blu-ray players and it seems it worked a lot better than the regular sony blu-ray players. I foolishly let my daughter make off with it because she pointed out I never played any games with it. I'm considering picking up a ps3 slim on craigs list to replace it. Since the ps4 was announced, used ps3 prices seem to have fallen.
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post #4 of 5 Old 07-11-2013, 01:46 PM
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Hi JamesGang,

I have a similar setup as your. I have the SH-AC500D with the analog outputs feeding three separate stereo amplifiers. Yes, the SHACs are old, but they work well for me, especially since most of my movie library is from DVDs.

S/PDIF only supports three formats of audio: Stereo PCM, DolbyDigital-5.1 and core-DTS. This was all that was available on DVDs, so the SHACs were fine prior to Blu-Ray.

When you have your players decode to PCM, you will only get stereo down the optical pipe. The SHAC can do either of two things with that stereo signal: It can play it as stereo ("Stereo" on the display), or it can play it using Dolby pro-logic (the surround format used on VHS tapes) to simulate surround ("Surround" on the display with the "Pro Logic" indicator lit).

If the BD player sends DolbyDigital-5.1 or DTS down the pipe, then the SHAC will decode it into all six discrete channels, and either the "Digital" or "DTS" indicators will light.

Now, to this question:
Quote:
Originally Posted by jamesgangnc View Post

But what's getting sent over that SPDIF connector when I play a blu-ray disk that has DTS-HD-Master Audio? Does the blu-ray player convert that to regular DTS? I'm guessing it must since my SH-AC500D recognizes the signal as digital and as DTS.
DTS is a "layered" audio format. It starts with standard (DVD style) DTS data, which is called the DTS-core. Then additional data is added to increase the fidelity, making the audio into DTS-HD. Then a third layer of data is added to make the audio lossless, and that is DTS-HD Master-Audio. When the BD player has DTS-HD MA selected, it will send all of the data down HDMI, but only the core down S/PDIF. So the SHAC will receive standard DTS, and should be happy with it.

Now the reason both BD players give you so little choice with selecting audio (either decode or bitstream) is because that choice is made within the Blu-Ray disc's code, on a per-title basis. The studios typically select either DTS or Dolby to include on the disc, and you rarely have a choice. So it's easy for the BD player to select a default. Since S/PDIF only supports DD5.1 and core-DTS (besides stereo PCM), that is what the player will be sent down S/PDIF.

But now I come up short as to why you are getting drop-outs and distortion. Since you don't get those dropouts with PCM (when the BD player does the decoding) then the likely culprit is the SHAC. There could be passages that the SHAC is not decoding properly. Then again, it could be that the players aren't extracting the DTS-core or the DD5.1 properly. Here are some tests you can try:

Are the drop-outs random or are they repeatable?
Do the drop-outs happen on DD5.1, DTS or both?
Do you have the problem on all discs or just some particular disks?
By the way, Are you using optical or coax S/PDIF?

Hopefully, it's not a problem with the SHAC.
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post #5 of 5 Old 07-11-2013, 03:31 PM
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When set to bitstream for output over optical or coax and the selected track is TrueHD or dts-MA, the player will send DD 5.1 or DTS instead. While they are the older lossy codecs used on DVD, they're output at considerably higher bitrates and some older processors have a hard time with them. What happens when you play a DVD as opposed to a Blu-ray? Do you get the same dropouts?
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