Stand-alone 5.1 decoders

[sburrell]

Hey,

Rather than have a huge AV amp like I have now, I had always hoped to simply buy a dedicated 5.1 decoder pre-amp that then fed into my main stereo amp - DTS and Dolby Digital are all I need, and I have no desire to use more than 2 speakers, I just want to be able to access the higher-quality tracks on my DVDs.

So far as I can discover, this does exist but tends to be very high-end - thus expensive.

Then I came across this:

AC3 DTS Digital Audio Decoder on eBay (http://www.ebay.co.uk/itm/AC3-DTS-Digital-Audio-Decoder-5-1-Audio-Gear-Decoder-/280732473378?pt=LH_DefaultDomain_0&hash=item415cf59822#ht_4266wt_883)

Only £37 including free postage from HK, which is not bad at all - assuming it's any good of course.

I don't suppose anyone's had any experience with this or a similar device? The one linked to uses 3.5mm jacks to output the stereo signals - so long as the 3.5mm stereo jack to RCA cable is of suitably high quality, should this offer decent quality output?

[Stratmangler]

Why do you need an AV decoder ? :scratch:

I understand that you've got a HTR sat there, but I'm finding it difficult to understand why you've got one when you're just running stereo.

My setup is Sky HD/Blu Ray player - Panasonic TV - Beresford DAC - NAD stereo amp - speakers, in that order, and things sound excellent.
The Sky HD/BDP/TV are all set up to pass PCM onwards to the DAC.
The TV is a hub for AV sources and sound, and the connection between the TV and Beresford is optical.

[Macca]

Asuming your DVD player has analogue outputs for sound? Most do - just connect them up to an auxiliary input on a 2 chanel amp - the DVD player will decode the Dolby Digital and DTS mix it down to 2 channels and output stereo - no need for another decoder on top of that. Then run your video signal straight into the telly from the DVD player.

[Stratmangler]

Martin

Many DVD players do not decode DTS internally.
Of the ones that do, how many can perform a two channel mixdown ?

[Macca]

Martin

Many DVD players do not decode DTS internally.
Of the ones that do, how many can perform a two channel mixdown ?

The decent ones will decode it but I see what you are saying if it is outputting to analogue the internal decoder will probably default to Dolby Digital mixdown. If so then no easy way around it but if you do not want multi-channel sound anyway not much point in worrying about it.

[sburrell]

Hey,

Well, those weren't the responses I was expecting, lol.

Oky-doky, well, first of all I don't own a DVD player, I have a laptop running OS X, and all I can say is that, since I started outputting the raw DD and DTS data to my AV amp instead of just presumably down-mixed stereo PCM, I've noticed a huge improvement in the quality of the sound. Even if it were possible to find some software that allowed precise control over how the chosen soundtrack is down-mixed, would I want to make my Mac do all that work (bear in mind the stated benefits of reducing load on the CPU and RAM in audiophile sound playback)? Finally, what is stereo down-mixing: is it just extracting the stereo front signal from the 5.1 sound; or does it somehow incorporate additional sounds form other channels too?

My TV source is a Grundig FreeSat HD, and unless I output to an AV amp, the DD signal remains pure and, through my Beresford, just becomes hiss and noise. There is no way that I can find within the Grundig's settings to tell it to force a stereo down-mix. But then, would I want to tell it to do that (see final question above)?

Finally, there's more to a DD and DTS signal than just multiple channels - it's also at a higher bit-rate and sampling frequency than a typical PCM signal. Surely down-mixing will also reduce the sample frequency and bit rate too, thereby reducing the final quality of the sound. Furthermore, who's to say how good the down-mixing process within any given component actually is - just because it's digital to digital doesn't mean it's a flaw-free process. And even if it is, surely DD/DTS > Analogue is better than DD/DTS > PCM > Analogue?

I really appreciate your responses. I guess in this circumstance it just doesn't apply, unfortunately. You have encouraged me to take a closer look at what OS X's DVD Player does to the chosen movie's soundtrack in order to output a PCM signal though - maybe you're right, and it's possible to get the high-quality stereo signal I'm currently enjoying without using an AV amp. But in the case of the FreeSat HD box, I'm confident there's nothing I can do, and it _will_ require an off-board decoder for the HD channels.

Interesting stuff, really :)

[sburrell]

I found a link to Apple's Final Cut Pro 7 documentation that outlines the concept of down-mixing multiple channels to stereo: link (http://documentation.apple.com/en/finalcutpro/usermanual/index.html#chapter=53%26section=9%26tasks=true)
From what I gather, down-mixing isn't a flawless process and results can depend on how you balance the channels with each other. This suggests that different A/V products may down-mix with differing results.
Also, am I wrong in thinking that DD and DTS surround channels may contain sounds that are unique to those channels (eg. rear and side) that, when introduced into a stereo down-mix, may have a detrimental effect (eg. making it sound cluttered)? This would make sense to me.
Anyone care to chip in? I'm finding this fascinating!

[Macca]

Hi Simon

Did not realise you were using a lap top as source, that is maybe a different ballgame, I really don't know. However I woulf suggest that whatever siganl you input into your A/V amp the amp is most likely converting to PCM and outputting two channels. DTS and DD are lossy formats, they are compressed so as to be able to get video and 5 channels of sound onto a DVD disc. They are generally EQ'd so as to make music sound punchy and crisp compared to the same track on CD or vinyl.

You will not just get the L/R front channels when playing in stereo mode you will get everything from all 5 channels output through just two.

Blu-Ray is a different matter - the Blu Ray music discs I have heard sound superb even with only 2 channels playing.

I agree that different A/V decoders give different presentations, it should not be a huge difference though.

[sburrell]

I woulf suggest that whatever siganl you input into your A/V amp the amp is most likely converting to PCM and outputting two channels.
Not sure what you mean. Isn't it more likely that it's converting each channel as it should directly into analogue, and making it available if the speakers are plugged in? I doubt there's any PCM stage.

DTS and DD are lossy formats, they are compressed so as to be able to get video and 5 channels of sound onto a DVD disc.
Compressed, yes, but seeing as it's very rare to get a strictly PCM stereo track on a DVD (I have some DVDs that are two channel, but still encoded in AC-3) it's a moot point.

They are generally EQ'd so as to make music sound punchy and crisp compared to the same track on CD or vinyl.
Aha, maybe this is where the confusion is arising. I'm listening only to movie DVDs, not Audio DVDs.

You will not just get the L/R front channels when playing in stereo mode you will get everything from all 5 channels output through just two.
No, that's only if the input is down-mixed. If I take in the DD or DTS data raw, I'm confident it's decoded directly into discrete channels and output as such.

I'm just thinking out loud, but, imagine a scenario where you had an external DD/DTS decoder feeding a Bantam Gold - that would be far less power-hungry than my current Marantz, and still give excellent DVD audio playback for films.

Hmm… With energy prices rising, that sounds quite a good idea actually… *scurries to eBay*

[Arfa]

Most AVR's will down mix a 5.1 signal to the channels that are setup. So if you tell the AVR you've just got two speakers plugged in, it will likely down mix everything to two channels.

If I understand you correctly, you're think of a setup that skips all down mixing and just plays two of the channels from a 5.1 mix, completely dropping the others, yes?

This may be possible on your AVR, if you setup it up to do such. Correct, a standalone decoder will let you glean off just the channels you interested in, i.e. only presenting the AVR with 2 channels. However, most DVD's tend to focus dialogue on the centre, music on the left/right and effects on the rears. Just listening to a couple of these channels in isolation will give you a very odd experience, undoubtedly, very quiet/muted dialogue. Hence why it preferential to downmix, so that you hear everything in the mix, even if it has been hashed together at the expense of audio purity.

I think you're better off spending your pennies on something like an old Sony BDP-S550 or similar bluray player with 7.1 analogue output to feed into your AVR etc.