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Old 19th September 2003, 10:43 AM   #1
cdl is offline cdl  Denmark
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Default Build an SPDIF-in for a DVD player

The possibility of DIY with DD, DTS etc. was discussed in this thread this thread but the interesting stuff somehow muddied away in boresome legal issues.

Now then, concluding that it isn't legal/possible to lay hands on a decoding chip, how about Kay's idea of buying a DVD player with DD and DTS decoding (which can be bought for almost no money now), and building an SPDIF input.

Surely, that can't that be illegal...? The thing was meant for playing DVD's, and in that process decoding the sound. Now, for some obscure reason, I want it to decode the sound of those very same DVD's, only reading the disc in an other drive than the built-in one. Licenses and all that crap have been payed, so what would anyone care - and who would ever find out anyway?

BUT - is it, even remotely, technically possible?

Shouldn't be too hard to trick the decoder into doing it's job regardless of whether a disc is loaded or not.

If yes, the analogue circuits after the decoding chip etc. etc. can always be tweaked, which should be able to bring fairly decent sound out.

Anyone got comments...?

/cdl
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Old 19th September 2003, 12:01 PM   #2
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I'm not sure I get the point of this.

You seem to be sujesting using one DVD player to decode the image from a DVD, whilst sending the audio signal to another player to be decoded in there.

What I want to know is why not just use the second player that is decoding the audio to to the images too? Or am I just missing the point here?
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Old 19th September 2003, 12:55 PM   #3
cdl is offline cdl  Denmark
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Hey Bigparsnip,

OK, let me explain.

My primary use (for now) would be for DVD movies etc.
I very much prefer using my computer to watch these, because of the drastically better picture quality (at least better than DVD/TV combo's I might be able to afford).

But I would like to decode the sound in a stand-alone box of a kind - for sound quality reasons as well as to avoid long analogue cables from the computer etc.

And I like the possibility of "feeding" whatever digital signal I like into the a stand-alone part of my HiFi. In the future, I think lots of other stuff than movies will be multichannel/DD/DTS etc.

Of course, we could all just go out and by a nice, upgradeable, multi$$$ multichannel/DD/DTS preamp from Thule, Holfi or others offering such things, but it's not DIY at all, and it's economically totally out of my league.

A cheap DVD for modding "along the way" suits my economy much better - and gives me more freedom to design the rest of my electronics as I wish!

/cdl
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Old 19th September 2003, 01:01 PM   #4
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Ok, that makes a little more sense. Well, seen as this is a diy forum, how about getting one of the cheeper decoders for DTS / DD that you can get with sets of computer speakers.

Once you had this, you could probably tap the data lines of the DSP from this and feed them into a set of DACs compleetly of your own design, so you can make them as good/cheep as you want.

I guess that this would be a little more complicated but it might work.

One other thing that i have seen (though not too often) which might be a good place to start from if you can find one on ebay or something, is a fairly low price dolby digital decoder and preamp from cambridge audio. Although, I can't remeber it's name at the moment, and I',m quite sure it didn't do DTS.
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Old 19th September 2003, 01:43 PM   #5
cdl is offline cdl  Denmark
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Glad I make sense now...!

Yeah, Parsnip that's a pretty big idea with those computer speakers... hadn't thought of that... will do some research into the subject. Of course, it's going to take a little adaptation to a setup without subwoofer, but still.

I'd be going for something with DTS though, now I'm at it... SBLive's DD decoding isn't great, but tolerable until I get hold of something with DTS.

Of course, the advantage of a DVD-player with SF-PDIF input and a modded analogue part would be playback of audio-DVD's without turning on the computer... but well, don't have any yet anyway!

off looking for digital pc speakers...

/cdl
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Old 19th September 2003, 02:37 PM   #6
cdl is offline cdl  Denmark
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Thumbs down Digital speakers....

Well - a quick check of the market here in DK shows that only a few DD/DTS compatible PC speakers are available. And that at prices equal to - or above - what cheap (complete) "hifi" DD/DTS recievers cost.

The latter are probably better constructed than whatever's powering the speakers anyway - and I'd prefer to pay for the larger PS etc. instead of flimsy plastic cases and cheap drivers.

BUT I already considered buying a cheap reciever and modding pre outs, however buying something priced around 2000DKK just to start pulling it apart seemed kind of stupid... less so with a 500DKK DVD player.

So as yet the digital speaker thing isn't the most attractive option - alas back to square one... possible to make an SPDIF in on a DVD...?

/cdl
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Old 19th September 2003, 03:31 PM   #7
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The external decoder idea is one of merit, but one has to understand that certain DSP's decode better than others... just like some ADCs and DACs sound better than others. And just pulling the DSP out of a cambridge audio speaker system or some cheap DVD players is going to be a challenge. Typically, these type of systems not only use "bad" (not bad really... just not as good persay) DSP's, but they are typically very specialized ASIC design. They are very difficult to integrate with for personal use as they are typically designed for very specific microcontrollers and typically have no published technical data or spec sheets.

Moto and Cirrus DSP's seem to be those that are commonly associated with being "the best" in my experience. Most DVD players nowadays use specialized ASIC designs (like Panasonic). In addition, a lot of DSPs require external memory interface to store the coefficients for doing DTS decoding. So, using the entire main decoding board would most likely be beneficial. I guess I'm saying you shouldn't just blindly buy a "cheap" DVD player or speaker system just to rip out a DSP decoder. It won't be as easy as you think. Do some research and find out where a common DSP is used (say a mid-fi receiver).

Something I have been debating over for quite some time is this computer decoder. See here. It uses a CS4926 decoder and integrates a CS4226 20 bit CODEC. One could tap off the data lines for the DAC system and put in some external DACs. There's even a remote integrated. One could just run some photo diodes somewhere to the front of a new case to allow remote interaction. The 4926 is one of the less demanding DSP's (as far as microcontroller integration goes). There are hundreds of pages of spec sheets and app notes that we went through on Cirrus DSPs alone during our senior design project.

Hope this helps a little...
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Old 19th September 2003, 05:15 PM   #8
cdl is offline cdl  Denmark
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Thanks, Tieftöner.

Of course, I'm not expecting a DIY/Mod-construction based on something cheapy to be as good as the Thule preamp at 20 times the price. But hopefully better than my soundcard.

I was considering using the main DSP board, only with "proper" analogue sections - and, possibly leaving the DVD transport in there for the occasional audio DVD. My general impression is that much quality can be won with decent analogue and power supplies.

You're right, should first do research into a model with an appropriate chip inside. If doable at all.

The Jazz computer decoder is definately nice, but also not that cheap - and I haven't found a danish distributor... wouldn't want to import tax that price here in DK

Anyone know whether the SBLive! issue has been solved in the newer DS-007 version?

/cdl
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