Dolby Digital/DTS to SP/DIF

[arniel]

There is one thing that I've never seen mentioned, and that is:
Is there any product on the market that can take in an encoded Sp/DIF bitstream, and decode it to three seapate sp/dif bitstreams i.e.

AC3/DTS/PCM -> decoder -> PCM Front L+R, PCM Real L+R, PCM Sub + Centre

which can then be fed to the audiophile DACs of choice.

Alternatively, do the commercial decoders employ generic DACs that can be tapped for the digital signals (maybe in I2S format, if not SPDIF)?

As another alternative, those of us with HTPCs should be able to do this in software, but it seems that there are no sound cards that have discrete (i.e. decoded) digital outputs instead of the cheesy 5.1 analogue outputs that come as standard.

This should be a fairly straighforward DSP project, apart from the DTS decode function, I would have thought, and would be ideal for a quality 2-channel setup that is to be expanded to a full home cinema.

Cheers,

Arnie

[jwb]

You want the <a href="http://www.cirrus.com/design/products/overview/detail.cfm?d=44">CS4923</a>

[BrianGT]

Originally posted by jwb
You want the <a href="http://www.cirrus.com/design/products/overview/detail.cfm?d=44">CS4923</a>

How exactly do you propose getting these chips to play around with? I would love to obtain them to make my own decoder, but have been unable to get ahold of them.

--
Brian

[jwb]

I think they are all covered under non-disclosure agreements, and you also have to license the software to make them do anything useful. But, you could conceivably pull the chip (a PLCC) and its EEPROM out of some existing equipment.

I'd propose to simply hack the right hardware and software into a home theater computer.

[arniel]

Hacking the hardware from an existing decoder looks like the only realistic option of obtaining these chips, unless anyone knows of a renegade Dolby licencee...

There is one small flaw - one of the main reasons for (me) doing this is to use my brain to save some money, that's not going to happen if the only source of components is a finished product.

The other option - using a PC to do the decoding, would just need a sound card that has three digital outputs, but it looks like all the cheap cards use the CMI8783(?) chip, which has just SP/DIF passthrough or analogue. And as far as I can tell, no software decoders have any way of sending each pair of channels to a different sound card (3 sound cards = 3 sp/dif outputs).

[johan]

Already the "old" Soundblaster Live soundcard included 4 digital outputs (exists at least on my SBLive Platinum), and the newer 5.1 version has 6 outputs. They are located on a separate bracket that is connected to the pins on the board. If I remember right it is a mini-DIN connector. It is supposed to be connected to an external digital speaker system.

[tiroth]

Originally posted by johan
Already the "old" Soundblaster Live soundcard included 4 digital outputs (exists at least on my SBLive Platinum), and the newer 5.1 version has 6 outputs. They are located on a separate bracket that is connected to the pins on the board. If I remember right it is a mini-DIN connector. It is supposed to be connected to an external digital speaker system.

Yep. Should work fine I imagine. I2S also available at the header i believe.

[arniel]

Please excuse my ignorance, but are you sure that these outputs are for all 6 of the output channels, rather than being e.g. one input sp/dif, one input toslink, one output sp/dif, one outpu tosling and one for a cdrom?

If so, that would be ideal! Except that I believe Soundblasters re-sample everyting to 48kHz internally...

I may have to have a look at the Creative site to check.

thanks.

[johan]

I do believe (from memory) that these are independent signals. I don't know how it works on the newer models, i.e. how do they encode six channels from normal stereo sound. My version at least is able to generate surround sound to the two rear channels.

And yes, all SBLive encode the sound internally to 48 kHz. It is stupid. If you record an external digital sound with 44 kHz, it internally re-encodes it to 48 then down to 44 again. And output is always 48 kHz. I heard some rumors that Audigy and Audigy 2 can output other sample rates, but I suspect that they internally use 48 kHz anyaway. This is ****, I am going to change to Terratec cards. They have true 24/96 support.

[arniel]

So, as I suspected, Soundblaster is still not an option.

It would be really nice to make the PC a home entertainment hub - there's enough processing power for sure, it's a case of getting suitable hardware peripherals.

My ultimate plan is to have the PC act as a control centre (using girder) and a/v digital decoder, outputting to dacs where appropriate.

For instance, it could be the controller for a complex source switcher of RGB, S-Video and audio components, but all analogue signals would be handled by an external relay box (connected by fibre so no PC noise would get into the analogue ground). Add an LCD module and some front panel buttons, and I would have a very powerful hub.

A digital tuner card would turn it into a lossless HD recorder (bye-bye VHS).

A sound card with 3 sets of sp-dif outputs would take care of audio decoding (bye-bye pro-logic decoder, hello stacked DACs).

And all controllable from the office. No more relying on the wife to set the video...

Apart from the sound card, I think I can put everything together.

As for software, one problem that I haven't sussed so far is how to use the PC as a pro-logic decoder IN REAL TIME from a line-in i.e. for terrestrial TV.

Any ideas?