Decoding Dolby Digital

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Decoding Dolby Digital

Postposted on Sat Apr 17, 2010 5:25 pm

I've got a set of Logitech Z5500s, which can decode DD on their own, but I use the optical connection for my PS3. I'd like to decode DD on my computer; if I buy a decent audio card will it be able to do that?

I'm not interested in software solutions (unless it's a DD-decoding driver for my built-in audio); my most common use case is iTunes, which includes a Dolby Digital audio track with HD TV downloads but can't decode it.
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Re: Decoding Dolby Digital

Postposted on Sat Apr 17, 2010 8:57 pm

Z-5500 has a coaxial connection that you can use with your motherboard. That is assuming your motherboard has a coaxial SPDIF port. If you aren't sure, the port looks like a RCA connector.

Otherwise, you have to acquire a sound card that has DDL support via analog.
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Re: Decoding Dolby Digital

Postposted on Sat Apr 17, 2010 11:34 pm

Krogoth wrote:Z-5500 has a coaxial connection that you can use with your motherboard. That is assuming your motherboard has a coaxial SPDIF port. If you aren't sure, the port looks like a RCA connector.

Unfortunately, no -- I checked that first. ;)

Otherwise, you have to acquire a sound card that has DDL support via analog.

Yes, that's what I'm after. I can't seem to find this listed or described as a feature on any of the cards (and reviews) I've looked at. Is it just poorly documented, and any good card will offer this?
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 1:17 am

Krogoth wrote:Otherwise, you have to acquire a sound card that has DDL support via analog.

What? DDL is a digital encoding to pass multichannel audio over digital connections. It's by definition not possible to pass via analog.

SNM wrote:I'm not interested in software solutions (unless it's a DD-decoding driver for my built-in audio); my most common use case is iTunes, which includes a Dolby Digital audio track with HD TV downloads but can't decode it.

There's no hardware accelerated decoder available for DD (heck, for any audio codec; thinking on it, how would you make one, unless it was a proprietary implementation for a specific card...?). If you need to decode DD (aka AC-3) audio, you can use something like FFDShow or the built-in one in MPC.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 2:48 pm

SNM I don't quite understand the whole situation: why do you want to decode DD on your computer at all? It seems you want to decode those iTunes tracks? Then all you have to do is have your coax out connected to the Z-5500s, and they'll take care of it.

Maybe I'm missing something here or misread, but I'm not quite understanding the scenario :)
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 3:12 pm

morphine wrote:SNM I don't quite understand the whole situation: why do you want to decode DD on your computer at all? It seems you want to decode those iTunes tracks? Then all you have to do is have your coax out connected to the Z-5500s, and they'll take care of it.

Maybe I'm missing something here or misread, but I'm not quite understanding the scenario :)

I don't have a coax out and I don't want to swap plugs around, so I want some way of decoding it before it gets sent out.

Of course, buying a card with a coax out and then using that is a simple enough workaround (or better solution) that I just hadn't thought of. I guess I'll look into that.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 3:14 pm

morphine wrote:SNM I don't quite understand the whole situation: why do you want to decode DD on your computer at all? It seems you want to decode those iTunes tracks? Then all you have to do is have your coax out connected to the Z-5500s, and they'll take care of it.

Maybe I'm missing something here or misread, but I'm not quite understanding the scenario :)

I think we're getting our acronyms mixed up.

DD: Dolby Digital. Also known as AC-3, this is an audio codec for multichannel audio.
DDL: Dolby Digital Live. This is a digital encoding scheme for passing digital multichannel audio down S/PDIF connections. It takes any decoded audio from the source and transcodes it to Dolby Digital format for transport. It is up to the receiver to decode the signal.

If you want to decode audio encoded by DD/AC-3, you need a decoder like FFDShow or something built into the media player. This will take the DD compressed audio, decode it, and output PCM to your sound card for conversion into analog signals.
If you want to transcode all the audio coming out of your system to DDL for decoding by a receiver, something like the Xonar will be able to do that.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 3:41 pm

It should be possible to capture the DD bitstream from a TOSLINK input on your computer (if it has one) and then retransmit on SPDIF Coax. By the way, SPDIF Coax can be rigged on any motherboard with the right header. Or, you can decode the DD bitstream on the computer via software and output on 3 analog jacks to the Z-5500.

It is likely easier to use this, or something like it. http://www.amazon.com/Digital-Optical-T ... B000I98ZQY

What I don't understand, though, is why you're going through this trouble when Logitech says the Z-5500 has SPDIF Coax and TOSLINK inputs?

EDIT: Oh, you want to record the output of your PS3 on your computer? I'm afraid that will require software to decode the DD bitstream.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 3:53 pm

bigfootape wrote:It is likely easier to use this, or something like it. http://www.amazon.com/Digital-Optical-T ... B000I98ZQY

Oh, you're awesome.

That said, I don't know how I've failed this badly at communicating but you guys don't quite get it. I download video files from the iTunes store. These files include Dolby Digital (or DD Live? I dunno)-encoded 5.1 tracks in case you want surround sound. iTunes doesn't decode this output; you need it done elsewhere (and it doesn't utilize external drivers to do it). My Z5500s can decode this output, but 1) my computer doesn't have coax out, and 2) the optical in on my Z5500s is used by the PS3 connection.

My computer does have optical out, though, so with an optical-to-coax converter I'm all set. Hurrah.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 3:59 pm

Or you could get a coaxial s/pdif output bracket and plug it in to the s/pdif header on the motherboard.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 8:53 pm

SNM wrote:
bigfootape wrote:It is likely easier to use this, or something like it. http://www.amazon.com/Digital-Optical-T ... B000I98ZQY

Oh, you're awesome.

That said, I don't know how I've failed this badly at communicating but you guys don't quite get it. I download video files from the iTunes store. These files include Dolby Digital (or DD Live? I dunno)-encoded 5.1 tracks in case you want surround sound. iTunes doesn't decode this output; you need it done elsewhere (and it doesn't utilize external drivers to do it). My Z5500s can decode this output, but 1) my computer doesn't have coax out, and 2) the optical in on my Z5500s is used by the PS3 connection.

My computer does have optical out, though, so with an optical-to-coax converter I'm all set. Hurrah.

So just install the correct software to decode DD/AC3. You've thrown everyone off by explicitly telling people to not bother offering up what is the normal way of doing things! Therefore people either assume you're trying to do something exotic or ignore your post.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 9:25 pm

mattsteg wrote:So just install the correct software to decode DD/AC3. You've thrown everyone off by explicitly telling people to not bother offering up what is the normal way of doing things! Therefore people either assume you're trying to do something exotic or ignore your post.

Yeah; that doesn't work when playing via iTunes. I've tried.
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Re: Decoding Dolby Digital

Postposted on Sun Apr 18, 2010 10:35 pm

SNM wrote:
mattsteg wrote:So just install the correct software to decode DD/AC3. You've thrown everyone off by explicitly telling people to not bother offering up what is the normal way of doing things! Therefore people either assume you're trying to do something exotic or ignore your post.

Yeah; that doesn't work when playing via iTunes. I've tried.
That may be, but asking "how can I get 5.1 in itunes" might have been a more fruitful way to phrase things, in retrospect. There are potential avenues to approach the problem that might not fit your criteria (and plenty of ways that what you are asking for could *not* work), and phrasing things that way might have made things clearer regarding what you were trying to do. I interpreted it as some sort of weird configuration where you wanted to input dolby surround via some means and then decode it on your computer - clearly not what you are trying to do.

In other news, why is itunes so far behind the times? This is functionality that should have already been old news a decade ago. It's amazing how rabid of a following a software product that's awful in so many ways can generate.
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Re: Decoding Dolby Digital

Postposted on Mon Apr 19, 2010 12:31 am

mattsteg wrote:In other news, why is itunes so far behind the times? This is functionality that should have already been old news a decade ago. It's amazing how rabid of a following a software product that's awful in so many ways can generate.
I can think of a few reasons, not the least being that including full AC3 decode support in iTunes would require Apple to pay licensing fees to Dolby. Videos from the Apple Store include AC3 soundtracks because that's what the Apple TV uses. No Macs come with 6 channel analog output anyways, so only support for s/pdif pass-through is necessary.

I've only tried to play video with AC3 audio in iTunes that I've encoded myself using Handbrake, with no luck. I just get silence for the audio, although I wouldn't be surprised if it was an issue with Handbrake.
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Re: Decoding Dolby Digital

Postposted on Mon Apr 19, 2010 6:08 am

Mediocre excuses at best. If you create your own ecosystem, you're responsible for the ways that it sucks.
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Re: Decoding Dolby Digital

Postposted on Tue Apr 20, 2010 12:04 am

Sorry about misunderstanding earlier, I didn't have a clear picture of what you were getting at.

It seems what you'd really like is some kind of driver shim which intercepts SPDIF and decodes it if it's Dolby Digital or maybe DTS as well. I can't say for sure that there isn't something which would do the job, but I'd try to avoid it because 1) it might be buggy and 2) it might not be supported through OS upgrades.

I would like to know, though, if your sound card can do Dolby Digital Live (which transmits normal sound as Dolby Digital to your speakers). Alternately, do you have media or games which aren't DD and aren't DTS which use your 5.1 outputs? For instance, some MP4 files have audio encoded in 5.1 AAC. Do you have anything like that?

If your motherboard can do DDL or you don't care about only having stereo (except for Dolby Digital and DTS tracks), just buy or build an SPDIF coax bracket, or use a converter and go all digital to your Logitech receiver. It's less hassle and provides a really good quality connection. I don't always find onboard motherboard DACs to be very worthy, and this way you won't be using them.
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