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[Vreejack]

Trying to pass DTS through TOSLINK to an external decoder by using MC loopback.

It mostly works, but the Realtek audio driver on my motherboard will only accept 5.1, so I suspect that DTS is being mixed down.

As it stands, my motherboard will only actually output 2 channels (which is why I got the USB-TOSLINK adapter), so the fact that it accepts 5.1 is surprising, but since I am using digital loopback is there a way to force the audio driver to accept eight channels?

[Hendrik]

DTS only supports up to 6 channels, at least the variant of DTS which works over SPDIF/TOSLINK.
You cannot pass 8 channels over such a link, it doesn't have enough bandwidth.

[Vreejack]

DTS only supports up to 6 channels, at least the variant of DTS which works over SPDIF/TOSLINK.
You cannot pass 8 channels over such a link, it doesn't have enough bandwidth.

Do you have a reference for that? I was under the impression that BNC was not as good as a short Toshiba cable. And BNC out is very rare.

[mwillems]

Do you have a reference for that? I was under the impression that BNC was not as good as a short Toshiba cable. And BNC out is very rare.

The limitation is not the cable type, but the SPDIF format; he wasn't suggesting BNC, but rather noting the limitations of any SPDIF connection (coax or optical).  If you want an 8 channel digital signal out of a PC the easiest way to do it is HDMI to an AVR or a direct USB connection if your device supports it.  

SPDIF (whether coax or optical) only has bandwidth for two uncompressed channels, or 6 channels compressed.  I've heard rumors that SPDIF can be made to support compressed 6.1, but I've never heard of a way to get 7.1 (compressed or otherwise) out of any SPDIF connection.

Check out this thread over at AVS for a rundown and some links (farther down in the thread): http://www.avsforum.com/t/1049146/true-7-1-high-def-sound-from-spdif

[Vreejack]

Okay then. My decoder has an HDMI input, but adding an output to my computer is a problem. Is there such a thing as USB to HDMI for audio? Seems sketchy.

[mwillems]

Okay then. My decoder has an HDMI input, but adding an output to my computer is a problem. Is there such a thing as USB to HDMI for audio? Seems sketchy.

I've never used one myself; the way that most folks here get an HDMI audio output is through their video cards as almost all modern graphics cards have at least one HDMI out (and many have two).   A cheap graphics card is probably the easiest and most reliable way to get an HDMI output if you have a spare PCI-E slot.

That said, there seem to be a large number of USB to HDMI converters on the market, and since it's a digital to digital conversion it has the potential to be seamless (if implemented correctly).  Here's one I found poking around that might do the job (but I don't know):

http://www.amazon.com/Cable-Matters-SuperSpeed-2048x1152-1920x1200/dp/B00ANTCU9M
(specifically the USB 3 to HDMI model on that page; the USB 3 to HDMI/DVI model only supports 2 channel audio)

I'm sure some of the other folks on the forum might have more direct experience with this kind of thing.  You might have good luck opening a thread over in the sound equipment or PC subforums asking for specific recommendations.

[Vreejack]

And... of course that adapter will no longer work with Windows.

I may finally understand this, please take a look:

Since you can only send two uncompressed audio channels via S/PDIF, how can I send compressed audio? Do I have this right?

• Not being able to compress my own DTS on the fly, how can I get applications to send DTS to a dummy audio driver that MC will pick up via loopback? I am guessing that if an app sees a 5.1 sound output then it will send compressed audio--if it has it--on two channels, and if I tell MC not to do any mixing then it will forward those two channels to my decoder via S/PDIF and DTS will actually work.
• If I start with more than two channels then I need to use the MC Dolby digital encoding, which I think gives four channels compressed to two.

So I need to be able to switch between two audio modes on MC. And my decoder as well, but that is a given. I also need to be more aware of what audio streams are available.

[mwillems]

And... of course that adapter will no longer work with Windows.

That's a bummer I didn't even think about that piece, because win 8 adoption has been spotty in these parts (in part because it broke some USB functionality).  That may get fixed in time.

Guess a cheap video card may be the solution if you've got space (older models can be had for $20 or $30).

• Not being able to compress my own DTS on the fly, how can I get applications to send DTS to a dummy audio driver that MC will pick up via loopback? I am guessing that if an app sees a 5.1 sound output then it will send compressed audio--if it has it--on two channels, and if I tell MC not to do any mixing then it will forward those two channels to my decoder via S/PDIF and DTS will actually work.

Is your dummy driver only two channel? The application needs to see six channels on the audio driver it's addressing to send 5.1 audio (compressed or otherwise).  But this may not be relevant because:

• If I start with more than two channels then I need to use the MC Dolby digital encoding, which I think gives four channels compressed to two.

My understanding is that Dolby Digital supports 5.1, the same as DTS http://en.wikipedia.org/wiki/Dolby_Digital.  So you're not losing channels by encoding to Dolby Digital.

So I need to be able to switch between two audio modes on MC. And my decoder as well, but that is a given. I also need to be more aware of what audio streams are available.[/list]

You might be able to achieve this via zones and zoneswitch.  Set up a zoneswitch rule sending all your DTS audio to one zone, and the rest to another zone with the appropriate bitsreaming/encoding settings.

JRiver will tell you (in the audio path dialog box) what kind of audio the source is, so you should be able to see what's providing DTS or DD or whatever.

It's possible that some combination of bitstreaming settings and output format settings would get you the right behavior all the time in one zone, but I'm definitely not an expert on that.  Possibly setting bitstreaming to "DTS" and output format to "encode to Dolby Digital" might work.  My understanding is that when bitstreaming's criteria are met, it skips all DSP (including output format), but when the criteria aren't met (i.e. a non-DTS source) then output format would kick in.  But that's just based on my understanding of how it works with DSD, you should try it with known sources and see what happens.  Hopefully some of the folks with more experience of external decoding will chime in.

DTS and DD over SPDIF are both lossy compression formats, BTW, so you might want to think about finding a way to get to HDMI as a long-term solution if that sort of thing (lossy compression) bugs you.

[6233638]

Okay then. My decoder has an HDMI input, but adding an output to my computer is a problem. Is there such a thing as USB to HDMI for audio? Seems sketchy.

Does your computer not have an HDMI or DVI port with audio support?

Not being able to compress my own DTS on the fly, how can I get applications to send DTS to a dummy audio driver that MC will pick up via loopback? I am guessing that if an app sees a 5.1 sound output then it will send compressed audio--if it has it--on two channels, and if I tell MC not to do any mixing then it will forward those two channels to my decoder via S/PDIF and DTS will actually work.

You need a soundcard with DTS Connect support to encode to DTS audio on-the-fly.
Otherwise, all you can do is bitstream audio which is already encoded to DTS, or use Media Center's Dolby Digital encoding. (which should do 5.1)

So I need to be able to switch between two audio modes on MC. And my decoder as well, but that is a given. I also need to be more aware of what audio streams are available.

There should be no need to switch formats if you set up Media Center to bitstream DTS/Dolby, and encode all other audio to Dolby Digital.
 
You can use Zones and Zone Switch for this, if you find it necessary though.
 
 
 
It might be easier to provide a solution if you can explain in detail what you are trying to accomplish.
E.g. what type of source it is you're trying to pass through Media Center - or are you just trying to send all system audio through Loopback?

[Vreejack]

Thanks for all the help. This audio stuff is starting to make sense. I don't mind lossy audio for videos.

My receiver feeds 7.1 speakers and can handle encoded 5.1 and has one hdmi port that I usually connect to the BD. Otherwise I am using TOSLINK for the computer connection.

My computer is a cheap Gateway that I bought for a song and a dance because I forgot to check its audio capabilities. It can handle stereo (Realtek) and has only simple audio in/out. Motherboard will not take cards, but it has a lot of USB 3.0 jacks. I can hook up a simple stupid USB-audio connector that looks like 7.1 and passes along TOSLINK to the receiver, but will this actually work? If I have an encoded AC-3 source, will the bitstream just pass on through from Silverlight without needing to use MC?

Maybe I should check again and see if my TV actually does pass bitstreams on through.

What do I do if I have a multichannel stream that is not compressed? Is that likely to happen? I am a little foggy on how many channels you start with in encoded streams and where the decoding is likely to happen. I suppose that the decoding takes place in the audio hardware, but of course I don't want any decoding at all because I am using an external DTS receiver.

If I am starting with two channels of compressed audio, then I assume that I do not need to worry about recompressing it. Should AC-3 just be thought of as a bitstream with a bandwidth that (hopefully) fits in S/PDIF?

I think I asked this in another thread, but my TV looks like a 2-channel system (like everyone's?) via HDMI, so wont the computer send it a stereo signal? If Netflix has AC-3 I want it to send AC-3, not some downgraded mess or audio that is missing the dialog track.

Oh, and here is the kicker. Silverlight is such a bloated cow that I can not really run it and loopback-MC at the same time. Not for HD video, anyway. You get what you pay for, I suppose.

Another option: Sound Blaster X-Fi Surround 5.1 Pro USB Audio System will compress it live in external hardware, saving precious machine cycles. But again, if I am starting with AC-3, why would I need this?

Back to tinkering. I found an AC-3 test file.

[Vreejack]

Maybe I am misunderstanding this. People are adamant that S/PDIF is 2-channel, but is what they really mean that it can only hand that much bandwidth? When you send AC-3 via TOSLINK is it actually in six channels of low bandwidth audio? Or is it sending six channels of audio squeezed into what would look like two channels of bad audio to a naive receiver?

[6233638]

Maybe I am misunderstanding this. People are adamant that S/PDIF is 2-channel, but is what they really mean that it can only hand that much bandwidth? When you send AC-3 via TOSLINK is it actually in six channels of low bandwidth audio? Or is it sending six channels of audio squeezed into what would look like two channels of bad audio to a naive receiver?

S/PDIF can only handle two channels of uncompressed audio. (up to 24/96 officially, though some new hardware will do 24/192)
 
Multichannel audio via S/PDIF uses lossy compression to fit into a two-channel audio stream.
This audio stream would just be random noise if you played it back without first going through a Dolby/DTS decoder.
 
 
To have the computer output 5.1 over S/PDIF, it has to be encoded to Dolby or DTS.
This requires a soundcard which supports either Dolby Digital Live, or DTS Connect.
Media Center also has the option of doing software-based encoding to Dolby Digital 5.1 via the Output Format section.
 
 
 
I don't know what your source is for this 5.1 audio going into loopback, but whatever player you are using, is decoding the Dolby/DTS stream (if there is one) to PCM for playback on your PC.
 
To output this over S/PDIF, it must be re-encoded as a Dolby/DTS stream.
There's no Dolby/DTS stream to pass-through, as the player has already decoded it before it reaches Media Center.
 

If Netflix has AC-3 I want it to send AC-3, not some downgraded mess or audio that is missing the dialog track.

Then the Netflix app has to output the bitstream to your sound card.
The app would be decoding the Dolby stream to PCM first if you were to pass it through Media Center. (or any other device)

[Vreejack]

Making more and more sense, thank you.  I just got an AC-3 test video to run properly by forwarding it through my TV's HDMI in --> TOSLINK out. It did not want to work at first, but then for no reason at all it worked perfectly.  I had to set the decoder to Dolby D. Hrm, maybe because I also had Dolby D encoding running on MC. But at least the S/PDIF passthru worked.

So it turns out that watching Silverlight or Flash I don't actually want to use MC at all if I can avoid it. But if Netflix has AC-3 as ....

#$%^ I just found out that Netflix does not even support surround sound on PC's. It did support Dolby Digital on my XBox-360, which recently broke, which is why I bought this cheap computer. 

Ah, but apparently there is a special app for Netflix for Windows 8. Here we go....   

[Vreejack]

No I was completely wrong about the audio through my TV. I was confused because of all the cable swapping and testing I was doing.  But you know what? I just pulled out my receiver (not an easy thing because of all the cables connected to it) and discovered it has two HDMI inputs and an output. I am off to Wal-mart to buy some cables. Fortunately it is three blocks away.