Originally Posted by DonTHB
Maybe dadudster can add to this.
Naw, you did a pretty good job summing it up. The problem, I think, is mostly legal. The RIAA and MPAA don't want you to be able to decode DD/DTS (or better) signals from an input stream on a computer because then it would allow you to make PERFECT digital copies of their media without having to "break the law" (by bypassing their encryption).
Although there is always the issue of lag with software decoding, computers these days could probably still software decode fast enough for the sound to sync basically correctly (although a slight lag will inevitably still exist). If you look at my "Inconvenient Truth" thread on this forum related to that problem, you will see that Media Player Classic is actually able to recode a 2 channel PCM stream into 5 channels without a noticable delay in the sound. Decoding an AC3 stream is really no more time/processor intensive then recoding a 2 channel PCM stream, so it is feasible that a software decoder (like ffdshow) could handle it in "real enough" time.
However, the bigger issue here has to do with DirectShow messaging flags that are set by the audio chipset specifying the input signal type. In the case of the Realtek chipset, while it can recognize the type of signal being input via SPDIF, the chip has been designed to "block" that signal from being accessible by software decoders by setting the DirectShow messaging flag to indicate that the signal is just a 2 channel PCM steam (instead of an AC3 stream wrapped in a 2 channel PCM stream). As a result, for most of these sound cards, the only option you are given is to simply pass-through the AC3 signals to the SPDIF-Out. The signal is still technically accessible to your computer (as demonstrated by the popping noises that come out of your speaker if you try to have ffdshow decode the signal in graphedit), but since software decoders use the DirectShow messaging flag in order to determine HOW to decode the signal, the result is that software like ffdshow ends up decoding the signal incorrectly because it thinks it is a standard 2 channel PCM stream instead of AC3.
If you were to design your own software solution that just ASSUMED the SPDIF-In signal was AC3, then you might be able to get something to work. However, then you would have the problem that ALL signals sent to the SPDIF-In would HAVE to be AC3 (and more specifically, would HAVE to be either DD or DTS, but not both).
Again, the technology is already there to do everything we want, but this is mostly a legal issue. My guess is that the music and movie industries are using the DMCA (POS law, btw) to threaten the hardware manufacturers and none of the hardware manufacturers want to risk being sued.
Originally Posted by blue389
What if I buy a soundcard that has coaxial SPDIF IN and can decode DOLBY/DTS 5.1? Will this allow me to receive surround audio from the cable box through spdif coax and use the WinAudio tool or Win7 to route the audio to the AMP card?
Now, IF you manage to find a hardware component that will support AC3 pass-through (even if only to a software decoder) then you will have accomplished what many of us have been unable to accomplish, and I will say "kudos to you." It has been claimed that these hardware components will allow for software decoding, but I have not been able to confirm this myself:
- Sabrent USB-SND8 8-Channel 3D USB 2.0 External 7.1 Surround Sound Box with Digital Output
($25) I am planning on testing this one out myself and will get back to you if it is successful.
- ASUS Xonar HDAV1.3 Deluxe 7.1 Channels 24-bit 192KHz PCI Express x1 Interface Audio/Video Enhancement Combo Card
($239!!!!) I don't have the money to afford one of these (although it does look REALLY cool).
One last thing to note, the D2Audio chip on the Media Live Diva does not have any DD/DTS decoding capabilities. While, from what I have been able to gather from JavaJack, the D2Audio chip has a lot of really cool functionality and capabilities, it seems like it is currently being underutilized by the Media Live Diva. It seems that the Media Live Diva relies mostly on the Realtek chipset for the majority of the handling of sound. The D2Audio chip is really only used to drive the 5.1 amp card (or 7.1 analogue pass-through card), and even then, the D2Audio chip isn't doing ANY additional signal decoding. It is basically just taking the "analogue" signal sent from the computer, applying some "filters" (via equalization, renormalization, etc.), and then passing that signal to the speakers.