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My HTPC project has been a frustrating and yet enlightening process. I'm beginning to develop an understanding of how audio/video renderers, codecs, and decoders work, but I'm not quite there yet. I use Media Portal (MP) and I am dealing with inconsistent sound quality so I am seeking to understand WTF is going on. Certain AVI (encoded with MP3 sound) and live television (encoded with nVidia audio renderer, I think)are very loud with my receiver (Onkyo TXSR 501) set volume to 18. AVI that I've encoded and used the AC3 stream are much quieter and I need to turn the reciever up to 30-35 to hear things clearly. I have few friends with home theater systems. I've only heard 1 other person's (they had an Onkyo HTIB) and it sounded markedly better than my setup.


My motherboard uses Nforce 2 Soundstorm sound (Asus A7N8E-Deluxe). It is my understanding that that the nVidia NF2 boards can encode Dolby Digital (DD) sound. In this case, if you are passing through an AC3 stream from an AVI, it would be improper to use the nVidia driver to convert the stream to DD because the AC3 stream already has the 5.1 information? Is this correct?


In MP, I can use several audio renderers such as nVidia Soundstorm, Default Directsound Device, Reclock. I also have AC3Filter installed. I'm confused as to what AC3 Filter really does. Is it a decoder or a codec or a filter? It seems like it would be a filter, given the name but I'm not sure how it interacts with MP. When you set up AC3 filter, does all audio get passed through it? AC3filter allows one to pass certain streams like DD or DTS through SP/DIF. I can't find options to selectivly pass through streams that are already encoded with the nVidia configuration like you can with AC3filter. So in this case is it possible to prevent Soundstorm from encoding DD?


I also have to configure decoders in MP. Let's take the Dscaler Audio Decoder, for example. There are options to set things such as DD passthrough, and DTS passthrough and the type of speakers you are sending (i.e 2.1 or 5.1). If I've encoded a movie to an AVI with AC3 stream, do I need to specify the output? And if I do am I decoding/encoding it 2 or 3 times? Does the receiver or Soundstorm know if a AC3 stream is already DD encoded?-->BecauseIt seems like I am configuring the same settings on different software components of my HTPC (i.e set the speaker type and output on AC3Filter, DScaler, and Soundstorm), and I'm virtually certain that my sound is getting decoded more than once, which is resulting in a sub-par listening experience.


I've been trying a lot of things and tweaking... andnow my live TV has picture but no sound and MP is not producing sound when I try to play MP3 from my music collection. Also, AVI encoded with the AC3 stream seems to be DD encoded, but AVI with MP3 sound are output to Stereo only. Obviously, I need to further my understanding of these audio components so that I can untangle my configuration mess.
 

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I don't have any experience with Soundstorm, but maybe I can help a little with the rest.


AC3filter is an AC3 codec like Dscaler Audio Decoder. Only it has many more options as far as tweaking, speaker dependant delays etc. whereas Dscaler has pretty much the option of decoding vs. passthrough and I believe a group delay. Codecs in general take the compressed data (whether it is MP3, Ogg Vorbis, DD, AAC, etc.) and decode them into uncompressed bitstreams that the soundcard will understand. A lot of times the codecs can do some form of volume attenuation, if you see any kind of volume slider anywhere, make sure to keep it pegged at the max (at least for starters) and control the volume on the receiver. Same goes for all of the volume sliders in the windows audio screen. Once you get everything straight, you can tweak individual sliders so that your MP3s don't blow you away when you switch over from watching a movie to listening to music.


Most of the options and tweakability of AC3filter is only useful if you run the audio from the analog outs of your soundcard. If you run a digital cable to your receiver then you just need to select "SPDIF" or "SPIDF Passthrough" (which is the same for most all of these audio codecs. When this is selected, the codec only takes the data and sends it along unmodified as an AC3 (or DTS) SPDIF signal to your receiver, the same as a STB dvd player would.


I believe that soundstorm will passthrough DD/DTS streams unmodified, but I am not positive. I would assume their is at least an option for it to do so. Normally there is a check box like "Decode AC3 streams" if this is unchecked then the streams are just passed along for the receiver to handle (this is just generally how soundcard drivers are setup, again soundstorm may be different.)


Again, it is my understanding that soundstorm will pass through DD/DTS streams un-modified but will take all others (like multi-channel AAC or Vorbis), after their individual codecs have decoded them, and re-encode them into a SPDIF compliant AC3 stream to be sent out to your receiver.


As far as why one program uses one set of codecs and another uses a different, well that's just Bill G's sense of humor... ...seriousely, it has to do with the priority ranking of various codecs in the directshow hierarchy, a good place to read up on that (in addition to here) is over at Blight's webpage for Zoomplayer www.inmatrix.com I believe.


Personally, I would suggest playing around with the various codecs and figuring out which ones you want to use (you only need one decoder for each audio codec type). Once you have everything figured out, re-install windows fresh and only load those codecs.


-Suntan
 
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