To understand why digital over S/PDIF is not automatic "awesome audio" (and hopefully to make this thread more useful for future references), you need to understand how the digital audio is being played back on today's systems. The digital audio reproduction pipeline, if you want to call it:
- Storage: Audio stored in digitally encoded (and most likely compressed) format(s)
- Playback: Digital data is decoded and decompressed to "raw" PCM data. There can be stages within called "transport" where the data travel along your setup in a variety of formats, involving transcoding and recompression, etc.
- Post-processing: I think this only works on decoded PCM data. Things like adding reverb, upmixing, etc.
- Digital to analog: digital PCM data need to be converted to analog electrical signals before reaching the speakers
- Listen: Speakers vibrate (or whatever other means to shake the air) to create the sound waves which your ears will receive them
Only the first and last stage are fixed. In between you can have all sorts of stages and variables which you need to figure out. Optimizing audio quality involves choosing components for the various stages and to choose where exactly in your pipeline you want stages (2)-(4) to happen.
So let's take two examples and run them through the pipeline: playing MP3's (or in-game audio from your computer) and playing movies from an optical storage medium.
Playing MP3's or any computer-generated audio: MP3 is a format that does
both compression and encoding. So when you fire up your player application they are already doing the decoding+decompression. The result is "raw" PCM audio data (think of it as the wav file you rip from CD's). If you have a discrete sound card, you can either: a) use the D/A converter circuitry on the sound card (and also any post-decode-processing features of the card before the D/A process) and output the analog signal to the receiver/amplifier/speakers, or b) output the PCM data directly to a receiver/processor that can do the D/A conversion for you. In this case, audio quality is determined by the post-decode-processing (if you are just playing regular audio you may not want any of that) and the actual D/A circuits. A discrete sound card should feature better D/A circuitry than onboard. For now, what you are doing is using the D/A stuff with the onboard audio chip and you are connecting to the analog output.
If you want to use the digital output onboard or from a discrete sound card, then what you are doing is to transmit the PCM data over to something else for later processing+D/A conversion. For playing regular 2-channel audio this is fine, because S/PDIF can carry 2-channel (already uncompressed) PCM data no problem. All you need to do is to set the drivers to pass that PCM data untouched instead of doing the D/A. However, things get messy when you want some processing in the computer, such as upmixing from 2-channel to 5.1, or your game is giving you 2+ channel of audio data. To transmit this larger amount of data over S/PDIF, you need to recompress+re-encode the PCM data into something like Dolby Digital or DTS, which the "DDL feature" of a discrete sound card (or the venerable nForce2) will do for you. However, compression in DD and DTS is lossy, meaning if you are playing back a MP3 audio file you are lossily compressing the audio twice which should most certainly affect quality. This is where HDMI comes in, because it can carry multi-channel PCM data to the next stage in your pipeline. So this is the case when blindly going S/PDIF digital may hurt audio quality.
Now, on to playing movies from an optical medium: the audio data are already stored in DD/DTS/TrueHD/DTS-HD MA so there are 2 ways of playing the audio. First is to decode+decompress it in your player application (we are talking HTPC here after all), which means you are dealing with PCM data already. How to send that PCM data? Refer to above. The second way is to transmit that encoded+compressed data
as is for something like a receiver to process. This is known as bitstreaming. So if your movie has a DD/DTS sound track, you can set the player application to bitstream the data directly outside the HTPC. For the new fancier TrueHD/DTS-HD MA tracks (they are encoded but
losslessly compressed audio data btw), you can only bitstream them over HDMI. Decoded multichannel PCM data can still be transmitted over HDMI as described above.
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For your current situation, if you are getting a Xonar, then what you are implying is to do everything up to the D/A conversion on the sound card. This is the cheapest upgrade that you can do because the Xonar should give you better audio quality compared to the onboard audio chip. The reason it is cheapest is because you can use your existing speakers. The reason for you to switch to S/PDIF digital is a) to bitstream DD/DTS data from your movies on optical medium, b) to send realtime DD/DTS-encoded (and lossily compressed) audio from games/music, and/or c) to send 2-channel PCM "raw" digital audio. Audio quality in some cases may be
worse because of transcoding so this is not a silver bullet. This also requires another stage in your pipeline to process the digital audio data, usually an A/V receiver. You are going to be investing in home theatre-grade speakers as well so receiver+speakers is a big jump in cost.
HDMI is another animal but think of it as a digital output like S/PDIF without the bandwidth limitations. The HD3650 claims to have support but it is sort of messy (IIRC you need to route S/PDIF from your sound card/chip to the video card and you need a special DVI-HDMI converter which means no new fancy 7.1 formats) unlike the HD4xxx series. I would say you need to budget for a video card upgrade (preferably with native HDMI output too) in addition to the receiver+speakers.
So to find an answer to your current scenario you have to consider a number of things:
- Do you need the flexibility of a receiver (primary example: multiple inputs) down the road? Like a console and other audio playback components?
- Are you going to get into the new fancier audio formats on Blu-ray anytime soon?
- Can you really distinguish the differences between what you have and what you think you are about to get?
- Budget
- And others...
Sorry if this is a long one. I hope I am not boring everyone to death.
Edit: FireGryphon has written another excellent
post on a similar topic, but he came from the angle of wires and DACs. This should complete the picture for you.
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