In your case, the Realtec codec is used for final output, and the MCP-T is used for sound calculations.
The ALC650 can take a combined digital signal from the MCP-T and break it up into separate analog channels for your speakers.
Without the MCP-T, the CPU would have to make such calculations based on software, and usually does a fairly poor job with a significant performance loss.
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Thank you for your answer. How will I then know that I use the superior Nforce Audio and not the slower realtek chip?
By reading the manual I get the impression that I can choose which soundchip to use by either connecting the audiocable directly to the motherboard or use the spdif-thingie:
http://www.tomshardware.com/mother [...] -audio.jpg
If you use analog, you're using both. The MCP-T is a DIGITAL sound processor, it does things like 3D imaging and so forth. All the Realtec codec does is divide that combined digital signal into separate analog signals.
If you use S/PDIF output, you take the digital signal directly from the MCP-T. If you use the analog outputs, you take the digital signal from the MCP-T THROUGH the Realtech codec.
So you see that either way you're getting sound from the MCP-T. All digital soundcards, even the Audigy 2, use codecs for converting the combined digital signal into separate analog signals.
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Don't worry the Realtek acts as a Codec here i.e. a combination of a Digital-to-Analog Convertor and an Analog-to-Digital Convertor. When playing any sort of sound on your PC, it gets processed by the MCP-T and then the digital signal that's produced is sent to the ALC-650 for conversion to analog so you can actually hear it (your ears can't 'listen to' a digital signal, now can they). It's the same as any other sound card. They have a sound processor (a special digital signal processor) normally (some rely on the CPU like the Hercules Muse series) and all of them have DAC/ADC chips that do the conversion. The only difference is that the Realtek ALC650 is a shitty quality convertor.
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