Most people think of HDTV as just improved picture quality, but another great improvement is the digital sound. Many HDTV shows are broadcast in Digital 5.1 surround sound and all HDTV tuners support 5.1 sound. The good news here is PC digital sound cards are cheap and can be either internal PCI cards or external USB devices. Prices range from around $20 to over $100 depending on the bell and whistles you want. The low end cards work just fine for most applications. You can connect them to a 5.1 PC surround sound system using three 1/8” plugs or some have S/PDIF optical output or coax output for connection to your home surround sound system.
Some of the newer graphics cards such as the ATI HD Series have HDMI output which includes 5.1 digial audio output as an option as well as video.
Here's
a review of one of the ATI HD cards that also is a digital tuner.
Many newer motherboards also have built-in digital output either S/PDIF or HDMI.
