When do you used and A/D converter and how....

How does the AD converter work, what's it for?
When would you need to use it?
One good example for the use of a analog to digital converter would be for converting a vinyl record to a CD or another digital source. A CD recorder has a A/D converter in it to change it to a digital format.Recording studios use them to record analog music to their digital recorders.All music starts out as analog.
I forgot to mention,digital synthesizers produce digital sounds that start out as digital.
most of your garage bands these days use digital recorders to capture their music. Typically the mic feed (which is analog) goes into an interface box where the A/D conversion takes place. Check out the professional musician sites and you'll find TONS of alternative A/D converters for this purpose. The quality of the microphone, the quality of the A/D converter and the sampling rate have a lot to do with the ultimate quality of the sound upon playback.

In a home stereo system, you may never need to convert to digital if you play analog sources such as vinyl LP's, reel-to-reel tape or FM radio (non HD). But if you want to put all your music on a computer, you'll need to convert those analog signals to digital somewhere. Hence the need for an A/D converter.

If all you do is playback, you probably don't need one.