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I've read they were hardware audio codecs. But how are they different from mp3, aac codecs?

mp3 and aac codecs can also be implemented in hardware (can't they?).

What key differences do AC'97 and High Definition Audio have?

How are AC'97 and High Definition Audio used most of the time?

P.S.: This article mentions AC'97 for modems. Why does modem need Audio codec?

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The software codec and hardware codec are two completely different things that you're confusing. Wikipedia says that the proper term should be endec (encoder/decoder) for the soundcard chip that does analog to digital conversion and digital to analog conversion.Take a look at this and this. Unfortunately I don't have time for a proper answer now. – AndrejaKo Dec 10 '11 at 23:01
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From Wikipedia:

AC'97:

AC'97 (short for Audio Codec '97; also MC'97, short for Modem Codec '97) is Intel Corporation's Audio Codec standard developed by the Intel Architecture Labs in 1997, and used mainly in motherboards, modems, and sound cards.

Audio components integrated into chipsets consist of two components: an AC'97 digital controller (DC97), which is built into the I/O Controller Hub (ICH) of the chipset, and an AC'97 audio and modem codecs, which is the analog component of the architecture. AC'97 defines a high-quality, 16- or 20-bit audio architecture with surround sound support for the PC. AC'97 supports a 96 kHz sampling rate at 20-bit stereo resolution and a 48 kHz sampling rate at 20-bit stereo resolution for multichannel recording and playback.

HD Audio:

Intel High Definition Audio (also called HD Audio or Azalia) refers to the specification released by Intel in 2004 for delivering high-definition audio that is capable of playing back more channels at higher quality than previous integrated audio codecs like AC'97. During development it had the codename Azalia.

Hardware based on Intel HD Audio specifications is capable of delivering 192-kHz 32-bit quality for two channels, and 96-kHz 32-bit for up to eight channels. However, as of 2008, most audio hardware manufacturers do not implement the full high-end specification, especially 32-bit sampling resolution.

The reason the MC'97 is tied to the AC'97 is to provide interaction between the MC'97 modem and the AC'97 speakers/mic, for using it like a "voice modem".

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Can you show example how you would use AC'97 or HDA?? – DrStrangeLove Dec 10 '11 at 20:44
a) You'd use it when you were integrating Intel chip-sets into the circuit board (most likely a PC motherboard) you are manufacturing. b) As a user, you'd install drivers from Intel for your OS and enjoy the music. :) Beyond that, I'm not sure how you'd expect to "use" it? – techie007 Dec 10 '11 at 20:47
about b) - i can also enjoy music played with software mp3 or aac codecs. How can i make sure that i enjoy music via AC'97/HDA ?? – DrStrangeLove Dec 10 '11 at 20:53
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You don't, your drivers do. I think the problem here is that you are thinking hard/firmware codecs (AC'97, HDA) are the same as a software-level file (compression) format codec (MP3, WMA, etc.). Perhaps check out Wikipedia's Codec Aticle for more info, as not all "codecs" are created equally. :) – techie007 Dec 10 '11 at 21:03
does AC'97/HDA use compression?? – DrStrangeLove Dec 10 '11 at 21:06
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