It's evident that the sound quality of the integrated audio depends on the analog part rather than digital (that is, on the AC'97 codec chip). According to the AC'97 standard, the codec consists of two functional units. The first is a codec control circuit and hardware interface to AC-Link that distributes audio data. The second is the analog part that includes mixing and amplifier circuits and supports inputs and outputs. The two units collaborate via special converters that translate signals from digital into analog format and vice versa. The set of functions every AC'97-compliant codec must have is described in detail in paragraph 2.3 of the specification. I guess there is no need to enumerate every standard function and specification item. It wouldn't be an easy read then.
It may seem that such a strict standard should make the driver universal, that is, operational with any “compliant” codec. However, some of the specifications describe numerous requirements, which are simply recommended for further implementation. So, it is the codec manufacturer that decides which recommended functions should be included. That's the main reason why we see several AC'97 codecs available in the market. For example, the codec chip may include the amplifier cascade, frequency filter unit (to control timbres), stereo-base enhancement unit (3D Stereo Enhancement), digital (S/PDIF) output support units and so on. According to the AC’97 standard, the codec is controlled by user commands entered along the GPIO bus (General Purpose Input/Output). This command-entry ability implies that you can vary codec settings from a software control panel.
Any codec must support codec cascading. For example, some devices achieve multi-channel support by coupling several stereo-codecs. Each codec is assigned a unique ID to “know” its audio content. All expansion cards with rear satellites and subwoofer connectors are made in this way.
Note that mainboards don’t practically please us with interesting and highly functional codecs. The major determinative for each manufacturer is very often the cost of the codec.
Most Popular Codec: You Know It
Well, if there are popular brands of cigarettes or cars, there should be popular codecs, don’t you think so? Having studied the audio insides of many contemporary mainboards, I arrive at a conclusion that the today’s top codec is ALC650 from Realtek. This is the chip used by an overwhelming majority of mainboard makers.
Realtek ALC650 is the most popular codec
ALC650 is an 18-bit full-duplex six-channel AC’97 codec developed specifically for PC-based multimedia systems. You may think this presentation rather suggests not very high sound quality of the chip, its orientation at the low-end market sector. That’s not quite right, as the today’s low-end differs dramatically from what we saw just yesterday. Just look: the specified signal/noise ratio is 90dB, which is not bad for a “mass-market” codec (you can see the spectrograms for ALC650 in the “Performance” section). It supports three pairs of audio outputs with independent volume control. The codec is equipped with its own amplifier (50mV/20Ohm) for headphones, which shortens the list of electronic components and, accordingly, the overall mainboard cost. Of course, the integration of the amplifier into a single DIP-package is only profitable from the manufacturer’s point of view.