AC '97 initiative targets 2 design areas

Electronic News, March 2, 1998 by Bob Grisamore

Emerging audio programming interfaces such as DirectSound, DirectSound3D and MIDI Wavetable synthesis, as well as the emerging audio decompression technologies such as Dolby Digital and MPEG-2, have the PC doing much more audio signal processing than before.

The AC'97 technology initiative, led by Intel, is an attempt to address these issues for the PC OEM and the hardware silicon manufacturer. The AC'97 architecture splits the audio function into the digital signal processing function and the mixed-signal codec function.

Standard AC'97 codecs have four stereo analog inputs and four mono analog inputs. They also have a stereo analog-to-digital converter (ADC) and a stereo digital-to-analog converter (DAC). Each analog input is independently mixable in the analog domain with the DAC analog output to generate the line output signal. The architecture of AC'97 codecs calls for a high quality analog mixing path and a high quality digital PCM output and input path.

The PCM playback path has been designed to play back audio from PC sources, which always require stereo output signals, while the PCM record path is generally used to record microphone, CD audio, or other audio inputs. This path has typically been overlooked in the past for PC audio, but its use is becoming more widespread as more applications involve users speaking into a microphone. Voice recognition and echo-canceling telephony software require a particularly high quality input PCM stream.

The split AC'97 architecture can meet high levels of audio performance much easier than the fully integrated controller/codecs in today's ISA-based products. Still, what does it mean to have a high quality PC audio system these days? According to the PC'98 audio requirements, the three major metrics are dynamic range, total harmonic distortion plus noise, and frequency response.

The dynamic range specification generally indicates the amount of noise in the audio system that a listener will perceive. The Microsoft PC'98 section on audio quality defines dynamic range as "the ratio of the full scale signal level to the RMS noise floor, in the presence of signal, expressed in dB FS."

Noise in an audio system comes in two types: either from the intrinsic properties of the analog-converter path which an IC designer must know and compensate for before designing, or from external effects which a system integrator can minimize by proper board level design and component layout practices. One of these external effects comes from the switching power supply of the PC. But, proper power distribution and design for high power supply rejection can minimize these effects.

The total harmonic distortion plus noise (THD N) specification generally indicates the accuracy a system exhibits when reproducing a sound. Designing systems to minimize the THD N requires some additional small, but important tricks. Meanwhile, frequency response at high frequencies is generally limited by the DAC and ADC converter performance, internal filter response and external filter response. For the system integrator, designing output circuits to achieve good high end response is done by properly setting the low pass filter pole. FCC out-of-band tests generally require low pass filtering of out-of-band signals. Oversampling DACs, typically used in audio codecs, may also require additional output filtering. As a result of this analog filtering, the digital filter response may need to compensate for this in-band filtering.

And in echo-cancelling audio systems or line cancelling telephony systems, group delay can be a very important consideration. Longer group delays will lengthen the required filter and that will increase processor DSP MIPS or hardware filter implementation costs.

In order to meet the demands for even better PC audio, Intel and many leading AC'97 licensees established a set of extensions for AC'97 and dubbed them AC'97 2.0 extensions. The extensions address limitations as well as define new features and the AC-Link slot infrastructure needed to support these features. With the extensions, the breadth of AC'97 functions and configurations makes designing ICs to implement these functions difficult.

Audio system designers must work very closely with their target OEM customers. But by proper IC design and system integration in AC'97 systems, we raise the performance bar for PC audio higher.

Mr. Grisamore is systems engineer in the PC Products division at Cirrus Logic.