Editor’s Pick: Automating audio codec filter design

Written by Kenton Williston December 04, 2009 Hits: 534 Favorites

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Kenton’s Score-O-Meter: 5 of 5. This MediaTek article is a fascinating look at the guts of audio encoder/decoder (codec) chips and the design process behind them. It explains why MATLAB’s automatic hardware generation beats hand-written RTL—including some rather surprising data points about area efficiency. This is a must-read for anybody designing the digital side of a mixed-signal chip.

Review: MediaTek tips its hand in this article on its audio codec design process. I found the whole affair absolutely fascinating. I learned a lot about the guts of codecs and got several surprises along the way. For example, the hand-designed DSP logic was surprisingly painful to lay out:

In our previous design methodology, we used a single multiplier-accumulator (MAC) and a finite state machine sequencer to implement the signal processing chain… …On occasion, we also found that the RTL code for the sequencer finite state machine was almost impossible to place and route. There were about 2,000 states in the finite state machine with more than 40 variables assigned in these states. This resulted in a very low area utilization ratio of 10% during placement and routing of the gate-level netlist. In other words, the proposed silicon area was 10 times greater than anticipated!

The article also makes an interesting point about the increasing area available for digital logic in mixed-signal chips:

Changes in fabrication technology are shifting the balance of analog and digital components on the chips that we design. While analog transistors must maintain a certain size to drive their required loads, digital transistors shrink when a new fabrication process is introduced. When migrating from a 0.25um to a 45nm fabrication process, for example, the overall area of a mixed-signal audio codec went from 50% digital and 50% analog to 25% digital and 75% analog.

There is much more terrific detail in the article.

Bottom line: This is a must-read for anybody designing the digital side of a mixed-signal chip.

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Kenton Williston