Meeting held June 18, 2003
The PNW Section held its June meeting and season-ending business meeting with elections on June 18, 2003. Some 50 people, including 20 AES members eligible to vote for section officers, found their way through some serious road construction to Rane headquarters in Mukilteo, WA.
Rick Chinn invited everyone to briefly introduce themselves. Elections for the 2003-04 officers and committee were conducted. The results are posted on the PNW website.
Dennis Bohn, principal partner and VP of Research and Development at Rane Corp. was introduced as the first speaker. Mr Bohn is the author of the Rane Pro Audio Reference Book, as well as the classic Audio Handbook, which he wrote while at National Semiconductor. He has also worked at Phase Linear Corp., and is a Fellow of the AES.
He began with a quick overview of first-year electrical engineering. "Electronic filters are all about the math....you WILL love the math..." his slide exhorted. He went on with the simplified LaPlace transform used to represent complex impedance , and the formulas used to model resistors, capacitors and inductors. Then, the transfer function and poles and zeroes, leading to filters and their orders. The characteristics of the basic low-pass, band-pass and high-pass filters were shown, then the Butterworth, Bessel and Chebyschev configurations. There was a comparison of active and passive designs and their common uses in audio, especially crossover networks and 1/3 octave equalizers and tone controls.
A special request from PNW Chair Rick Chinn was an examination of the need for low-cut and high-cut filters in an equalizer. One cannot expect to get decent results from just pulling the end sliders on a graphic EQ all the way down. It was pointed out by Dennis that pulling down an end slider results in a shelving response, and not very effective at low or high-end roll-off.
Dennis continued with a comparison of implementing filters in the analog domain versus digital with DSP. Probably the major advantage of analog filters is speed of response, however, digital filters allow incredible flexibility. Such flexibility led Rane to develop an answer to an old problem - the "truth in slider position" of the graphic equalizer. The interaction of 1/3 octave graphic EQ filters creates a response curve that belies the position of the sliders. Various methods have been used to minimize the effect, but only the introduction of DSP filter techniques has led to such a good result.
A refreshment break was held, and door prize winners were drawn:
Rick Jeffs then explained how theory was put into practice in the Rane DEQ 60. Rick is a senior design engineer for Rane, having previously been a Rane test and manufacturing engineer, and an amplifier design specialist at Carver Corp. The DEQ 60 looks pretty much like an ordinary graphic equalizer on the outside, but implements all filtering by DSP. Jeffs demonstrated the difference in sound and operation of the DEQ 60 from traditional analog EQ designs, which he attributed to the elimination of passband filter interaction error. If desired, the unit can operate with the traditional error factor.
The Pacific Northwest Section extends its thanks to the presenters for the fascinating session, and to Rane Corporation for providing the meeting location and generously donating the many door prizes.
-Reported by Gary Louie, AES PNW Secretary
Last modified 2/5/2004.