Chris Deckard (l) and Bob Smith (r) led discussion on compressors at PNW Section Nov. 6, 2014 meeting.
PNW AES Section's November meeting featured a dynamic give-and-take about audio compressors. Presenters were Christopher Deckard and Bob Smith. The meeting was held at Shoreline Community College (Shoreline, WA) and about 50 people attended (20 AES members).
Chris Deckard, who recently left a job as a Mackie DSP engineer, realized that many people use compressors as a tonal tool as well as a dynamics tool. He showed a compressor analogy diagram (by Rick Chinn), showing a guy sitting on a hot plate, changing the volume based on how hot his butt gets.
Chris covered basics of compressors, like ratio, knee, attack/release times, as well as detectors - peak vs RMS, and different VCA topologies using feedforward, feedback, and lookahead configurations. Participants discussed the old trick of playing an analog tape backwards with a compressor/limiter or noise reducer, then using the result forwards, offering processing of reverse transients that might be compared to lookahead in some ways.
Much interaction with other attendees with experience in commercial compressor designs gave many additional insights, as did those asking about fundamentals of operation.
Looking for more insight into what makes a compressor sound like it does, Chris described some circuit configurations that are used for their sound. Some contributors to different sounding compressors include the optical system, like incandescent lamps, neon lamps, LEDs, electroluminescent devices, use of FETs, different VCAs, VariMu, use of transformers, tubes, or solid state. There was a lot of audience discussion of circuit nuances, and Chris noted that some critical parts used in the circuits have highly variable performance even when new.
Chris wrote a Matlab compressor simulator with graphic interface for research. He did several demos with different parameters, showing performance changes like THD with various settings. Active audience discussions included differences in limiters vs. compressors, and knee characteristics. The Matlab app could also render a sound file using the desired settings.
After a break, door prizes were awarded.
Next, Bob Smith (Physio-Control and BS Studios) performed some practical hardware compressor demos using an RNC 1773. He changed settings and showed results on waveform screens as the audio demos played. He noted that such processing exposes noise problems from poor quality hardware/technique.
