Dr. AIX's POSTS — 29 July 2014

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Compression vs. Compression? Part I

The issue of compression can be confusing. We certainly saw that in the Harman sponsored film “The Distortion of Sound”, which seem to dwell on the ills of MP3 data compression as a primary culprit in the inability of artists to get the “soul” of their music to their fans and then provided a very weak demonstration of audio compression instead. I know the engineers at Harman and JBL are some of the smartest in the business so how did the message in the film hit the wall and burst into flames? I’m still waiting to hear back from some of my friends at Harman…but the best I can say so far is the filmmakers were asked to exaggerate the facts to make their point.

So today, I thought I would provide the facts about audio compression. I’ll follow this up with a post on data compression soon (the lossy kind that was referred to in the 22-minute film).

Audio compression is a pretty simple concept. I usually explain it to my students as an automatic “turner downer” mechanism built into the signal path. If the compression/limiter circuit detects a signal that it knows will cause an overload at some subsequent stage in the signal path, it automatically attenuates (turns down) the amplitude of the signal by a specific ratio or amount. Light compression uses ratios from 2:1 to around 8:1. For ratios of 12:1 or 20:1 it’s referred to as limiting. Seems simple enough, right?

In the days of analog recording on tape, one of the first things a good engineer would do is ask the ensemble, conductor or band to run the loudest section of the piece to be recorded (for a live concert this could be the loudest section of the loudest piece in the program). He or she would then set the microphone preamplifier and/or the recording level of the analog deck so that those peaks would remain safely below the overload level of the meters…usually 3-6 dB below to guarantee that the performance adrenaline wouldn’t kick it up past an acceptable level.

Knowing that the dynamic range of analog tape is around 65-72 dB, pieces with very loud and very soft sections could be problematic. In order to ensure that the loudest sections were safely recorded meant that the softest ones might find themselves in the noise floor of the tape. I hate to admit it but there have been times during a live concert recording when I realized the level that I set was too high and that I was going to overload the tape if I didn’t turn down the record level (which definitely a no-no in the world of concert recording…after all I’m screwing with the dynamics that the conductor or performer is producing). So I would very slowly turn down the recording level during the recording. This is same concept as a compressor, although a compression circuit does this automatically and much more quickly.

To be continued.

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About Author

Dr. AIX

Mark Waldrep, aka Dr. AIX, has been producing and engineering music for over 40 years. He learned electronics as a teenager from his HAM radio father while learning to play the guitar. Mark received the first doctorate in music composition from UCLA in 1986 for a "binaural" electronic music composition. Other advanced degrees include an MS in computer science, an MFA/MA in music, BM in music and a BA in art. As an engineer and producer, Mark has worked on projects for the Rolling Stones, 311, Tool, KISS, Blink 182, Blues Traveler, Britney Spears, the San Francisco Symphony, The Dover Quartet, Willie Nelson, Paul Williams, The Allman Brothers, Bad Company and many more. Dr. Waldrep has been an innovator when it comes to multimedia and music. He created the first enhanced CDs in the 90s, the first DVD-Videos released in the U.S., the first web-connected DVD, the first DVD-Audio title, the first music Blu-ray disc and the first 3D Music Album. Additionally, he launched the first High Definition Music Download site in 2007 called iTrax.com. A frequency speaker at audio events, author of numerous articles, Dr. Waldrep is currently writing a book on the production and reproduction of high-end music called, "High-End Audio: A Practical Guide to Production and Playback". The book should be completed in the fall of 2013.

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