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.