About Compression

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9. About digital 10. Ab. compression 11. About saturation 12. About filters 13. About delays 14. About chorus 15. About switching 16. Synthesis

We have said above that dynamics is a critical characteristic of a sound that allows the human ear to recognize an instrument. Dynamics allows hearing an instrument within a mix, or a voice amongst instruments.

The other characteristic is the timbre: the proportions of harmonics added to the fundamental frequency of a note. But altering the timbre (spectrum) of sound is possible, and the human ear will still recognize it’s a guitar, a piano or a cymbal.

What about altering the dynamics of a sound? If one suppresses the envelope of a sound, or changes it in a manner it cannot be recognized, it will, of course, be impossible to tell what the original instrument is. The best example is certainly the “violin effect” that a guitarist can achieve by raising his volume pot from zero at the beginning of every note (listen to “Survival” by Yes for an excellent example).

Respecting the dynamics of a sound is therefore critical. But respecting this dynamics doesn’t mean it’s impossible to control it. Most of the modern instruments – drums, electric guitar, electric bass – have huge dynamics: the amplitude variations can go much beyond what the human ear likes to receive.

Dynamics control is all about time constants:

  • Tweaking the volume over the duration of a song is just a subtle effect, used to add life to the music
  • Change the volume of an instrument within a few bars will be heard as a technical problem
  • Change the strength of notes within a bar will be heard as a instrumental problem
  • Alter the volume within the duration of a note will radically change the sound of the instrument

There are three basic dynamics processors:

  • Compressors
  • Expanders
  • Gates

Since the major issue with modern music is to tame the huge dynamics of instruments, expanders are rarely used. Gates are very specific tools that can be used to modify the sound of a drum or a cymbal, for example, and are not used with guitars.

Joe Meek is certainly the man who has explored and settled the foundations of this process, and his circuits are still in use today. Compression is widely used in modern music, for the best and the worst.

Compression for the worst. A compressor can kill the life of an instrument track, cause playing soft or strong is a critical means of expression for a musician. It can also kill the musical message of a track, or of an album. The radios that broadcast music must compress the music, for technical constraints. The DJ’s have to compress too, to get this high volumes that people want in a disco. But when you really want to listen to music the way it was meant to be, you don’t want any compression.

Compression for the best. Compare drum tracks recorded in the sixties or the early seventies with what we do today. First of all, the miking techniques are much more sophisticated than before, and almost each instrument of a drum kit is now recorded separately. This allows applying compression, but also gating and expansion on a case by case basis (and reverb, echo, EQ…). Drum tracks have gained expression, musicality, and most of all, clarity.

Of course, top-rank drummers don’t really need that: people like Manu Katche, Steve Gadd, or basically, every experienced jazz drummer have a sufficient control over their hands and feet to sound good even with two mikes only in front of them. But in the real life, where drummers are human beings and not aliens, dynamics processing and compressors are almost mandatory when a clean and strong drum track is desired.

Compression for guitars. A compressor can work as a limiter (put the peaks under control), as a sustainer (increase the notes decay time), or both. With a short response time, a compressor allows leveling all the notes in a guitar part: this is extremely interesting for arpeggios, for example, or during a very fast lead part, when the right hand can “miss” the pick of a note.

Tom Scholz knew all that, of course, when he started thinking about building new gear for Boston. He has registered a patent which is, in my opinion, one of his most interesting publications: US 4,627,094

Plug a guitar directly in the soundcard of your PC, and record what gets out of it. You won’t like it… the sound will be thin, even if you boost it. Boost it again, and it will clip and sound even more horrible.

Take a compressor, and do it again: you will lose in expressivity if the compression is too strong, of course. But all in all, it will sound much better. More “natural”. The sound is almost acceptable. Add some EQ, some reverb, and you’re done: you have a real guitar sound. No amp, no saturation, no speaker, no mike: just a little box with a few components…

The explanation is fairly simple, and is explicit in Tom Scholz’s patent: “On a guitar, the first sound or pulse that comes out can be a huge peak which is almost always much stronger than the signal which follows within a few milliseconds. A guitar amplifier tends to smooth out these sounds because it cannot respond to them fast enough, because it clips (distorts) large signals, and because the speakers have a slow response” (Scholz – US 4,4627,094 – col.7 – l.25 to 30).

This paragraph is probably the most important for someone who wants to understand the Rockman line. Tom Scholz has created the first realistic solid-state amp simulation, because all the Rockman preamps, from the first Rockman headphone amp to the last SR&D product – the Ultimatum Distortion Generator, start with a built-in compressor.

This compressor acts both as a limiter and a sustainer, but the limiter aspect is what gives this natural feeling and this loudness: since the peaks are smoothed out, the rest of the notes can be boosted without clipping. The sustainer function is only additional comfort if you have weak pick-ups, but it doesn’t have the same criticity: getting pick-ups with a high output level will provide better results.

The rest – distortion, filters, BBD’s – was pretty straightforward in comparison: that’s what all the other manufacturers actually do, with different approaches and results. SR&D has “only” developed high-quality devices. But the compressor inside every Rockman preamp is the key of SR&D’s specificity.

Compression was the basis of the Rockman DI approach, thus allowing playing without amps nor cabs.

Copyright Rockman.fr 2007