Sunday, 9 February 2014

A Limiting Solution part 1

Feedback comb filter structure
Feedback comb filter structure (Photo credit: Wikipedia)
Comber 0v01, my first MaxForLive comb filter suffers from a problem common to many systems with variable gain that include feedback. It has a tendency to go into oscillation, and because of the many modulation sources, the exact point at which it becomes unstable isn't easy to predict. Worse, the best sounds tend to occur just before it goes into the red.

One of the traditional techniques that are often used to solve this is to use a compressor or a limiter in the feedback loop, so that the gain variation is controlled. The implementations vary - one old method involved using an incandescent light-bulb and a light-dependent-resistor. Max provides library objects that do limiting and compression, but they have a User Interface (UI) issue: too many inter-dependent controls. Limiters and compressors are powerful tools, but adding lots of controls to Comber just to make it more stable would make an already complex UI even more busy. So how could I add the minimum of controls but keep lots of versatility?
I decided to try using one control to make several changes simultaneously in a limiter. As the input gain increased, the limiter threshold would be reduced, and the output gain would increase. This way, the maximum level stays the same, but the amount of limiting that is applied can be controlled. I also enabled one additional control for time response, and here the library object provided fast and slow modes. So I had one rotary control and one switch, plus a 'Bypass' switch to remove the limiter from the output.

The new controls could just be squeezed into Comber, and the testing showed that it made the effect much easier to use, although the limiting does alter the sound slightly, and on the higher settings, it does alter the dynamics quite a lot. And that's how Comber 0v02 was created!

Using Comber

Comber has a lot of controls, so here's a quick guide to getting the most out of it.

  1. Start with the Feedback controls set to zero, the LFO Depths set to zero, and the Pan Depths set to zero. Dry/Wet should be at 100% (Wet), Make-up Gain should be about 100%, and the Comb Gain controls should be at their default 50 position.
  2. Try the Comb 'Freq' (frequency) control. This moves the notches up and down in pitch, and can make big changes to the timbre of the input.
  3. Experiment with increasing the Comb Feedback controls from zero, and see how they change the sound.
  4. Use the LFO Depth control to move the Comb Freq(uency). Moving combs always sound much better than static ones! 
  5. The Pan Depth controls move the pan position of the two channels.
  6. The Limiter control starts out with no limiting at zero, and increases the amount of limiting (lower threshold) as you increase it towards 100. The maximum is a very non-subtle effect! 
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