Monday, April 8, 2013

Comb Filtering

(I'm writing this article as part of the Introduction to Music Production course on Coursera.)

Comb filtering is an interesting phenomenon. When you play audio back with itself at a very slight delay, it causes interference patterns. Most often, this occurs when a reflective surface is near your microphone.

In my studio, I took a speaker and played white noise from my computer through it. Then, I placed a microphone near the speaker, and sent that audio back into the computer. I used the channel EQ plugin to look at the frequency response of the audio after it went back into the microphone.

Then, I took a reflective surface (a dry erase board, in my case) and gradually brought it closer and closer to the microphone and speaker. As the board gets closer, it takes an increasingly shorter time for the sound to bounce off the surface and get to the microphone. As the delay gets shorter, we can start to hear the filter effect more and more.

If you watch the frequency response displayed on the screen, you'll see notches and peaks begin to appear at regular intervals. This is where comb filtering gets its name, because the frequency response looks something like a comb!

Monday, April 1, 2013

How dynamic processors work

(I'm writing this article as part of the Introduction to Music Production course on Coursera.)

In my last blog article, I mentioned dynamic effects as one of the main categories of plugins, and explained that they function as a sort of automatic volume control. Today, we'll look at the four main working parts of dynamics processors: threshold, ratio, attack, and release.

The threshold setting describes when the plugin kicks in. It is usually given in dB units. In a downward compressor, when the audio signal reaches the threshold, the compressor begins reducing the signal's gain.

The black curve represents the original audio
signal, and the red line represents the threshold.
The ratio setting describes how much the sound is modified. A 2:1 compression will reduce the signal above the threshold by half. This means that if the input signal is 2 dB over the threshold, the compressor will reduce it to 1dB over the threshold.

The blue curve represents the compressed
audio,  reduced by a ratio of 2:1.
The attack and release parameters control how quickly the plugin reacts. In a downward compressor, the attack is how long it takes for the compressor to start reducing gain after the threshold is reached, and the release is how long it takes for the compressor to stop reducing the gain once the signal drops below the threshold.

Compressors, limiters, expanders, and gates all have these four parameters. Limiters function very similarly to compressors, but have extremely high ratios. Basically, they never allow the audio signal to get more than a little bit above the threshold. Expanders are sort of like the opposite of compressors. The reduce the signal when it drops below the threshold, which increases the dynamic range, making quiet signals quieter. Gates are basically extreme expanders. Whenever a signal drops below the threshold, the gate will allow no signal through.