A filter in audio synthesis is “a device or process that removes from a signal some unwanted component or feature”. Filters are used all over the place, and have the ability to fully transform audio in a variety of ways.
Csound comes with several types of filters, though this example focuses on the four most common: low pass, high pass, band pass and band reject. A low pass filter removes/attenuates frequencies above a specified cutoff frequency, while allowing frequencies below the cutoff to “pass through.” A high pass does the opposite, allowing the highs to pass, while removing/attenuating frequencies below the cutoff. A band pass filter can be thought of as a combination of the two, as is it removes high and low frequencies, leaving a band of frequencies at the cutoff. The band pass has an additional parameter; The band width of a band pass determines how large a band will be. A wide band will allow more frequency information to pass through, while a very narrow band will nearly produce a sine wave. A band reject filter is the opposite of a band pass, as it removes frequencies at the cutoff.
Download filters.csd here.
The listening example plays bursts of white noise through these filters with various settings to the cutoff or band width parameters of the filters. The first group of notes starts with a low pass fully open, with each new burst being filtered lower than the previous. The second group of bursts are processed with a high pass. The filter starts open, until only the high end is left.
The second two groups of notes maintain a cutoff of 440Hz, while the width of the bands are changed. The band pass starts open, then narrows until only a dirty sine is left in the signal. Due to the loss of gain from this process, I’ve used the balance opcode to make up for lost amplitude so you can hear it. Finally, the band reject removes frequencies at the cutoff, creating a hole in the white noise. In the last couple of notes, you can hear a distinction between the high noise and the low noise.