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## Prefabricated time-varying band-pass filter

Time-varying band-pass filtering, as often used in classical subtractive synthesis (Section 8.4.1), can be done using the `vcf~` object, introduced here:

: a "voltage controlled" band-pass filter, similar to `bp~`, but with a signal inlet to control center frequency. Both `bp~` and `vcf~` are one-pole resonant filters as developed in Section 8.3.4; `bp~` outputs only the real part of the resulting signal, while `vcf~` outputs the real and imaginary parts separately.

Figure 8.29: The `vcf~` band-pass filter, with its center frequency controlled by an audio signal (as compared to `bp~` which takes only control messages to set its center frequency.

Example H04.filter.sweep.pd (Figure 8.29) demonstrates using the `vcf~` object for a simple and characteristic subtractive synthesis task. A `phasor~` object (at top) creates a sawtooth wave to filter. (This is not especially good practice as we are not controlling the possibility of foldover; a better sawtooth generator for this purpose will be developed in Chapter 10.) The second `phasor~` object (labeled "LFO for sweep") controls the time-varying center frequency. After adjusting to set the depth and a base center frequency (given in MIDI units), the result is converted into Hertz (using the `tabread4~` object) and passed to `vcf~` to set its center frequency. Another example of using a `vcf~` object for subtractive synthesis is demonstrated in example H05.filter.floyd.pd.

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