Subtractive Synthesis and Filters

1. Subtractive Synthesis and Filters Lowpass, Highpass, Bandpass, Band reject (notch), Badwidth, Q Music Tech 1, Fall 2012

2. Subtractive Synthesis • What is it? • Subtractive synthesis is the act of removing spectral content from a sound. • Additive synthesis used multiple elementary components to create a complex sound. Subtractive does the opposite • Starts with a spectrally rich sound and removes components from the frequency spectrum to create new sounds.

3. Removing content • How do we remove spectral content from a sound? • Through the use of filters? • What is the function of any filter? • Filters are used to remove content from something and leave something else behind. Coffee filters, air filters, oil filters, etc.

4. Sound Filters • This same concept can be applied to sound. Using various types of filtering we can sculpt sounds to become something new, something recognizable, or to improve the quality of a recording. • 4 Types of filters: • Lowpass • Highpass • Bandpass • Band reject (notch)

5. Frequency response • To understand filters we must understand the concept of frequency response. • Frequency response is short for “amplitude-versus-frequency-response” and is a graphic representation of how frequencies are heard within an audio system. • A flat frequency response curve is most desired for audio systems, as it means that there is no boosting or attenuating of audio signals across the frequency spectrum.

6. Frequency response

7. Lowpass filter • Cuts high frequencies and allows low frequencies to pass. Hence the name low-pass • These are useful for removing high pitched hiss and buzzing from audio files and unwanted high white noise.

8. Lowpass filter • The graphic shows the frequency response of a lowpass filter

9. Highpass filter • Cuts out low frequencies and allows higher frequencies to pass. Again, it’s all in the name, high-pass • Useful for eliminating low rumbles as a result of unwanted room noise or system noise from audio recording equipment.

10. Highpass filter • The graphic shows the frequency response of a highpass filter

11. Bandpass filter • Allows only a selected band of frequencies to pass. • Useful for isolating a specific (usually small) range of the frequency spectrum of a sound. • The same affect can be achieved through combining highpass and lowpass filters together, but a bandpass filter give greater control over frequency response and quality of the filtering (more on that in a minute).

12. Bandpass filter • Graphic of a bandpass filter.

13. Band reject (notch) filter • Opposite of a bandpass filter. Selects a portion of the frequency band and attenuates selected frequencies. • In the same way that the bandpass filter allows only certain frequencies to be heard, a notch filter will remove that same band of frequencies and allow what is left to be heard. • Like inverting a bandpass filter.

14. Notch filter • Graphic of a notch filter

15. Shelving filters • Allows for boosting or attenuating frequencies at a certain threshold. • Shelving filters don’t actually remove frequency content, but alter the presence of a band of frequencies to bring them out or reduce them in the resultant sound. • Names are inverse of what they do. • High shelving filters act like lowpass filters • Low shelving filters act like highpass filters • Why? Because of where they are located on the frequency response graph. High shelving filters are at the high end of the spectrum (but cut high frequencies) and low shelving filters are at the low end of the spectrum (cut low frequencies).

16. Components of filtering • There are a number of factors and terms that are applied to filtering, such as: • Passband • Stopband • Cutoff frequency • Half-power point • Bandwidth • Center frequency (bandpass and notch only) • Q (bandpass and notch only) • Slope

17. Passband/Stopband • The passband is the band of frequencies that are still heard after filtering is applied • High/low frequencies left after highpass/lowpass filtering • Band of frequencies heard in bandpass filtering, or frequencies left over after notch filtering • The stopband refers to the frequencies that have been removed from the sound. In other words, the frequency band that has “stopped” being heard.

18. Cutoff/center frequency • In highpass and lowpass filters, the cutoff frequency is the frequency at which attenuation begins. • Notice that there isn’t a dramatic cutoff, but a gradual attenuation of frequencies. The CO frequency is where attenuation begins. • In bandpass and notch filtering, the equivalent frequency is called the centerfrequency, as it resides directly in the center of the pass/stopband.

19. Half power point • The cutoff frequency is also referred to as the half-power point. • The cutoff frequency is the point at which the filter reduces the power to .707. • Power = proportional to the amplitude of the signal squared. • .7072 = .5; the CO frequency resides at the point at which power is at .707, or half the power of the original signal • It is also called the 3 dB point, because .707 to 1.0 is relative to a reduction of 3 dB from the signal.

20. Bandwidth and Q • Bandwidth and Q apply only to bandpass and notch filtering. • Bandwidth = the difference between the higher and lower cutoff frequencies in a bandpass filter • Q = ratio of center frequency to bandwidth; defines the “quality” of the bandpass filter • Q = CF/high cutoff – low cutoff OR Q = CF/BW

21. More on Q • Q represents a degree of “resonance” within a bandpass filter • A low Q coincides with a wider bandwidth, but a high Q value is usually focusing on a frequency response at a sharp peak frequency. • When a high Q is excited by a signal near its center frequency the filter rings at the resonant frequency for some time after the signal has passed.

22. Slope • Slope is the degree of attenuation in a filter. • A gentle slope will gradually attenuate frequencies past the cutoff frequency or center frequency • A sharp or steep slope will more abruptly attenuate frequencies past the cutoff or center frequency.

23. Slope