Amplitude-modulation (AM) means, that one oscillator varies the volume/amplitude of an other. If this modulation is done very slowly (1 Hz to 10 Hz) it is recognised as tremolo. Volume-modulation above 10 Hz leads to the effect, that the sound changes its timbre. So called side-bands appear.
Example 04C01_Simple_AM.csd
<CsoundSynthesizer> <CsOptions> -o dac </CsOptions> <CsInstruments> sr = 48000 ksmps = 32 nchnls = 1 0dbfs = 1 instr 1 aRaise expseg 2, 20, 100 aModSine poscil 0.5, aRaise, 1 aDCOffset = 0.5 ; we want amplitude-modulation aCarSine poscil 0.3, 440, 1 out aCarSine*(aModSine + aDCOffset) endin </CsInstruments> <CsScore> f 1 0 1024 10 1 i 1 0 25 e </CsScore> </CsoundSynthesizer> ; written by Alex Hofmann (Mar. 2011)
The side-bands appear on both sides of the main frequency. This means (freq1-freq2) and (freq1+freq2) appear.
The sounding result of the following example can be calculated as this: freq1 = 440Hz, freq2 = 40 Hz -> The result is a sound with [400, 440, 480] Hz.
The amount of the sidebands can be controlled by a DC-offset of the modulator.
Example 04C02_Sidebands.csd
<CsoundSynthesizer> <CsOptions> -o dac </CsOptions> <CsInstruments> sr = 48000 ksmps = 32 nchnls = 1 0dbfs = 1 instr 1 aOffset linseg 0, 1, 0, 5, 0.6, 3, 0 aSine1 poscil 0.3, 40 , 1 aSine2 poscil 0.3, 440, 1 out (aSine1+aOffset)*aSine2 endin </CsInstruments> <CsScore> f 1 0 1024 10 1 i 1 0 10 e </CsScore> </CsoundSynthesizer> ; written by Alex Hofmann (Mar. 2011)
Ring-modulation is a special-case of AM, without DC-offset (DC-Offset = 0). That means the modulator varies between -1 and +1 like the carrier. The sounding difference to AM is, that RM doesn't contain the carrier frequency.
(If the modulator is unipolar (oscillates between 0 and +1) the effect is called AM.)
If the modulator itself contains more harmonics, the resulting ring modulated sound becomes more complex.
Carrier freq: 600 Hz
Modulator freqs: 200Hz with 3 harmonics = [200, 400, 600] Hz
Resulting freqs: [0, 200, 400, <-600->, 800, 1000, 1200]
Example 04C03_RingMod.csd
<CsoundSynthesizer> <CsOptions> -o dac </CsOptions> <CsInstruments> sr = 48000 ksmps = 32 nchnls = 1 0dbfs = 1 instr 1 ; Ring-Modulation (no DC-Offset) aSine1 poscil 0.3, 200, 2 ; -> [200, 400, 600] Hz aSine2 poscil 0.3, 600, 1 out aSine1*aSine2 endin </CsInstruments> <CsScore> f 1 0 1024 10 1 ; sine f 2 0 1024 10 1 1 1; 3 harmonics i 1 0 5 e </CsScore> </CsoundSynthesizer> ; written by Alex Hofmann (Mar. 2011)
Using an inharmonic modulator frequency also makes the result sound inharmonic. Varying the DC-offset makes the sound-spectrum evolve over time.
Modulator freqs: [230, 460, 690]
Resulting freqs: [ (-)90, 140, 370, <-600->, 830, 1060, 1290]
(negative frequencies become mirrored, but phase inverted)
Example 04C04_Evolving_AM.csd
<CsoundSynthesizer> <CsOptions> -o dac </CsOptions> <CsInstruments> sr = 48000 ksmps = 32 nchnls = 1 0dbfs = 1 instr 1 ; Amplitude-Modulation aOffset linseg 0, 1, 0, 5, 1, 3, 0 aSine1 poscil 0.3, 230, 2 ; -> [230, 460, 690] Hz aSine2 poscil 0.3, 600, 1 out (aSine1+aOffset)*aSine2 endin </CsInstruments> <CsScore> f 1 0 1024 10 1 ; sine f 2 0 1024 10 1 1 1; 3 harmonics i 1 0 10 e </CsScore> </CsoundSynthesizer>
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