Yesterday we learned how to incorporate parameters into our instrument such that the amplitude and frequency could be altered from our score. However this only allows us to control the loudness of a new note; what if we want to alter the loudness during the course of a given note? Also did you notice that unsettling audio-pop glitch between successive notes?
Firstly some theory. In Csound we have three types of variables:
- i-rate variables which change at the note rate,
- k-rate variables which change at the control signal rate, and
- a-rate variables which change at the audio signal rate.
Thus, i-variables are modifiable in the score, and k-variables and a-variables are modifiable in the orchestra, where a-variables produce sonic output, and k-variables can manipulate particulars of these audio functions.
We want to create an amplitude envelope which is controllable for each note of the score, and luckily Csound has a built-in function, linen
kr linen kamp, irise, idur, idec
which produces a k-variable from a given amplitude, attack, duration, and decay value. We can then modify our instrument as follows:
<CsoundSynthesizer> <CsInstruments> sr = 44100 ;sample rate kr = 4410 ;control signal rate ksmps = 10 ;samples per control signal nchnls = 1 ;number of output channels instr 1 k1 linen p4, p6, p3, p7 ;p3 = duration, p4 = amp p5 = freq a1 oscil k1, p5, 1 ;p6 = attack time, p7 = decay time out a1 ;output endin </CsInstruments> <CsScore> f1 0 4096 10 1 ;table containing a sine wave ;ins strt dur(p3) amp(p4) freq(p5) attack(p6) decay(p7) i1 0 1 10000 440 0.5 0.5 i1 1 1 5000 528.64 0.75 0.25 i1 2 1 2000 594.39 0.9 0.5 i1 3 1 5000 660 0.2 0.9 i1 4 1 10000 792.86 0.5 0.5 </CsScore> </CsoundSynthesizer>
In this case, we firstly design the amplitude envelope such that for a given maximum amplitude, attack time, decay time, and overall duration, we can calculate the instantaneous value of the line function at a control rate interval (i.e. 4410 times per second). This variable k1 is then used as the amplitude of the oscillator a1. As before, p5 is then used as the corresponding frequency.
Notice that those audio-pop glitches are gone? Using an amplitude envelope, we can avoid those instantaneous audio jumps, and thus audible glitches, an invaluable tool for all compositions (unless you purposely want them, of course :p).
In this third tutorial, we have discussed the practical use of an amplitude envelope and how to implement it. In the next tutorial we will discuss altering the timbre of our instrument.