BP2 makes it possible to use Csound orchestra instructions such as
kr tablei kndx, ifn
to make continuous parameters vary according to a function table (indexed ‘ifn’) that BP2 produces and insert in the Csound score when necessary.
Below is a typical instrument in which volume and pitchbend are controlled either by linear interpolation, using start and end values, or by function tables.
- p5 and p6 are the start and end values of volume
- p8 and p9 are the start and end values of pitchbend
- p7 contains the index of the volume function table. If it is 0, then volume varies by linear interpolation.
- p10 contains the index of the pitchbend function table. If it is 0, then pitchbend varies by linear interpolation.
Indexes of function tables are assigned automatically by BP2, and each table is stored in the score as a ‘f’ statement just preceding the ‘i’ statement relating to an instrument using the table. BP2 takes care of not overwriting tables that might be refered by Csound instrument and enlisted in the Csound instrument description file.
In any case, it starts indexing from 100 onwards.
sr = 22050
kr = 2205
ksmps = 10
nchnls = 1
ik1 = 32767. / 127.
ik2 = log(2.) / 1200.
ifvol = p7
ifcents = p10
kvol line p5, p3, p6
if (vol <= 0) goto volumelin
ilenvol = ftlen(ifvol)
kndxvol line 0, p3, ilenvol
kvol tablei kndxvol, ifvol
volumelin: kcents line p8, p3, p9
if (ifcents <= 0) goto pitchbendlin
ilencents = ftlen(ifcents)
kndxcents line 0, p3, ilencents
kcents tablei kndxcents, ifcents
pitchbendlin: kpitch = cpspch(p4) * exp(kcents * ik2)
kamp = kvol * ik1
a1 oscil kamp, kpitch, 1
This instrument is currently saved as “BP2test.orc”.
Function tables might invoke either GEN07 (straight lines, p.62) or GEN08 (cubic spline, p.64).
If for instance the function table index is 102, and GEN07 is used, the following instruction will be inserted by BP2 into the score:
f102 time size -7 v1 n1 v2 n2 v3 n3 … vmax
with n1 + n2 + … + n(max-1) = maxpoints
‘size’ is a power of two equal to or greater than the actual number of points ‘maxpoints’.
‘time’ is exactly parameter p2 of the following ‘i’ statement which makes actual use of the table.
Writing ‘-7′ instead of ‘7’ after ‘size’ inhibits rescaling.
GEN08 looks better in terms of smoothing the table, but it has the drawback of setting first derivatives to 0 on the first and last points of the table. It also slows down Csound’s sound file generation.
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This is part of old documentation calling for updates and explanations…