csound-expression-0.0.3: Csound combinator library




Guide to instrument-making



Instruments are functions from some signal representation to signal. Score is a Functor, so to play on instrument means to apply instrument to Score of its notes. Instrument can be made with opcodes. Translator derives p-fields from instrument structure. There are only two explicit p-fields itime and idur (p2 and p3 in csound).

Signals / Types

Signals are represented with trees. Tree contains information about how signal was build.

There are four types for signals (CsoundExpr.Base.Types).

Arate is audio rate signal

Krate is control rate signal

Irate is init value

SignalOut is no output at all (it's produced by opcodes like out, outs, xtratim)

There are two classes to allow csound's polymorphism : X and K

X = Arate | Krate | Irate

K = Krate | Irate

Csound's S - signal is represented with String. Ftable is represented with Irate.

There are two special types MultiOut (for opcodes that may produce several outputs, see CsoundExpr.Base.MultiOut) and SideEffect (for opcodes that rely on number of appearances in csound code, like unirand, see CsoundExpr.Base.SideEffect)


Naming conventions : Opcodes are named after csound's counterparts usually. Some opcodes in csound can produce signals of different rates by request (oscil, linseg). Those opcodes are labelled with suffix. Suffix defines output rate of signal (oscilA, oscilK). Some opcodes in csound have unfixed number of inputs due to setup parameters, almost all of them. Those opcodes have first argument that is list of setup parameters.


oscilA :: (X a, X b) => [Irate] -> a -> b -> Irate -> Arate
oscilK :: (K a, K b) => [Irate] -> a -> b -> Irate -> Krate        

Imperative style csound code

Most of csound opcodes can be used in functional way. You can plug them in one another, and make expressions, but some of them behave like procedures and rely on order of execution in instrument. Module CsoundExpr.Base.Imperative provides functions to write imperative csound code.

outList - to sequence procedures

'(<=>)' - Assignment

ar, kr, ir, gar, gkr, gir - named values, to produce signal with specified name and rate.

Functional style :

exmpInstr :: Irate -> SignalOut
exmpInstr pch = out $ oscilA [] (num 1000) (cpspch pch) $ gen10 4096 [1]

Imperative style :

exmpImper :: Irate -> SignalOut 
exmpImper pch = outList [        
        ir "amp" <=> num 1000,
        ir "cps" <=> cpspch pch,
        ir "ft"  <=> gen10 4096 [1],
        ar "sig" <=> oscilA [] (ir "amp") (ir "cps") (ir "ft"),
        out (ar "sig")]


You can use polymorphic operations to do some arihmetic on signals from CsoundExpr.Base.Arithmetic. And Signal is Num. Eq is undefined though.

exmpArith :: K k0 => k0 -> SignalOutSource


To see what will come out of an expression you can print it. Signal is Show.

User Defined opcodes

You can add your own opcodes to library, see CsoundExpr.Base.UserDefined


There are two ways to tell csd to include instrument in csound file. Instrument can be a part of Score or it can be midi instrument, then it should be mentioned in massign or pgmassign function. If you want to play midi-instr for some time t, you can tell it to csd function by invoking csd with rest t in place of EventList value.

flags = "-odac -iadc  -+rtmidi=virtual -M0"

header = [massign [] 1 instrMidi]

instrMidi :: SignalOut
instrMidi = out $ oscilA [] (num 1000) cpsmidi $ gen10 4096 [1]

-- play instrMidi for 2 minutes
exmpMidi = print $ csd flags header (rest 120)


Song, see src