In Csound, you must define "instruments", which are units which "do things", for instance creating a sine wave as audio signal and play it (= output it to the audio card). These instruments must be called or "turned on" by a "score". The Csound "score" is a list of events which describe how the instruments are to be played in time. It can be thought of as a timeline in text.
A Csound instrument is contained within an Instrument Block, which starts with the keyword instr and ends with the keyword endin. All instruments are given a number (or a name) to identify them.
instr 1 ... instrument instructions come here... endin
Score events in Csound are individual text lines, which can turn on instruments for a certain time. For example, to turn on instrument 1, at time 0, for 2 seconds you will use:
i 1 0 2
Note that orchestra and score are two completely different types of code. The orchestra contains the actual Csound code.1 The instruments are written in the Csound Programming Language. The score is mainly a list of events. The Score Language is poor and offers only some very basic tools.
In modern Csound code, the score often remains empty. The events derive from orchestra code,2 or from real-time interaction, like MIDI, OSC, mouse clicks or any other live input.
A Csound document is structured into three main sections:
Each of these sections is opened with a <xyz> tag and closed with a </xyz> tag. Every Csound file starts with the <CsoundSynthesizer> tag, and ends with </CsoundSynthesizer>. Only the text in-between will be used by Csound.
EXAMPLE 02B01_DocStruct.csd
<CsoundSynthesizer>; START OF A CSOUND FILE
<CsOptions> ; CSOUND CONFIGURATION
-odac
</CsOptions>
<CsInstruments> ; INSTRUMENT DEFINITIONS GO HERE
; Set the audio sample rate to 44100 Hz
sr = 44100
instr 1
; a 440 Hz Sine Wave
aSin poscil 0dbfs/4, 440
out aSin
endin
</CsInstruments>
<CsScore> ; SCORE EVENTS GO HERE
i 1 0 1
</CsScore>
</CsoundSynthesizer> ; END OF THE CSOUND FILE
; Anything after a semicolon is ignored by Csound
Comments, which are lines of text that Csound will ignore, are started with the ";" character. Multi-line comments can be made by encasing them between "/*" and "*/".
"Opcodes" or "Unit generators" are the basic building blocks of Csound. Opcodes can do many things like produce oscillating signals, filter signals, perform mathematical functions or even turn on and off instruments. Opcodes, depending on their function, will take inputs and outputs. Each input or output is called, in programming terms, an "argument". Opcodes always take input arguments on the right and output their results on the left, like this:
output OPCODE input1, input2, input3, .., inputN
For example the poscil opcode has two mandatory inputs:3 amplitude and frequency, and produces a sine wave signal:
aSin poscil 0dbfs/4, 440
In this case, a 440 Hertz oscillation with an amplitude of 0dbfs/4 (a quarter of 0 dB as full scale) will be created and its output will be stored in a container called aSin. The order of the arguments is important: the first input to poscil will always be amplitude and the second input will always be read by Csound as frequency.
Since Csound6, the code can be written in a way which is knows from many other programming languages:3
aSin = poscil(0dbfs/4,440)
Many opcodes include optional input arguments and occasionally optional output arguments. These will always be placed after the essential arguments. In the Csound Manual documentation they are indicated using square brackets "[]". If optional input arguments are omitted they are replaced with the default values indicated in the Csound Manual. The addition of optional output arguments normally initiates a different mode of that opcode: for example, a stereo as opposed to mono version of the opcode.
A "variable" is a named container. It is a place to store things like signals or values from where they can be recalled by using their name. In Csound there are various types of variables. The easiest way to deal with variables when getting to know Csound is to imagine them as cables.
If you want to patch this together:
Sound Generator -> Filter -> Output,
you need two cables, one going out from the generator into the filter and one from the filter to the output. The cables carry audio signals, which are variables beginning with the letter "a".
aSource buzz 0.8, 200, 10, 1
aFiltered moogladder aSource, 400, 0.8
out aFiltered
In the example above, the buzz opcode produces a complex waveform as signal aSource. This signal is fed into the moogladder opcode, which in turn produces the signal aFiltered. The out opcode takes this signal, and sends it to the output whether that be to the speakers or to a rendered file.
Other common variable types are "k" variables which store control signals, which are updated less frequently than audio signals, and "i" variables which are constants within each instrument note.
You can find more information about variable types here in this manual, or here in the Csound Journal.
The Csound Reference Manual is a comprehensive source regarding Csound's syntax and opcodes. All opcodes have their own manual entry describing their syntax and behavior, and the manual contains a detailed reference on the Csound language and options.
In CsoundQt you can find the Csound Manual in the Help Menu. You can quickly go to a particular opcode entry in the manual by putting the cursor on the opcode and pressing Shift+F1. WinXsound , Cabbage and Blue also provide easy access to the manual.
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