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Csound: CsoundSyntax


Orchestra and Score

In Csound, you must define "instruments", which are units which "do things", for instance playing a sine wave. 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...

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

The Csound Document Structure

A Csound document is structured into three main sections:

  • CsOptions: Contains the configuration options for Csound. For example using "-o dac" in this section will make Csound run in real-time instead of writing a sound file.1 
  • CsInstruments: Contains the instrument definitions and optionally some global settings and definitions like sample rate, etc. 2 
  • CsScore: Contains the score events which trigger the instruments.

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



; Set the audio sample rate to 44100 Hz
sr = 44100

instr 1
; a 440 Hz Sine Wave
aSin      oscils    0dbfs/4, 440, 0
          out       aSin

i 1 0 1

</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.

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.

Using the Manual

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.

  1. Find all options ("flags") in alphabetical order at www.csounds.com/manual/html/CommandFlags.html or sorted by category at www.csounds.com/manual/html/CommandFlagsCategory.html .^
  2. It is not obligatory to include Orchestra Header Statements (sr, kr, ksmps, nchnls, etc.) in the section. If they are omitted, then the default value will be used:
    sr (audio sampling rate, default value is 44100)
    kr (control rate, default value is 4410, but overwritten if ksmps is specified, as kr=sr/ksmps)
    ksmps (number of samples in a control period, default value is 10)
    nchnls (number of channels of audio output, default value is 1 (mono))
    0dbfs (value of 0 decibels using full scale amplitude, default is 32767)
    Modern audio software normal uses 0dbfs = 1

    Read chapter 01 to know more about these terms from a general perspective. Read chapter 03A to know more in detail about ksmps and friends. ^
  3. The third and fourth input are a table containing the waveform, and the starting phase. They are optional. If not specified, they use default values: a sine wave, and phase zero.^

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