Header Statements and Global Space

The Orchestra Header contains global information that applies to all instruments and defines aspects of Csound's output. It is sometimes referred to as instr 0, because it behaves as an instrument, but without k- or a-rate processing (i.e. only opcodes and instructions that work at i-rate are allowed).

The most essential statements given in the orchestra header are:

sr the sample rate (default 44100)
ksmps the number of samples for one control period (default 10)
NOTE: It is recommended to specify ksmps and let Csound calculate kr. ksmps must be an integer and should usually be a power of two.
nchnls the number of audio channels (default 1)
NOTE: This refers to both, output and input channels. Use nchnls_i in case input and output do not have the same number of channels.
0dbfs the number which serves as amplitude value for zero dB full scale (default 32768).
NOTE: It is recommended to set 0dbfs to 1 as this is the standard in all audio applications nowadays.

A standard Csound header may look like this:

sr = 48000
ksmps = 32
nchnls = 2
0dbfs = 1

Other common header statements:

nchnls_i the number of input channels (if different from nchnls)
A4 the frequency set to pitch A4 (default 440)
seed sets the global seed for most random generators
NOTE: The syntax is seed 0 or seed(0) but NOT seed = 0
massign assigns incoming midi to a specific instrument
pgmassign assigns a MIDI program number to a specific instrument
ctrlinit sets the initial values for a set of MIDI controllers
ftgen generates a function table

The instrument header is part of the global space. This space continues between and after the instrument code, and is evaluated as a whole before any instrument is running.

Typical statements in the global space other than the orchestra header are:
- User Defined Opcode definitions
- Macro definitions
- #include statements for external files
- setting or initializing global variables, e.g. gaReverb init 0
- declaring software channels, e.g. chn_k("watchdog",3)