SPECTRUM(integer-expression)

Returns the current level of a frequency band from the Retro Rocket audio spectrum analyser.

The analyser continuously monitors mixed audio output and splits it into 32 frequency bands using a Fast Fourier Transform (FFT). SPECTRUM() allows BASIC programs to read these levels in real time to create visualisers, audio-reactive effects, VU meters, and similar displays.

The return value is always in the range:

0 = no activity in the band
Approximately 1023 = maximum detected energy

Frequency bands

Retro Rocket divides the audio spectrum into 32 bands meaning valid band numbers are in the range 0-31:

Lower band numbers represent bass and low frequencies.
Higher band numbers represent treble and high frequencies.

For example:

SPECTRUM(0) may respond strongly to kick drums and basslines.
Mid-range bands respond to vocals, guitars, and synths.
High bands respond to cymbals, hiss, and sharp transients.

The exact frequencies represented by each band depend on the analyser configuration and FFT size.

Behaviour in Retro Rocket

SPECTRUM(0) returns the current level of the lowest frequency band.
SPECTRUM(31) returns the current level of the highest frequency band.
Values automatically decay over time when frequencies disappear.
Bands update continuously while audio is playing.

If no audio is active, all bands gradually return to zero.

Notes

The analyser monitors final mixed audio output, not individual streams.
Values are already scaled for visualisation and are suitable for direct use in graphics code.
Frequency energy is not evenly distributed in music. Bass-heavy material naturally produces stronger low-frequency bands.
Reading SPECTRUM() is inexpensive and suitable for use every frame in animation loops.

Examples

Simple bar graph:

FOR B = 0 TO 31
    H = SPECTRUM(B)
    LINE B * 10, 200, B * 10, 200 - H
NEXT

Simple colour-reactive effect:

BASS = SPECTRUM(0)
 
GCOL RGB(BASS, 0, 255 - BASS)
CLS

See also SOUND · STREAM · DECIBELS