This opcode dynamically modifies a gain value applied to the input sound ain by comparing its power level to a given threshold level. The signal will be compressed/expanded with different factors regarding that it is over or under the threshold.
icomp1 -- compression ratio for upper zone.
icomp2 -- compression ratio for lower zone
irtime -- gain rise time in seconds. Time over which the gain factor is allowed to raise of one unit.
iftime -- gain fall time in seconds. Time over which the gain factor is allowed to decrease of one unit.
asig -- input signal to be modified
kthreshold -- level of input signal which acts as the threshold. Can be changed at k-time (e.g. for ducking)
Note on the compression factors: A compression ratio of one leaves the sound unchanged. Setting the ratio to a value smaller than one will compress the signal (reduce its volume) while setting the ratio to a value greater than one will expand the signal (augment its volume).
Because the results of the dam opcode can be subtle, I recommend looking at them in a graphical audio editor program like audacity. audacity is available for Linux, Windows, and the MacOS and may be downloaded from http://audacity.sourceforge.net.
Here is an example of the dam opcode. It uses the files dam.orc, dam.sco, and beats.wav.
Example 88. An example of the dam opcode compressing an audio signal.
/* dam.orc */ ; Initialize the global variables. sr = 44100 kr = 4410 ksmps = 10 nchnls = 1 ; Instrument #1, uncompressed signal. instr 1 ; Use the "beats.wav" audio file. asig soundin "beats.wav" out asig endin ; Instrument #2, compressed signal. instr 2 ; Use the "beats.wav" audio file. asig soundin "beats.wav" ; Compress the audio signal. kthreshold init 25000 icomp1 = 0.5 icomp2 = 0.763 irtime = 0.1 iftime = 0.1 a1 dam asig, kthreshold, icomp1, icomp2, irtime, iftime out a1 endin /* dam.orc */
/* dam.sco */ ; Play Instrument #1 for 2 seconds. i 1 0 2 ; Play Instrument #2 for 2 seconds. i 2 2 2 e /* dam.sco */
This example compresses the audio file “beats.wav”. You should hear a drum pattern repeat twice. The second time, the sound should be quieter (compressed) than the first.
Here is another example of the dam opcode. It uses the files dam_expanded.orc, dam_expanded.sco, and mary.wav.
Example 89. An example of the dam opcode expanding an audio signal.
/* dam_expanded.orc */ ; Initialize the global variables. sr = 44100 kr = 4410 ksmps = 10 nchnls = 1 ; Instrument #1, normal audio signal. instr 1 ; Use the "mary.wav" audio file. asig soundin "mary.wav" out asig endin ; Instrument #2, expanded audio signal. instr 2 ; Use the "mary.wav" audio file. asig soundin "mary.wav" ; Expand the audio signal. kthreshold init 7500 icomp1 = 2.25 icomp2 = 2.25 irtime = 0.1 iftime = 0.6 a1 dam asig, kthreshold, icomp1, icomp2, irtime, iftime out a1 endin /* dam_expanded.orc */
/* dam_expanded.sco */ ; Play Instrument #1. i 1 0.0 3.5 ; Play Instrument #2. i 2 3.5 3.5 e /* dam_expanded.sco */
This example expands the audio file “mary.wav”. You should hear a melody repeat twice. The second time, the sound should be louder (expanded) than the first.