This tutorial assumes that the user is familiar with Csound programming.

We will build a small module for filtering sounds. This module will feature a choice of low-pass, hi-pass, bandpass and band-reject filter, along with a time-function to control the cut-off and/or center frequency. We will also offer a post filter gain slider, a selector for choosing the soundfile to process and an automatic balance switch to reestablish power to the filtered sound.

Normally, we would first build the orchestra in Csound and then add the interface, but since we already know what we want to do, we will first build the interface.

We will therefor need to define the following interface objects:
 

1. a selector to choose to sound to be processed
2. a graph for the time varying cut-off frequency of the filter.
3. a graph for the bandwidth of the filter (for band-pass and band-reject variants).
4. a popup to select the type of filtering.
5. a slider for the output gain.
6. a toggle for reesetablishing the level of the sound.

 

• Under the File menu in the Main window, select New... and then Module...

 
• Open the tk_interface editor pane by clicking on its button or choosing it in the Sections menu.

 
• Enter the first graph definition by typing this line:

cgraph  freqc -unit Hz -label "Cut-off Frequency" -rel log -min 50 -max 20000 -init 2500

• Enter the second graph by doing Button-3 or Command-Click in the tk_interface pane to bring up the inserter, Choose graph freq under the Graphs cascade. The following pre-formatted line is inserted:

cgraph freq -unit x -rel lo -min 20 -max 15000 -init 500

But we will change the parameter range to -min .001, max 1 and init to 0.1 because this function will be used as a multiplier for the center frequency (freqc) in the orchestra to give us the bandwidth of the filter. We also change -rel log to -rel lin for a linear function. While we are here, let's give it a label of "Bandwidth".

• Enter the fileselector by using the same inserter method and choosing the standard filein under Filein.

 
• Enter the toggle. Change the name to "select" and give the label "Balance with Input".

 
• Enter the slider with the inserter. Choose slider10 under Sliders. Change the name to "gain" and give a label "Post-Filter Gain". Change to init value to 1.

 
• Enter the popup with he inserter. Change the name to "type" , the label to "Select Filter Type" and give this list for value: "lo-pass hi-pass band-pass band-reject".

 

cfilein  name   -label "Sound to process"
cgraph   freqc  -label "Cut-off Frequency" -rel log -min 50 -max 20000 -init 2500
cgraph   freq   -label "Center  Frequency" -rel lin -min .001 -max 1 -init 0.1
cslider  data10 -label "Post-Filter Gain" -res .1 -min 0 -max 10 -init 1
ctoggle  gain   -label "Balance with Input" -init 0
cpopup   type   -label "Select Filter Type" -value "lo-pass hi-pass band-pass band-reject"

• Open the stereo pane in the CECILIA editor.

 
• Enter the following text in the pane:

      instr 1

a1,a2 soundin   "[name]", [offname] ; name from fileselector
                                    ; and offset substituted here

if    gktype != 0 goto hp           ; using if / goto and the value
ag    butterlp    a1, gkfreqc       ; of the "type" popup to select
ad    butterlp    a2, gkfreqc       ; the correct kind of filter
goto bal

hp:
if    gktype != 1 goto bp           ; the freqc graph controls the
ag    butterhp    a1, gkfreqc       ; cut-off frequency
ad    butterhp    a2, gkfreqc
goto bal

bp:
if    gktype != 2 goto br           ; the freq graph controls the
ag    butterbp    a1, gkfreqc, gkfreq*gkfreqc ;bandwidth
ad    butterbp    a2, gkfreqc, gkfreq*gkfreqc
goto bal

br:
ag    butterbr    a1, gkfreqc, gkfreq*gkfreqc
ad    butterbr    a2, gkfreqc, gkfreq*gkfreqc

bal:
if gkgain == 0 goto nobal           ; gkgain toggle decides to
ag    balance     ag, a1            ; balance or not
ad    balance     ad, a2

nobal:
outs  ag*gkdata10, ag*gkdata10      ; the data10 slider controls
                                    ; the final gain

      endin

• Enter the score in the score editor pane. In fact, the only score that we will need here is the #min keyword. Since the only note we want is on instrument 1 and we do not need to pass it any parameter. The duration of the note will always be the duration of the soundfile. This is a case where the #min score is perfect. Make sure you study the other types of scores available in CECILIA.  Studying the BuiltIn CECILIA modules is a good way to develop an understanding for the powerful possibilities of CECILIA score pre-processing.

 
• Save the module, choose a sound with the file selector, define time-profiles with the graphs and press Play!

 
• Studying CECILIA's Builtin modules is highly recommended!