Hello,
this is because some synthesis type make use of sample files (granular synthesis) or need additional parameters (subtractive).
for subtractive synthesis you need to set the cutoff frequency from the blue channel, a value of 1 set the cutoff at the fundamental frequency of each playing oscillators (also called key tracking / follow), a value of 2 will double that frequency.
Example, key tracking cutoff :
synthOutput = vec4(l, r, 1., 0.);
You can switch between waveforms through the integer part of the Alpha channel :
synthOutput = vec4(l, r, 1., 1.);
Note : Beware of unexpected results due to the addition of slices output, all slices you place on the canvas will be added as one unless they use a different channel so a RGBA value of 1, 1, 0.5, 0.25 become 2, 2, 1, 0.5 with two slice on the same channel, i recommend to experiment with one slice first.
for granular synthesis you must first place audio files in the grains folder /usr/local/share/fragment/grains/ then reload samples through the interface button, Fragment make a guess of the sample pitch but it is often inaccurate so if you don't hear anything you must increase the actual frequency and if you hear a high pitched noise decrease it. For full control of the initial pitch try renaming the audio sample filename with note name such as flute_#A4.wav
for PM/FM the actual implementation is a work in progress and has changed alot, i don't recommend to use it, it also use a filter so you just have to set the cutoff frequency :
synthOutput = vec4(l, r, 0.5, 0.);
The modulation is represented by the integer part of the alpha channel which set the modulator frequency to the corresponding oscillator index. It is pretty limited right now as you can't set the modulation amplitude.
You can find more about those additional parameters here :
https://www.fsynth.com/documentation/tutorials/synthesis_methods/
Be careful with the resonance parameter as it may be unstable in current build.
Next release this year will address all non-additive synthesis issues which were experimental implementations as of now.
