Range Question for LPF and others....

stefan · 15 Dec 2004

The filter itself takes its input in hertz, hence the "pitch to frequency" converter in the vcf module of the example.osm The 0-48 range is pitch input, not frequency.

jorge · 15 Dec 2004

yeah, stefan is right.

48 in pitch means 48 semitones or 4 octaves as each octave has 12 semitones.

toadology · 15 Dec 2004

Thank you for your quick reply.

I do not understand how pitch can be converted to frequency. I've always understood pitch to be someone's interpretation of frequency. As in a low frequency sound is perceived as having a low pitch. I've never thought of pitch as being something quantifiable.

From what you said earlier, the converter would take an amount in pitch and then output that to frequency. What is the conversion ratio? What frequencies does 0 - 48 pitch include?

Thanks for your help...

jorge · 15 Dec 2004

when you have a frequency of 1KHz, convert it to Pitch, add 12 semitones, back to frequency, the result should be 2KHz ( one octave above ).

when you have a frequency of 5KHz, convert it to Pitch, add 12 semitones, back to frequency, the result should be 10KHz ( one octave above ).

so in both cases you added 12 semitones but in the first case it added 1KHz to it and in the second case it added 5KHz.

The human ear perceives pitch and not frequency so its more natural to control pitch. When I work with pitch, I normally picture a piano keyboard with each piano key being a semitone or 1 pitch unit. While the piano keys are positioned linearly, the frequency they correspond to grows up exponentially as you go up the keyboard.

toadology · 15 Dec 2004

Thanks for clearing up the pitch part of my question.

So the Low Pass Filter in the Example.osm changes the frequency of its "Low Pass Cut-off" frequency when a differnet note/key is pressed? In other words, when a higher note is pressed a higher frequency is used as the cut-off frequency for the filter. An interesting concept to have the low pass cutoff frequency relative to the input.

For a simple LPF: If I wanted the cutoff to be 5000 Hz, could I just have a Float value of 5000 connected to the cutoff input of the LPF? therefore bypassing the whole pitch conversion relative to note issue...

Hopefully I have this right. Thank you for all your help.

T

jorge · 16 Dec 2004

I think that having the low pass cut-off frequency relative to the note played is the normal thing to do eventhough I'd like to point out that I'm not an expert in synth design as I spend a lot of time writing code and very little time actually creating schematics.

If for example you were playing piano and had a low pass with cutoff at 2KHz, the highest notes in the piano would be completely silent and as you'd move down the piano, harmonics would be added changing the shape of the waveform played. That piano would play very different sounds as you'd move up and down. You normally want a synth sound to retain similar sounding characteristics as you move up and down the keyboard except for pitch of course.

No only you should control the cutt-off relative to the note played, but it should be also in terms of pitch and not frequency. For example a note at 1KHz with relative cutoff of 2.5KHz making it a total of 3.5KHz would have two harmonics passing through the filter at 2KHz and 3KHz while a note at 5HKz with the same relative cutoff of 2.5KHz would have only the fundamental passing through. Once again as you'd move up and down the keyboard you'd have differently shaped waveform with different attenuations for each harmonic. If you worked in pitch instead, the attenuation of the harmonics would be similar independently of the frequency of the note played.

A useful example that I normally think of in order to understand frequency and pitch is that multiplying frequency by 2 is the same as adding 12 semitones as pitch, both causing a move up of 1 octave. In general multiplying frequency is equivalant to adding pitch.

If you want the cutoff of the LPF to be fixed at 5KHz, you can connect a float to the cutoff with a value of 5000/22050=0.226757... note that sound values go from 0 to 1 with one being 22050 Hz ( half sampling rate ) when dealing with frequency.

I hope I'm not confusing you.

toadology · 16 Dec 2004

I completely understand now. 😉 Thank you all very much.

Toadology

lalo · 16 Dec 2004

🙂 good explaination jorge... anyway the so called key-following in the cutoff frequency is a special feature.. old analog synthesizers had a big knob to set the frequency of filters and nothing more... so i think that the pitch to cutoff design should be not the standard... in modern synthesizer you have a parameter called 'key-following' when you can set the amount of influence that the note played has on the cutoff frequency... not all instruments have the same pitch/cutoff ratio...

lalo