This page last modified: Sunday April 28, 2013

FS-1 Frequency Shifter


Sound clip 1 - bass played through FS-1 (mp3 file, 1.3MB)

Sound clip 2 - drum loop, stereo (mp3 file, 2.7MB)

The FS-1 Frequencu Shifter is created by Jürgen Haible (the schematics and stuff that I used should still be here). It is a facinating unit for sure. Not to be confused with the Pitch Shifter, which shifts your pitch preserving the harmonic structure in the signal; the Frequency Shifter shifts your signal preserving the frequency structure. Neither to be confused with the ring modulator, which produces both sum and difference frequencies from the original signals; the Frequency Shifter gives you separate sum and difference signals. Confusing? Probably others can explain it better.

For me, the most useful and beautiful effect is the slow phasing like sound produced when we use a very low-frequency modulation (near zero hertz) - and especially in stereo, with up and down shifted signals fed to seperate channels. Listen to the first sound clip, arond 2:10 there is a simple demonstration of the mono phaser effect: slow modulator and 50-50 blend of dry and wet signals produce that kind of sound. Listen to the second sound clip (the stereo one), around 0:56 I switch channel 2 to down-shift with fully wet signals - in the begginning you hear a phasing sound. Later in the second clip, around 1:52 I have a 50-50 blend with separate up and down shift, you can hear a very interesting stereo swirling effect.

I should try to explain what is happening inside the box. I will assume that you know how a ring modulator works. (basically a ring modulator multiplies two voltages - if you give it two sine curves of different frequencies, the ring modulator will produce the sum and difference frequencies.) There is inside the FS-1 a very interesting little oscillator called the Quadrature oscillator. It produces two sine waves of the same frequency but one is phase shifted 90 degrees compared to the other. Sine and Cosine, basically. The input signal also needs a 90 degree phase shifted counterpart, this is done by pushing it through a dome filter - two strings of filters that will give us two signals where one is shifted 90 degrees compared to the other. Sine and Cosine again. Then the two Sines are multiplied (the oscillator signal and the input signal) and same for the Cosine pair. Now we have two signals: one is carrier Sine multiplied by signal Sine, and the other is carrier Cosine multiplied by signal Cosine. Now when we sum and difference these two signals we get up-sifted and down-shifted output signals.

An interesting thing is the ability for the modulation oscillator to go through zero. This is because the 90 degree phase shift signifies a direction. So by inverting one of the two (either the Sine or the Cosine) you will get a reverse direction carrier signal, which means that we shift the other way.

Another interesting thing is that if you frequency shift something down, then the bass frequencies will come up on the "other side" before the treble frequencies, and therfore they will be on top. So by shifting down you will basically flip the frequency spectrum upside down. This is illustrated in the second sound clip, around 0:25 I start shifting down and through zero, and at 0:30 you can hear the bass drum coming out on top - ad 0:44 you can hear that the bass drum has a higher frequency than the cowbell. Also in ther first sound clip about 1:31 I play harmonics from high to low, and continue doing so while shifting down through zero, and at 1:56 you can hear the up-side down result.