Analogue Systems recently added the RS240 Frequency Shifter ($795) to its roster of RS Integrator analog synth modules. The RS240 is based on the coveted Bode 735 frequency shifter, created by Harald Bode in 1970. However, the RS240 fits the 3U, Eurorack form factor, making it compatible with Analogue Solutions and Doepfer modular synths, among others.
The RS240 requires a power supply (±12 VDC) and a rack. The review unit came in an attractive walnut case that included an RS80 VC-LFO ($119) and a power supply. (The price for such a standalone setup varies depending on the module accompanying the RS240.) The desktop configuration made it easy to integrate the RS240 into my studio.
A hardware frequency shifter works in the same way as a ring modulator: it takes two signals and gives you the sum and difference tones, called sidebands. Unlike a ring modulator, a frequency shifter gives you independent outputs for the sidebands (called “up shift” and “down shift”). A hardware frequency shifter requires a lot of parts, and consequently, it's an expensive and somewhat complex module to build.
The difference between pitch shifting and frequency shifting is simple: pitch shifting transposes a tone by multiplying it by a fixed number. The pitch-shifted tone and its associated harmonics can therefore be transposed equally in musical intervals (such as a minor third or a perfect fourth). That allows the harmonics in the overtone series to maintain their original harmonic relationships, preserving much of the character of the sound.
Frequency shifting, on the other hand, adds or subtracts a fixed amount to or from the tone and its harmonics. It is a mathematical, linear shifting of the frequency spectrum, which changes (compresses or expands) the mathematical relationships of the overtones, thereby distorting the overtone series. The result is added timbral complexity. Put the frequency shifter under voltage control, and you have a powerful analog processor.
Plug and Play
The RS240 has a simple front-panel layout, featuring unbalanced ⅛-inch (3.5 mm) jacks for audio and CV I/O, five knobs, and a switch. The module has one audio input, three CV inputs, separate outputs for the upper (Out B) and lower (Out A) sidebands, and three mix outputs.
The largest knob controls the amount of frequency shift, with an exponential range of ±2 kHz and a linear range of ±50. The Scale control has four pitch-range settings, as well as Exp (exponential response) and Zero. The Scale ranges — 5 (±5 Hz), 50 (±50 Hz), 500 (±500 Hz), and 5K (±5 kHz) — determine the amount of shift for each positive and negative voltage swing. For example, in the 5K setting, each volt at the CV input causes a 1 kHz shift in the signal (up shift and down shift). The higher the setting, the wider the range and the more pronounced the timbral effects will be. In the Exp setting, each volt adds an octave change to the source input.
The Mixture knob sets the balance between upper and lower sidebands at the Mix outputs. The Squelch on-off switch and Squelch Threshold control are for gating system noise when the input signal is low. I found them to be especially useful because signal from the VC-LFO seemed to bleed into the RS240 when there was no external signal present.
On the subject of noise, the outputs are noisy around 3 kHz. If you're processing simple, low-level audio sources, you may find yourself reaching for the EQ.
The Zero Adjust control is used to calibrate each setting on the Scale control. The closest you can get to hearing the input signal unprocessed is to set the Scale control to Zero, position the large knob at 12 o'clock, switch off the Squelch, and set the Zero Adjust so that the LED goes dark. In both cases, finding the zero point in this way was difficult. Each time I tried, the light would drift on and off slowly. However, this setting was great for creating slow-moving spatial effects, when the signals from Out A and Out B were hard panned left and right, respectively. Ah, the joys of analog processing.
The RS240 is a powerful module that's easy — and fun — to use. In extreme settings, it can make your tracks unrecognizable. For example, I used it to create complex rhythmic material by dramatically shifting talk-radio broadcasts until the words disappeared.
The RS240 is also great for subtle processing, such as adding 2-channel movement to a mono signal. Although it's a bit pricey, the RS240 is a worthwhile investment if you want a processor with teeth.