The Noise Engineering Ataraxic Translatron is a linear feedback shift register oscillator similar to those used in the first generation of home video game consoles and many classic arcade games.
Linear feedback shift registers are an ingenious way to produce a variety of sounds with an extremely small amount of hardware. The Atari VCS used only around 35 logic gates to produce all of its sounds. The complexity of tone for relatively minimal hardware made this synthesis technique common for sound where hardware costs were the primary development constraint. As video games entered popular culture these sounds became iconic but have seldom made it out of the video game world except when sampled from the games themselves or as their own genre of music “chiptunes”. The Ataraxic Translatron gives you classic arcade sounds in Eurorack format to be used just like any other VCO.
13 patches vary from a simple square wave to white noise with your favorite arcade sounds in between. All tones are available in 6 octaves range. A standard 1 volt per octave pitch control and CV control of the current patch are squeezed into a compact 4HP. An external clock mode that allows an external clock to drive the shift register allows for additional tone generation, modulation and gate generation.
- Pitch Knob - The pitch knob allows for adjusting the pitch through two octaves range when not in clock mode.
- Tone Knob - The tone knob selects which feedback network is used and therefore what tones the module produces. The AT has 13 patches that increase in harmonic complexity as the patch increases. When used with an external tone CV the tone knob scales the tone CV. The current patch is displayed on the seven segment display.
- Tone CV - The tone CV allows for voltage control over the current tone. It has an approximate range of 0-6 Volts across the entire patch range. Any over/under voltage is safely clamped.
- Pitch CV - The pitch CV provides a standard 1 volt per octave control over the output pitch. The valid input voltage range is 0-8 Volts. Any over/under voltage will be safely clamped to the maximum/minimum values. The module produces six octaves of perceptibly continuous pitch. Due to quantization errors at high speeds some patches will have a slight pitch quantization at higher frequencies. This is itself an interesting effect. The pitch CV input is also used as the clock input when used with an external clock. See the clock source switch section for a description of this usage.
- Clock Source Switch - The clock source switch selects if the module is using the internal timebase from pitch
- CV or an external clock. When the switch is in the CV position the pitch input works as 1 Volt per Octave. When in clock mode the pitch CV input becomes a binary clock that drives the shift register oscillator on rising edges. The input is a Schmitt trigger with a low threshold of around 2 Volts. Input in this mode is over and under voltage protected so it can be safely driven with any standard Eurorack signal. The maximum frequency which the LFSR can be driven is around 160 kHz.
- Output - The output is DC coupled digital waveform that varies from approximately 0 to 6.5 volts. Try using it as a randomized clock source too!
The core of the Ataraxic Translatron is a 16 bit linear feedback shift register with adjustable taps which are XORed together to produce the output tone. Most patches correspond to maximal length sequences. The details of the taps are in the table to the right. The input pitch CV is converted from 1v/8va to a period for the desired pitch which is then scaled by the current patch’s cycle length. This is used to set a timer that controls when the LFSR update happens. When in clock mode this update happens on any rising clock. All patches are maximal length except E which has the interesting property that the tone that is produced is based on what the shift register state is at the time it is selected.
Calibration of Tuning
The pitch CV response of the Ataraxic Translatron is controlled by an linear resistor divider network. To calibrate the tuning attach a volt meter (preferably 4+ digit) to the test points TPCV and TPGND on the rear panel and adjust the trim pot. The voltage measured should be 0.075 times the input voltage applied to the CV input. A reasonable way to tune the scale is to use an adjustable voltage source to generate 3.00 volts then adjust the AT tuning trim until the test points read 225 mV. It is also easy to use a stroboscope and a calibrated voltage reference to adjust the linearity control by tuning using an octave interval.