A1891 Voltage Controlled Bit Modifier / Bit Cruncher
Module A1891 is a voltage controlled bit modifier. It offers several voltage controlled algorithmic functions like voltage controlled bit crunching, bit shifting (with/without carry over), bit exchange, rectifying, absolute value and calculating operations like addition, subtraction, multiplication or division.
The module has two control units both with manual control and CV input with attenuator:
 one for the algorithmic function according to the selected mode (e.g. number of shifted bits): BC and BC CV
 and one for the sampling rate (SR and SR CV)

The signal input is equipped with an attenuator. As the module is DC coupled even control voltages can be processed. The mode (e.g. bit crunching, bit shifting, bit exchange) is selected by a 16position rotary switch:
 Bit crushing: BC controls the number of bits (bit reduction)
 AND: Signal AND BC
 OR: Signal OR BC
 XOR: Signal XOR BC
 bit shift right: BC controls the number of bit shifts
 bit shift left: BC controls the number of bit shifts
 multiplication: Signal x BC
 compare and complement: if signal > BC then output = bit complement of the input signal, otherwise unchanged
 compare and absolute: if signal > BC then output = absolute value of the input signal, otherwise unchanged
 addition: output = signal + BC (with overflow/clipping)
 addition with BC swap: same as 10 but with nibble swap of BC (nibble = half byte, i.e. four bits)
 short delay 1 with dynamic normalization: BC controls the length of the delay memory
 short delay 2: same as 12 but different length/feedback
 short delay 3: same as 12 but different length/feedback
 short delay 4: same as 12 but different length/feedback
 four stages FIR filter: BC controls the filter coefficient
Remark: Module A1891 is the first derivative of the universal AD/DA module. More modules derived from this basic idea are planned for the future (e.g. CV delay, CV modifier including Hz/VtoV/Oct, digital waveshaper/multiplier, clocked audio delay).
Reviews
Sound examples
These are the first sound examples of the A1891 prototype we will show at the NAMM in January 2008. Christian Assall has programmed 16 different operating modes that are selected by the 16position rotary switch. The simplest mode (mode #1) is the standard bit crunching function. The other modes are different bit manipulation modes. These are the very first trials and the modes of the final version of the module may be different. For all demo sounds the triangle output of a VCO was used as signal input for the A1891. For clarity no other sound processing modules are used. The sound is generated only by an VCO and the A1891. The VCO pitch is controlled by an A155 sequencer.
The first example shows the effect of manually changing the sampling rate in mode #1:
A1891_sampling_rate.mp3
The second example is a presentation of the 16 modes available in the prototype. One after another each of the 16 modes is selected. The sampling rate is fixed, only the manual BC control is operated from ccw to cw and back to ccw for each mode:
A1891_16_modes_manually.mp3
The third example shows the effect of dynamic sample rate modulation. An ADSR is triggered by one of the trigger rows of the A155. The ADSR output is connected to the SR CV input to modulate the sampling rate by the ADSR envelope. The BC parameter is fixed.
A1891_dynamic_sample_rate.mp3
In the next example the ADSR output is connected to the BC CV input. This example shows the effect of dynamic BC modulation. The sample rate is fixed:
A1891_dynamic_bc_01.mp3
In this example a VCA has been added between the VCO output and the signal input of the A1891. The ADSR output is connected to the CV input of the VCA. This example shows the effect of dynamic input level modulation. In addition the sample rate has been reduced a bit manually at the end of the example:
A1891_input_VCA.mp3
The following samples arised from playing around with the A1891. The patch is the same as above but the ADSR is used for both controlling the VCA and the BC CV input of the A1891 (inverted output of the ADSR for BC). Several parameters have been changed manually during the examples (mainly envelope amount and offset controls of the VCA and the A1891).
A1891_playing_around_01.mp3
A1891_playing_around_02.mp3