Tiptop Audio / Buchla 296t Spectral Processor

The Model 296t Programmable Spectral Processor is a set of sophisticated filters with a variety of outputs and special functions.

The Model 296t Programmable Spectral Processor is a set of sophisticated filters with a variety of outputs and special functions. The most obvious function is the 16-band “Graphic Equalizer.” The 16 bands are displayed in hexadecimal (0-F) at the top and bottom of the module, and the center frequency of each band is displayed above each slider attenuator.

This is a modified version of the Buchla Synthesizer User Guide by Daniel J. Schedit (November 16, 1982) for use with the 296t.

■ATTENUATOR OUTPUTS
The Signal Inputs jacks in the lower right are for “even” (eight even-numbered bands), “all” (all 16 bands), and “odd” (eight odd-numbered bands) signal inputs. Note that these represent even or odd numbered bands, and not even or odd harmonic bands. The ATTEEUNATOR OUTPUTS on the right of the module also have signal outputs for “odd”, “even” and “all”.
When a signal is patched to the “all” signal input and taken out of the “all” ATTEEUNATOR OUTPUTS, the module functions as a 16-band graphic equalizer. Note that -3dB is the maximum range of the attenuator output, and even when the slider is in the highest position, there is actually -3dB attenuation of the frequencies in that band. In the lowest position, the signal in that frequency band is completely cut off. The signal output jacks located directly above each slider output the signal present in that frequency band at ±0dB. These outputs are not affected by the position of the sliders. The COMB FILTER outputs signals from all the “even” or all the “odd” frequency bands, and is not affected by the sliders, with a gain of ±0dB.

■CONTROL VOLTAGE OUTPUTS
The control voltage outputs labeled ENVELOPE OUTPUTS are envelope follower outputs for each frequency band. In other words, the voltage from these outputs represents the amplitude of the signal present within each band. The “short”, “combo” and “long” switches in the upper right corner select the decay time of the envelope. These outputs are not affected by the position of the sliders.

■PROGRAMMED OUTPUTS
The 296t can manipulate parameters in a variety of ways using voltage control. The signals processed by voltage control are output to the PROGRAMMED OUTPUTS. The knob on the left of the PROGRAM CONTROL section can be used to “sweep” a wide frequency band (in the same way that a band-pass filter can “sweep” a frequency spectrum). This knob can also be voltage-controlled via the control voltage input labeled FREQUENCY. The knob next to the voltage input jack is an attenuator. To the right are the WIDTH control knobs, which determine the width of the 16 frequency bands. The labels above each attenuator indicate the approximate center frequency of each band. Note that as the bandwidth becomes very narrow, “gaps” appear between each band, and the passband disappears completely, so that no signal can pass through. At the maximum setting, each band is so wide that it encompasses the entire frequency spectrum, and the FREQ control has no effect. This knob controls the voltage, but there is no attenuator for the control voltage. In addition, the LOCAL PROGRAM INPUTS allow you to independently control the signal level of each frequency band with voltage.

■SPECTRAL BIAS
The knob and switch pair labeled PROGRAMMED SPECTRUM TRANSFER has a function related to what is often called a “vocoder” circuit. When the switch on the left is turned on, each “even” envelope follower output is internally connected to the adjacent “odd” control voltage input.
In other words, the 296t spectral processor analyzes the spectrum of the signal present at the “even” input, and sends the frequency spectrum to the “odd” band. If the signal present at the “odd” input has a sufficiently wide frequency spectrum, the timbre of the “odd” program output will be a copy of the “even” signal. The switch on the right has the same function, internally connecting from “even” to “odd”.

If you analyze the signal from the microphone and combine it with a harmonic signal from the oscillator, you can reproduce the vowels of the microphone signal. This is generally known as a “vocoder” patch. In order to optimize the operation of the vocoder, the input signal requires special equalization. This is the role of the two knobs (pre-emph.) next to the switches. Turning them to the right boosts the high frequencies of the “even” and “odd” input signals. These affect both the ATTENUATOR and PROGRAMMED outputs, so they should be turned down unless you are using the “vocoder” patch. Also, when using these switches, be aware that the BANDWIDTH and FREQUENCY settings are still active. In particular, to get the best results with the “Vocoder” patch, you will need to set both BANDWIDTH and FREQUENCY to their minimum settings.

■LEDS
The LEDs are driven by the envelope follower and indicate when a 10Vpp signal (such as from the 258t) has reached a certain frequency in the band, causing the envelope follower to ramp up to its maximum CV value of 10V. When using the “Vocoder” patch, the output signal gain may be low. You can increase the output gain by using the LED display to set the VCO to a position near the center of the band.
The LED lighting is not a clip indicator for the input signal.

Width: 52HP
Depth: 52mm
Current consumption:
+12V: 360mA
-12V: 350mA