Quad Anti Aliasing Filter (QAAF)

Description

The  QAAF was conceptualized as the first expansion for the Phase Displacement Oscillator. It is designed to eliminate aliasing and stair-step distortion from the Digital to Analog converters. We opted to make this a seperate module because it is useful by itself as well as coupled with the PDO.

There are four independent filters inside the QAAF.

Each -24dB/octave Butterworth filter runs at exactly the same frequency. The Butterworth filter has a small amount of resonance and a great sound. Each input jack is normaled from the expansion header that connects to the PDO, but any signal can be filtered by plugging in.

The frequency control circuitry uses a vactrol to control the clocking speed of each filter, slowing down the response to transient signals. This brings life and character to the sound.

Features

• Four 4-pole filters
• Channel to Channel Freq +-0.5%
• Switched Capacitor Design
• Butterworth Response Curve
• CV Input
• Frequency Offset Knob
• Clock Input/Output
• Mix Output
• Low-Frequency Limit Jumper
• Great for Pre & Post Filterin

Controls and I/O

CV Input – This input is roughly calibrated to 1v/octave. It drives a vactrol that will cause the QAAF to open quickly and close slowly.

Freq Knob – This knob controls the cutoff directly or acts as a bias/level shift control for the CV input.

CLK – This jack serves as a clock input or output, directly controlling the filter’s frequencies at a ratio of 100:1. A jumper on the back (lower jumper) selects it’s mode. Jump for clock output, leave open for clock input.

Inputs 1-4 – Audio Inputs, these are normaled from the expansion header going to the PDO. This normal is broken by plugging into these jacks.

Outputs 1-4 – These jacks are for each channel’s outut.

Mix – This output is a sum of each channel’s filtered input.

Frequency Limit Jumper – This rear jumper (upper set of pins) will limit the low frequency cutoff to about 300Hz. This will stop the QAAF from producing aliasing noise itself as it’s clock frequency will be limited to 30kHz. Jump for limiting, leave open to disable.

Specification:-

• Current consumption: 35mA +12 rail; 35mA -12 rail.
• Module depth approx: 40mm.
• Panel size: 4HP