PT2399 Overall Configuration

This PT2399 overall view shows the basic echo circuit configuration. As you can see, the manufacturer has exposed some of the pins to the internal operational amplifiers so that the constructor can connect resistors and capacitors to form low-pass filters. As you can see, the delay line is the main centre block responsible for generating the delay. However, on either side of the delay line (at the input and output ends) there are low-pass filters. The low-pass filter at the input end (LPF1) is an anti-aliasing device that eliminates all input signals outside the designed frequency range. This filter is very important because with echo type circuits, any noise that gets into the echo loop becomes prominent and lowers the quality of the overall audio.

The low-pass filter at the output end usually takes the signal from the digital to analogue converter (DAC) and removes any clock break-through. The analogue signal from the DAC converter is usually stepped (also known as quantized) and the low-pass filter smoothes it to appear like a regular analogue signal. It does this by filtering out the steps which are usually high frequency components. Without this filter the sound will have a metallic quality to it. This could of course be useful as a sound effect to perhaps make a robotic sound.

As you can see, the low-pass filter 2 (LPF2) is within the echo feedback loop and its output fed to a potentiometer which forms the repeats control. The delayed signal from this potentiometer is then fed back to the input stage around LPF1, which also behaves somewhat as a multi-feedback filter and input mixer. Hence it mixes the initial input signal with the delayed signal, and consequently we take the final output from pin 15.

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