BBD Box Analogue Multi processor
This is the circuit y'all have been asking for. It was basically an experimental test bed for the possibilities using a CCD based, Bucket Brigade audio delay line.

It flanges, it choruses, it chops, it dices. It even produces a crude Surround sound output. Various kinds of input schemes have been employed here and can be electronically switched.

The basic premiss is that it should be a pseudo stereo processor. That is to say one that will give the impression of a stereo field from a monaural source. Only with a twist. The source is also stereo. The idea is that it will cause the monaural component of a stereo source to be widened out to give the impression of a stereo source. Lets run by that again.

If the signal is stereo then it passes through the unit largely untouched. Of course some signal changes will occur but this is not generally a problem. However if a mono signal is passed through the unit, (IE: both left and right posses the same signal with the same phase) then it will be comb filtered by the BBD and spread out to give the impression that it's stereo.

It was originally used to process the output of a sampler which had limited resources in terms of sample RAM and mix. For example a Sample of a drum kit would be taken. Say 4 toms, kick, snare and metal work. The kick and snare would have ambience in the form of gated reverb on them but it would be mono which sounded dead and in the centre of the mix. Not very ambient in reality. On the other hand the toms were panned across the stereo field such that the high toms would be left, progressing through the low times in the right. All this in the one stereo mix and not separately processable (Should such a word exist)

This box was the solution, It could allow the toms to be placed where ever in the stereo field whilst the reverb on the Kick and snare sounded like stereo reverb rather than a mono sample.

Pseudo stereo is simple enough in concept. It is often used as the output stage on synths. It is simply stereo chorus. Where a mono signal is delayed ever so slightly, preferably by a slowly changing delay time (LFO) and then this signal is fed back to the original signal causing what is known as a comb filter. In stereo chorusing, the original signal is split left and right, but the delayed signal is fed, 180 degrees, out of phase to on or the other channels. Therefore what is cancelled out in one channel is boosted in the other and visa versa. The ear gets the impression of a wide stereo image with a central source. It is common for even high end digital stereo reverb units to use a similar approach.

This unit differs in two ways. You can get any kind of time related effect from it except actual reverb and it gives you a variety of phasic pre-processing into the delay line it self. For example you can have it simply sum the two channels together to become a mono version of the original stereo signal. Or you can derive the BBD's signal by differencing. As used in the basis for surround sound. This is where the left and right signals are subtracted from each other leaving only a signal that is not present equally in both channels.

Further subtraction and addition can be conjured such that only those sounds that are common to both left and right will pass through to the BBD and signals that are different or un-equal will be rejected. (In theory) and this is what sets this beast apart from any other processor. In theory, it will reject the toms because they are different in each channel but will process the Kick and snare because they are the same. The result is that it will give the impression that you have bucket loads of digital reverbs. :)

Beyond that, this box can provide flanging and general stereo chorusing and some truly wild special FX. It has an additional surround sound output in case you need it and the in/out switching is provided by a set of VCA's such that everything happens smoothly. You can go from dry signal to an extremely wet one seamlessly such that if you're wearing headphones, it feels like you're ears are being torn from your skul.

Of course these days, AEK uses some more sophisticated digital processing but for anyone wishing to experiment with the concepts, this may be a good starting point.

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