5 Properties of the Lexicon 480L large hall algorithm

Have you ever listened to the tail of the 480L large hall algorithm without dry signal?

Where does all this phasing come from? It sounds ugly - far from the IR you get from any major hall - but magic happens when mixed with the dry signal in the mix. Whats going on?

To test the phasor hypothesis, I mixed a decent reverb with its phase inverted signal on a creamware platform. It cancelt out 100 per cent. Good. Creamware doesn’t fool me with random block processing like MAX or PD. Then I rised delay of the inverted signal slowely. 1 ms is a huge step and sound changes dramatically. At 10-11ms the filtered (I think) reverb tail was allmost indistinguishable from the Lexicon’s. Modulating the 11ms delay didn’t get much closer. Modulation of delays seems to happen inside the core algorithm.

5.1 What core algorithm?

Schroeder reverbs and FDNs don’t give as much control over the onset and shape parameters compared to the nested allpass structures proposed by Gardner. Gardner published 3 different algorithms: small, medium und large hall. Griessinger (Lexicon founder) talked about Gardner. It seemed to be a good fit. Gardner’s large hall is my personal favorite anyway and it can be made to cause high frequency losses internally which sounds much more natural than the loss filters commonly found in FDN designs.

5.2 Psychoacoustics

When modulating delays the overall shape of the reverb stays more or less constant but the fine structure changes continuously. As it does in real halls. A concert hall with humans on stage is a time variant system. But the 480L’s hall changes more drastically and in different ways. That’s key.

A constant repetitive stimulus is inhibited in seconds - so called Clifton effect. To counteract this the reverb level could be raised, but that would muddy the sound. Exaggerated time variance and randomness helps enormously in the mix, especially in the decay phase when envelopment is lost otherwise, simply by calling attention to a varying stimulus - which by then sounds sufficiently different to be recognized as a separate auditory event - the reverb tail.

The 480L is commonly not used on completely dry signals but on acoustic instruments. It therefore enhances the salient properties of the recorded hall.

5.3 ClassicReverb

Finally I got close enough to use my own reverb on a daily basis, called ClassicReverbVST. Instead of tuning each delay separately I took the lazy approach and modulated the phased output stage. It could easily be tweak’d further, but hey, it works for me. It is available on this website as download and source code.