Airwindows TapeDither Redux (and 'one more thing')

VST, AU, etc. plug-in Virtual Effects discussion
990 posts since 7 Apr, 2007 from Bellows Falls, VT

Post Sun Aug 09, 2020 6:00 pm

TL;DW: TPDF Dither voiced for a tape-noise-like effect.


(also, here is the One More Thing, a theory on nodal tempo 'flavor')
I thought I had it all worked out. Put out a proper, well-coded TPDF dither, the highpassed variation on it I've called PaulDither, and move on to the fancy boutique stuff.

But there was this one experiment I had in mind. If you could do the highpass by delaying and then subtracting the random noise (and it gained you a bit of efficiency in the bargain), why not delay it more? It'd create comb filtering, a cancellation node. Why not keep delaying until the cancellation node dropped right down to around 1.5K?

Surely this would give me a nice cancellation notch right where the ear is most sensitive, and heightened clarity. What could go wrong?

Nope! I made a prototype, called it 'NodeDither', began experimenting, and immediately found that I'd made… a flanger! The long delay settings were useless. It made obvious overtones just out of the dither noise, a blatant tone color cast that wouldn't produce the desired effect. It didn't even produce an obvious notch in the response where I wanted it. The only thing it did do, was continue to function as a working TPDF dither no matter what the setting was (more on this later).

But, something else turned up in the experiments, and that's what brought you TapeDither.

If you use one sample of delay and inverting the noise, you get PaulDither: simple one-pole highpass. If you use two samples of delay, you get another sort of texture: kind of silky, but still digitally bright and intrusive. Using lots of delay, such as ten samples, starts to sound like the flanger, undesirable.

But, there's something interesting about powers-of-two delay times. One, two, four, eight and so on, these delay times are slightly less 'colored' in tone than the others. I think it has to do with interactions with the sample rate: they seem to line up more neatly, making it slightly more easy not to hear the pitch of the 'flangey' quality.

And four samples of delay (and then inverting the noise) produces something rather special: a noise profile that closely resembles what you hear off reel-to-reel tape.

I can't specify particular brands because (a) I hate when people do that to brand names not their own and (b) it's a technical discovery, not some complicated way of forcing digital audio to mimic a particular brand. It's no specific tape stock or tape machine. But what it is, is a voicing for TPDF dither that rolls off in an obvious way, around where tape noise rolls off. There's another little bump past that, which many people won't be able to hear, and then it begins to roll off again as it reaches the Nyquist frequency beyond where digital audio can't go. Compare that to any normal flat, TPDF, or highpassed dither. Those keep putting out noise energy right up to the frequency limit.

TapeDither is every bit a TPDF dither, technically correct and flawless as far as dither goes. But it also is a highpassed dither with a softer tonal voicing that resembles good tape machines, and that doesn't affect the dither performance at all. It doesn't attenuate the audio content at all. Only the background dither noise is turned into what you'd get off a tape deck, all while the audio is protected from truncation and digital artifacts.

The update (redux) to this plugin adds a 16/24 bit switch (defaulting to 24, like the original) and a DeRez control that lets you either monitor the effect more easily, or bit-crush tracks using this to dither the result. Mind you, like any proper TPDF dither, this completely removes the quantization noise so it becomes just raising the noise level" but hey, it might come in handy.

These experiments are paid for by Patreon, so if you would buy these plugins you can instead support the Patreon. It helps me scale up my whole deal, as you can see :)

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