As has been mentioned, yes you have to use feedback loops to send signals "backwards". The problem with not doing it is... sometimes it works! So then you think it should always work. Alas no. At the time I was working on this, I may have thought about how to detect a backwards-going signal, but I never implemented anything to prevent it.
I've probably said this on and off over the years, but SpinCAD Designer is not really geared to reverb design. I had some ideas about what you might do if you wanted to have a more reverb-builder focus. Ultimately I ran out of energy to maintain it and I do still think that doing reverbs in FV-1 assembler is probably best once you know what the building blocks are, just because there are SO MANY little parameters to adjust, if you really want to get to that level of granularity.
Also, just so you know, the Chirp block was a basic element in Julian Parker's Spring Reverb paper that I've referenced a few dozen times. But it's not a fully-realized chirp. It's just the stretched all-pass stage.
https://acris.aalto.fi/ws/portalfiles/portal/13004391/art_10.1155_2011_646134.pdfThis block lacks the low pass filter at the first transition frequency so you will actually get "mirrored" chirps up to the Nyquist point theoretically. It is probably preferable to put a low pass there. You'll have to figure out where it needs to go. Unless you can figure it out mathematically I'd just measure it using the spectrogram in Audacity along with some click/snare drum source material. Parker's paper recommends something like a tenth order filter, so good luck implementing that on an FV-1. I know you can do it as a series of two-pole stages, but still... there aren't even any examples of how to do direct-form filters. At least, I haven't seen any and I've read the FV-1 knowledge base many times. You can also just stack 2nd order SpinCAD filters at the corner frequency, but this will most likely suffer response anomalies compared to a properly designed high order filter.
Also it lacks the "chirp straightening" which is essentially a crossover well below the first chirp frequency, so that DC and low frequencies are not chirped. But yes, even with those shortcomings, it does provide time smearing in an interesting way and is one of my biggest interests in audio DSP. I really thought about trying to do all that on an FV-1 but it doesn't have enough instructions to do it accurately. There is a Linkwitz-Reilly crossover in one of Spin's examples. For cheap you might try the high/low outputs of a state variable filter but it may have other effects.
Double checking. OK they really are stretched all passes, I think. These are the way all all passes are done in the FV-1. I never thought about the mirroring. I will double check it.
OK here they are. Yes they are gloriously mirrored. Input pulse is on the bottom, chirp output on the top. This was done using the "File Output" mode of SpinCAD's simulator.
Here's the FV-1 code. It's a series of chained "chirp" blocks all with the same settings, taking 124/128 of the FV-1's instructions. With these particular settings, the chirp transition frequency is a little below 3 kHz and the chirp is about 50 msec long.
; mega-chirp.spcd
; null
; Pot 0:
; Pot 1:
; Pot 2:
;
;
; ----------------------------
;------ Input
;------ Chirp
RDAX ADCL,0.5000000000
RDA 12,0.38
WRAP 0,-0.38
RDA 25,0.38
WRAP 13,-0.38
RDA 38,0.38
WRAP 26,-0.38
RDA 51,0.38
WRAP 39,-0.38
RDA 64,0.38
WRAP 52,-0.38
RDA 77,0.38
WRAP 65,-0.38
RDA 90,0.38
WRAP 78,-0.38
RDA 103,0.38
WRAP 91,-0.38
RDA 116,0.38
WRAP 104,-0.38
RDA 129,0.38
WRAP 117,-0.38
RDA 142,0.38
WRAP 130,-0.38
RDA 155,0.38
WRAP 143,-0.38
RDA 168,0.38
WRAP 156,-0.38
RDA 181,0.38
WRAP 169,-0.38
RDA 194,0.38
WRAP 182,-0.38
RDA 207,0.38
WRAP 195,-0.38
RDA 220,0.38
WRAP 208,-0.38
WRAX REG0,0.0000000000
;------ Chirp
RDAX REG0,0.5000000000
RDA 233,0.38
WRAP 221,-0.38
RDA 246,0.38
WRAP 234,-0.38
RDA 259,0.38
WRAP 247,-0.38
RDA 272,0.38
WRAP 260,-0.38
RDA 285,0.38
WRAP 273,-0.38
RDA 298,0.38
WRAP 286,-0.38
RDA 311,0.38
WRAP 299,-0.38
RDA 324,0.38
WRAP 312,-0.38
RDA 337,0.38
WRAP 325,-0.38
RDA 350,0.38
WRAP 338,-0.38
RDA 363,0.38
WRAP 351,-0.38
RDA 376,0.38
WRAP 364,-0.38
RDA 389,0.38
WRAP 377,-0.38
WRAX REG1,0.0000000000
;------ Chirp
RDAX REG1,0.5000000000
RDA 402,0.38
WRAP 390,-0.38
RDA 415,0.38
WRAP 403,-0.38
RDA 428,0.38
WRAP 416,-0.38
RDA 441,0.38
WRAP 429,-0.38
RDA 454,0.38
WRAP 442,-0.38
RDA 467,0.38
WRAP 455,-0.38
RDA 480,0.38
WRAP 468,-0.38
RDA 493,0.38
WRAP 481,-0.38
RDA 506,0.38
WRAP 494,-0.38
RDA 519,0.38
WRAP 507,-0.38
RDA 532,0.38
WRAP 520,-0.38
RDA 545,0.38
WRAP 533,-0.38
RDA 558,0.38
WRAP 546,-0.38
WRAX REG2,0.0000000000
;------ Chirp
RDAX REG2,0.5000000000
RDA 571,0.38
WRAP 559,-0.38
RDA 584,0.38
WRAP 572,-0.38
RDA 597,0.38
WRAP 585,-0.38
RDA 610,0.38
WRAP 598,-0.38
RDA 623,0.38
WRAP 611,-0.38
RDA 636,0.38
WRAP 624,-0.38
RDA 649,0.38
WRAP 637,-0.38
RDA 662,0.38
WRAP 650,-0.38
RDA 675,0.38
WRAP 663,-0.38
RDA 688,0.38
WRAP 676,-0.38
RDA 701,0.38
WRAP 689,-0.38
RDA 714,0.38
WRAP 702,-0.38
RDA 727,0.38
WRAP 715,-0.38
WRAX REG3,0.0000000000
;------ Output
RDAX REG3,1.0000000000
WRAX DACL,0.0000000000
RDAX ADCL,1.0000000000
WRAX DACR,0.0000000000
DL
