Blue Sky "shimmering" sound

[boga]

Hi!

I am curious what is going on in the algorithmic sense when reverb adds this "shimmering", "synth" like sounds to the guitar sound? Like Strymon Blue Sky or other ambient reverb effects are doing? Can somebody explain or does someone even have some code to share?

All the best!

[Blackaddr]

A reverb is can be thought of basically as a multi tap delay, where each delay represent a longer echo reflection time. A naive approach to implementing this however won't sound very good. Having multiple "echoes" isn't enough. You need to provide some filtering since echoes typically lose a lot of frequency content depending on the room. In a cathedral, often lows are lost and thus as the highs in the echoes add up you get an increase in overall high frequencies, sometimes referred to as "presence".

If you take a look at the well known freeverb algorithm https://ccrma.stanford.edu/~jos/pasp/Freeverb.html

You'll see both comb filters as well as all-pass filters meaning they phase shift different frequencies by different amounts but don't directly attenuate them. The comb filtering and phase shifting combined with recombination causes attenuations. This results in a much more natural sounding reverb as frequency filtering is achieved through destructive interference, as apposed to directly filtering reflections.

I've not examined the freeverb algorithm in detail so the above is probably a little oversimplified, and a little-wrong. Either way, studying that algorithm is a great place to start for playing with reverbs.

[audioartillery]

To add to Blackaddr's concise summary of artificial reverb, I believe the "shimmer" type reverbs use an octave shift (up) to give it that high frequency content.  I haven't built one but I think I would start with the octave up and just reverb that and see what it sounded like.  In practice it's probably filtered heavily and mixed with a non-shifted reverb as well.

Reverbs aren't all that scary to implement with DSP, it's a worthy exercise if you're interested in reverb and have some basic programming skills.

[gena_p1]

several years ago found this pic in the internet



main idea : you need 1+ octave up and some +2 octave up with some clean - send to high decay reverb and you will get basic shimmer.

Ice-9 could say correctly, he is one of pioneers of shimmer on fv-1 chip

[DrAlx]

Take reverb output, put it through an octave-up pitch shift and an extra delay, and then feed it back to the reverb input.
Putting a delay on the pitch shift gives more control on how the shimmer effect builds up.
The general idea is that you get a slowly building and overlapping set of octave-ups.

See https://m.youtube.com/watch?v=BDO22SP8OJA

[Ice-9]

I think the FV-1 chip is the easiest for the DIY shimmer, yes it is a digital DSP rather than an analogue option but this makes it a simpler circuit to play with and develop.


A little off topic i suspect but here is a complete code for a newer shimmer I was working on some time ago. I will see if I can make a little sound clip in a while.

Code Select Expand


;New Shimmer Reverb Program
;from 3k Room
;09/01/2013 rev 1.01 Mick Taylor
;22/11/2015 rev 2.1 Set pre Delay Reverb freq. response and gain changes
;                    pre delay removed for shimmer code space             
;07/12/2015  Shimmer code added Mick Taylor

;Pot0 = Shimmer
;Pot1 = reverb level
;Pot2 = reverb time

mem shimdel 4096 ;delay for shimmer
mem stemp 1
mem idel 4000 ;initial sound space  122mS
mem iap0 11
mem iap1 27
mem iap2 43
mem iap5 171
mem iap6 296 ;thickening all passes embedded in initial delay

mem ap1 134         ;4.1mS
mem ap2 256         ;7.8mS
mem ap3 562         ;17.1mS
mem ap4 763 ;reverb loop input all passes

mem lap1a 1421       ;43mS
mem lap1b 1945       ;59mS
mem d1 2434       ;74mS
mem lap2a 1894       ;58mS
mem lap2b 1767       ;54mS
mem d2 2645 ;80.7mS   loop constants

;write constants registers

equ kd -0.5 ;damping coefficient for shelving

;write-first registers

equ dry reg0
equ rev_in reg1
equ kirt reg2 ;coefficient to scale initial sound
equ krt reg3 ;coefficient to affect RT of loop
equ apout reg4 ;output of loop input all passes
equ temp reg5 ;temp register for filter routines
equ gain reg6 ;adjust gain with RT
equ revout reg7
equ    pitchout  reg8    ;octave up output

;read-first registers;

equ lf1 reg20 ;reverb loop filter 1
equ lf2 reg21 ;reverb loop filter 2
equ hf1 reg22 ;loop high pass 1 fixed
equ hf2 reg23 ;loop high pass 2 fixed
equ lfin1 reg24 ;LPF for imbedding in initial delay
equ lfin2 reg25 ;LPF for imbedding in initial delay
equ lf reg26 ;input low pass shelving with kd
equ lpfp reg27

equ    lpfk    0.3      ;lpf coefficent for lpfp after pitch shifting 1.85kHz
equ    lpfs    -0.5      ;Shelving coefficent for lpfp

;clear read-first registers

skp run,endclr
wrax lf1,0
wrax lf2,0
wrax hf1,0
wrax hf2,0
wrax lfin1,0
wrax lfin2,0
wrax lf,0
endclr:

;initial sound tap positions 30.5uS/location, 100=3.05mS

equ ld1 874 ;first tap, left   26.7mS
equ rd1 874 ;first tap, right 26.7mS
equ ld2 1156 ;and so on...    35.3mS
equ rd2 962                    ;29.3mS
equ ld3 1345                  ;41mS
equ rd3 1121                ;34.2mS
equ ld4 1456                ;44.4mS
equ rd4 1423            ;43.4mS
equ ld5 2121            ;64.7mS
equ rd5 2124            ;64.7mS
equ ld6 3245              ;99mS
equ rd6 3646              ;111.2mS

;initialize sin LFO

skp run,endset
wlds sin0,25,100
wldr RMP0,16384,4096    ;load octave up
endset:
;--------------Off and Running Program Loops to Here--------------------------
;prepare decay pot Reverb Time

rdax pot2,0.97 ;get pot, limit to less than infinite
wrax krt,1 ;write loop decay time
sof 0.4,0.6 ;scale Pot to 0.6 to 1.0 range
wrax kirt,1 ;write impulse filter gains changed to 1 from 0 MT 22-11-15 gain always +0.99 before=too high
sof -0.88,0.99          ;scale to decrease gain with RT need to assess the -1 & the 0.99 range offset for  gain Vs RT now changed to -0.88
; Range allowed=-2.0 to +0.9999389 e.g. From scale above if pot2=0.6 then 0.6* -0.88+0.99=0.46;if pot2=1 then gain=0.11 23-11-2015
wrax gain,0 ;write gain factor and clear ACC

;------------------------------Octave up------------------------------------

cho rda,RMP0,REG|COMPC,shimdel
cho rda,RMP0,,shimdel+1
wra    stemp,0
cho rda,RMP0,RPTR2|COMPC,shimdel
cho rda,RMP0,RPTR2,shimdel+1
cho sof,RMP0,NA|COMPC,0
cho rda,RMP0,NA,stemp
mulx    POT0
rdfx    lpfp,   lpfk      ;Freq coef
wrhx    lpfp,   lpfs      ;Shelving coef.
wrax    pitchout,0

;-------------do inputs to predelay-------------------
;rdax pitchout,1
rdax adcl,0.5
rdax adcr,0.5 ;get inputs sum & divide by 2
wrax dry,1 ;22/11/2015 write dry input signal to dry register and keep in ACC for mulx next instruction:
mulx gain ;Acc=Acc* reg& give greater gain to short RT See code above for adjusting this 23-11-2015 Steve
wrax rev_in,1 ;22/11/2015 write gain adjusted dry input to rev_in register and clear ACC Reg1
wra shimdel,0
;--------------------read predelay and write initial all pass response delay------------------

rdax pitchout,1
rdax rev_in, 0.5              ;use 97mS delayed signal divided by 2 for reverb input
rda iap0#,0.5              ;Read from end of initial all pass memory0 divide by 2 adding to rev_in data
wrap iap0,-0.5 ;complicate input to initial delay
wrax temp,1                  ; Write ACC to register;multiply ACC x 1.         
rdfx lf,0.404              ;Low pass <2.7kHz
wrhx lf,-1                      ;Register=ACC; ACC=ACC*&#40;-1&#41;+previous contents of ACC
mulx kd                          ;ACC=ACC*Reg  kd = damping coefficient for shelving from POT0
rdax temp,1 ;low pass filter entire input
wra idel,0 ;write initial sound delay  clear ACC

;complicate initial sound

rda idel+500,1            ;read from 15.2mS position Retain ACC
rda iap1#,0.5                ;read from end of iap1 delay stream divided by 2
wrap iap1,-0.5                ;write to beginning of iap1,
wra idel+500,0            ;data at delay ram address=ACC; ACC=ACC*0 , i.e. clear ACC

rda idel+1000,1            ;read from 30.5mS position
rda iap2#,0.5                ;read from end of iap2 delay stream divided by 2
wrap iap2,-0.5                ;write to beginning of iap2,
wrax temp,1 ;save filter input
rdfx lfin1,0.2                ;0.2=~1.2kHz perhaps too high for Abbey Road reverb try 600Hz = 0.109
wrhx lfin1,-1 ;make HP filter
mulx kd ;multiply by negative shelving coef
rdax temp,1 ;add back input shelving LPF
wra idel+1000,0          ;now modify idel+1000 but clear ACC

rda idel+2500,1          ;read from 76.3mS position retain ACC
rda iap5#,0.5
wrap iap5,-0.5
wrax temp,1 ;save filter input
rdfx lfin2,0.2                ;0.2 =~1.2kHz
wrhx lfin2,-1 ;make HP filter
mulx kd ;multiply by negative shelving coef
rdax temp,1 ;add back input shelving LPF
wra idel+2500,0          ;data at delay ram address=ACC; ACC=ACC*0 , i.e. clear ACC


rda idel+3000,1          ;read from 91.5mS position retain ACC
rda iap6#,0.5
wrap iap6,-0.5
wra idel+3000,0

;do reverb input all passes

rda idel,0.9 ;leave some headroom
rda ap1#,0.5
wrap ap1,-0.5
rda ap2#,0.5
wrap ap2,-0.5
rda ap3#,0.5
wrap ap3,-0.5
rda ap4#,0.5
wrap ap4,-0.5
wrax apout,0                  ;Save all pass out to

;do reverb loop and sum all outputs

rda d2#,1                    ;Read from end of d2, retain ACC
mulx krt                        ;krt = Reverb Time coefficient
rdax apout,1
rda lap1a#,0.5
wrap lap1a,-0.5
rda lap1b#,0.5
wrap lap1b,-0.5
wrax temp,1 ;save filter input
rdfx lf1,0.404                ;2.7kHz
wrhx lf1,-1 ;make LP filter
mulx kd ;multiply by negative shelving coef
rdax temp,1 ;add back temporary filter input keep ACC
rdfx hf1,0.01                ;ACC=ACC+reg-ACC*0.01
wrhx hf1,-0.5 ;roll out lows in loop
wra d1,0                      ;Write sum to d1 location clear ACC

rda d1#,1                  ;Read from end of d1 memory
mulx krt
rdax apout,1
rda lap2a#,0.5
wrap lap2a,-0.5
rda lap2b#,0.5
wrap lap2b,-0.5
wrax temp,1
rdfx lf2,0.404              ;Again use 2.7kHz
wrhx lf2,-1
mulx kd
rdax temp,1
rdfx hf2,0.01
wrhx hf2,-0.5
wra d2,1.99               
rda d1,1.99               
mulx pot1                     
mulx pot1
wrax revout,0              ;Reverb output saved to register, ACC cleared

;do reverb smoothing

cho rda,sin0,sin|reg|compc,d1+100
cho rda,sin0,sin,d1+101
wra d1+200,0

cho rda,sin0,cos|reg|compc,d2+100
cho rda,sin0,cos,d2+101
wra d2+200,0

;now combine to output

rdax dry,1                      ;
rdax revout,1
wrax dacl,0