Optical delay

[samhay]

There has been a few threads lately discussing short and/or alternative methods of generating delay.
When the discussion turned to bouncing a signal of the moon, I figured it was time to play along.

Ultrafast (sub nanosecond-ish) spectroscopy makes use of optical delay lines - some movable separation between 2 or more mirrors - to set very short time delays. I figured I would do the same using a phototransistor as the receiver, which we can drive via a distant LED. This is what I built:


(click for larger version).

It works, and gives surprising little distortion or noise.
The phototransistor has peak response ~ 900nm and I have some NIR LEDs, but opted to use a red LED so I could see that I was pointing it at the phototransistor - I can't see 900nm light.

I then set to play around with the distance between LED and phototransistor - every 3 cm (~1") should give ~100 picosecond delay. This didn't work very well over the distances I tried. It got progressively more difficult to bias the phototransistor as the distance increased (less light absorption) and further, the delay I measured was always ~50 microseconds (us). I am guessing this is limited by the LED and/or phototransistor response.

However, as a bonus/consolation prize, you can get a few dB of voltage gain, which I wasn't expecting.  If you put 2 LEDs in series and point them both at the phototransistor, you get more gain, so I guess this may arise due to constructive interference (i.e. loud nodes) between the 2 LED beams?

So, if you want a really short, ~50 us delay and/or a rather unique kind of booster (put 2 red LEDs in series where D1 is), then give it a try.

p.s. the frequency response of the delayed signal is not great either. Treble is rolled off quite significantly, again probably due to LED response. However, this could be seen as a colourful new kind of mojo...

[deadastronaut]

cool...could this be used to create the 'haas'' effect...?

where say a right channel can be ever so slightly delayed...

https://www.youtube.com/watch?v=-PFgCrvwByo


edit: this was a good episode of 'rough science' where the guy used an opened transistor
to communicate with light...very cool.

https://www.youtube.com/watch?v=e-wrF728ZNo&t=1135s

[balkanizeyou]

now you only need 1000 stages of this to get a long delay!

but in all seriousness, cool stuff.

[samhay]

> https://www.youtube.com/watch?v=e-wrF728ZNo&t=1135s
That's the same idea. They have a bigger light than I do.
The same approach is almost certainly how the 'light lead' works too (but they using an optical fibre, which is probably a better idea than trying to directly beam your signal across the stage): http://www.iconicsound.com/

Rob - I have an idea brewing for a Hass effector, but it won't be using this approach. You want 10's ms, and this is giving you 10's us, so your field will be a few mm closer to 1 ear than the other. Not sure this will cut it.

I did (briefly) consider using multiple stages as an optical BBD-a-like, but you need a few ms delay to be audible (AFAIK), so I would need ~40 stages for 2ms, which sounds like a lot of work...

[R.G.]

One could also use a speaker driving the end of a garden hose, with a mike at the other end. Delay is one millisecond per foot.

[samhay]

^yes, that was mentioned recently.
http://www.diystompboxes.com/smfforum/index.php?topic=116204.msg1075044#msg1075044

[ElectricDruid]

One could also use a speaker driving the end of a garden hose, with a mike at the other end. Delay is one millisecond per foot.

So if I build a tube with a speaker at one end, and a microphone at the other attached to a long piston-arm that makes the microphone shoot up and down the tube, I could organize it so that I got delays between (say) 3 inches and 10 feet (hey, it's a big wheel) and then I'd have the world's weirdest flanger? Also the world's largest flanger. In fact, several new Guinness world records would probably have to be created for such a device.

T.

[samhay]

^It would be cooler if you built a giant trombone playing robot and used that as the variable delay, but yes.

[R.G.]

^yes, that was mentioned recently.

And long, long ago, and many times in between. 

There are issues, not the least of which is the resonance of the chamber between the mike and speaker, and the damping, or lack thereof.

A spring reverb is the same thing, but with magnetic "speaker" and "mike" on the ends. The speed of sound in metals is between several and many times the speed of sound in air. But there are the issues with resonance and reflections still gets in there.

The speed of light in air is about a half-nanosecond per foot, so you need ~2000 feet of travel per mS of delay. The loss of signal level is brutal, and so is the need for LONG travel paths. Maybe you could use a few km of optical fiber on a spool and put better "speakers" and "mikes" on the ends. A km of fiber is about 3281 ft, so you might get on the order of  units of milliseconds of delay out of a 1km fiber.  A good multimode optical fiber will have attenuation in the range of 3-6db per km, and a single mode fiber will be as low as 0.15db per km, so there's hope on the attenuation and signal loss front.

But a km spool of fiber is still big and heavy. 

[samhay]

>But a km spool of fiber is still big and heavy.
and very expensive - I did consider this briefly.

[Tony Forestiere]

Maybe Wavley will kick in with his "Cistern" reverb. I can't locate the pictures.

[Ben N]

Bottom line: Little Cistern wont do what her Big Cistern do.

[mac]

One could also use a speaker driving the end of a garden hose, with a mike at the other end. Delay is one millisecond per foot.

Put the hose into the fridge. As c drops with T you can have more ms per foot

Or use the hose rubber, c is 60m/s.

mac

[wavley]

Maybe Wavley will kick in with his "Cistern" reverb. I can't locate the pictures.

Somewhere I have pictures, but 20mS delay time is a bit to ask of my cistern that is the only thing that I miss about my old house.  Nice, natural sounding reverb time though!

A while back I was working with one of the HAM operators at work trying to get his dish working again so that I could bounce my guitar off the moon in honor of the anniversary of the moon landing, but it never happened and now he's unlikely to do moon bounce until he sells his house and moves in with his daughter who is apparently building a HAM shack for him.

Bottom line: Little Cistern wont do what her Big Cistern do.

You're right there!  Mine was 20x10x6 but this one...

[dbp512]

hmm, looks like it might work as a double tracker. I've had my eye on the keeley 30ms, but I don't like buying things when I could instead try and build them. Would the difference between 30ms and 50ms have much of an effect; or can we shrink the delay time just a touch?

[chuckd666]

The delay time is 50 MICROseconds.

[dbp512]

The delay time is 50 MICROseconds.

Well, this is super awkward. I'm in pharmacy school, i *really* should know the difference between micro and milli...

[chuckd666]

You're in deep trouble now! 

[dbp512]

You're in deep trouble now! 

Just don't tell my prof, and pretend you didn't see this. Remember, you're only in trouble if you get caught!