help me replicate Cubase Bit Crusher effect in a pedal!

[ian87]

anyone familiar with this? i've been applying the Bit Crusher liberally to my drum trax recently, with perfectly awful results.

has anyone built a "downsampler" in a pedal?

if not, how would i go about it?

thanks!

-ian

[Peter Snowberg]

I have a commercial design in the pipes so I'm not going to be too specific, but with that in mind....

Bit crushing (in general) and downsampling are two different effects. Bit crushing limits the amplitude domain information where as downsampling limits the time domain info. The limiting of the time domain info has the additional "benefit" of creating aliasing of input frequencies which can have almost ring-mod type sounds associated with it. Using both destruction forms together can be downright fun!

Get an 8 bit (or 10 bit) A/D and D/A with parallel interfaces. Use the oscillator from a PLL like the 4046 to clock the conversions and use a bunch of switches to remove various bits from the parallel interface between the converter chips. You can also re-route bits to create octave effects.

The digital bit crushing is best done with the aid of a microcontroller or a dedicated ALU/barrel shifter chip for maximum flexibility. Such designs get painful quickly if you use dedicated ALU hardware. One such chip is the http://assets.zarlink.com/DS/zarlink_PDSP1601A_NOV_98.pdf but microcontrollers allow for much more in less space with less power consumption.

I hope that help at least a little.
-Peter

[ian87]

thanks for your response, peter. it's gonna take me a while to decode all that, but i appreciate it.

now you've piqued my curiosity about your own design! can you at least say what the application is? pedal? pro audio? something else entirely??

[Peter Snowberg]

It's for a guitar pedal.

I'm using 12 bit A/D and D/A with an Atmel AVR microcontroller to shift and manipulate the bits, but here is the basic concept:



As you remove bits going towards the MSB (most significant bit), you get closer and closer to making a square wave converter. The sample rate control allows you to interject aliasing noise if the anti-aliasing filter that's normally on the front of a sampling circuit is not adjusted along with the sample rate. The Nyquist theorem says that you can't feed an A/D with any frequency over 1/2 of the sample rate without generating "aliasing artifacts" which sound dreadful to "normal" people. For this reason some of the classic analog delays (at least a couple of them) actually swept the filter frequency to avoid aliasing as it has a very metallic sound. If you filter the sound after the D/A, it gets much better.

If you re-route input bits to different output bits (e.g. bit 2 -> bit 6 and so on) you fold the waveform over and get all kinds of octave madness.

Take care,
-Peter

[ian87]

WOW.  :shock:

ok, since you've clearly already done all the hard work....

.... when will this thing be available??? how much? can i beta-test it for you???



-ian

[Tim Escobedo]

Both Paul Perry and Alesis make gizmos that do this.

[Peter Snowberg]

Good questions....

I need cash right now and I'm moving to Sausalito in a couple of months so it's going to be a little while. I can't afford to get boards made at the moment.

I'll let you know when it's ready for beta testing but if you hold your breath you're going to turn blue.

It looks like Paul Perry already sells something of this flavor:
http://www.frostwave.com/sonicalienator/
I've never had the pleasure of playing with one, but they look similar in ways. If you want something short term, I would buy his.

Take care,
-Peter

[mattv]

Ian, one of those boxes you borrowed from Chris is strikingly similar in design to what Peter drew up. I wonder how it sounds compared to the plug-in...?

[ian87]

hey matt! long time.

yeah, i didn't play with the 8-bit fuzz for long enough to get a good feel of what it was capable of. but thanks to tim, i just picked up an Alesis BitRman. Compression, distortion, dual phasing, plus a 4th effect that can either be a comb, decimator, bit reducer, frequency modulator, ring modulator, or frequency shift.

can't beat that for 50 bucks!!

thanks tim, and peter, and matt.

[Paul Perry (Frostwave)]

That's the first time I ever was put in the same sentence as Alesis  :roll:  
but, yes, the Frostwave "Sonic Alienator" uses a PIC micro for the digital crushing (24 algorithms to choose). Also a low pass filter (analog, based on the MS20 LM13600 filter), also variable sample clock rate.

[Dan N]

Ian, one of those boxes you borrowed from Chris is strikingly similar in design to what Peter drew up. I wonder how it sounds compared to the plug-in...?

Yes, very similar:




That was a fun pedal, although I pretty much left the switch on the first MSB on and had my fun playing with the sample rate.

[mattv]

If I ever get around to building it, I'd like to try rerouting the bits, as Peter mentioned. I'm sure that would be interesting.