PT2396 pitch shifter?

[earthtonesaudio]

Taking a look at the PT2396's datasheet, it seems one could build a pitch shifter by connecting a 4-bit synchronous up/down counter to pins 4-7.

Just the PT2396, a CD4029, and a variable frequency square wave clock would be all you need for a DIY whammy.

...well maybe not a whammy but perhaps something along the lines of a Clari(not).

[anchovie]

I too once thought this was a nice idea...then I saw page 8 of the full datasheet that says there's a 525ms mute period between delay time changes!

[earthtonesaudio]

Oh... well now, that changes things. 

[Taylor]

I am currently working on a pitch shifter using PT2399s, lots of opamps and some OTAs. It's basically a [------]* implementation of the standard digital pitch shift algo, using ramp waves in quadrature to bend 2 delay lines, and then crossfading between the outputs of the delays. I know it's not "practical" (especially since I have my own DIY DSP pitch shifter) but I think it will be a lot of fun, and especially because the whole architecture is open rather than in code, so you can easily modify it to do weird stuff.

*What's the word I'm looking for? "Discrete" is wrong, because it uses chips. "Analog" is wrong, because the delay lines are digital. But there should seemingly be a word to differentiate a device which uses a monolithic DSP chip, from one which uses a ton of chips, where each chip provides a discrete function. The only antonym for monolithic I could find was "manifold" which is no good.

[earthtonesaudio]

What's the word I'm looking for?

That's a really good question.  I want to say "exploded" because it takes all the bits that are normally done in software and does them in (mostly) hardware, but that doesn't capture the whimsical, just-for-the-hell-of-it, impractical but captivating aspect.

Perhaps we need a new word.  Dispragmatic?

[Top Top]

I am currently working on a pitch shifter using PT2399s, lots of opamps and some OTAs. It's basically a [------]* implementation of the standard digital pitch shift algo, using ramp waves in quadrature to bend 2 delay lines, and then crossfading between the outputs of the delays. I know it's not "practical" (especially since I have my own DIY DSP pitch shifter) but I think it will be a lot of fun, and especially because the whole architecture is open rather than in code, so you can easily modify it to do weird stuff.

*What's the word I'm looking for? "Discrete" is wrong, because it uses chips. "Analog" is wrong, because the delay lines are digital. But there should seemingly be a word to differentiate a device which uses a monolithic DSP chip, from one which uses a ton of chips, where each chip provides a discrete function. The only antonym for monolithic I could find was "manifold" which is no good.

I have actually heard some pitch shifting on old albums, like one of the early Residents records, and I think on one song on The Beach Boys Smiley Smile. Would this be how that was done, but with BBD chips? It's definitely a pitch shifted sound, not varispeed tape tricks. It has that chopped up sound that a pitch shifter makes.

[Taylor]

According to Wiki, BBDs were developed in 1969, and Smiley Smile came out in 1967, so I would say no in that case. A/DA had an analog pitch shifter in the 70s or 80s using BBDs, but I believe it used a single delay line, and chopped the sound off during the vertical part of the ramp wave, so with its pitch shift came a choppy tremolo sound which you'd recognize. (I never used or heard one, just going on descriptions I've read of the sound)

[Top Top]

Hmm...

http://www.youtube.com/watch?v=ix1-Coa7qQQ

Check out around :50 - 1:10

I wonder how it was done. When it gets into the higher pitches (from about 1:04 on) you can hear a choppy/vibrato modulating sound.

I have always thought it was weird/wondered how they did it since I first heard it. If it didn't have that vibrato-like sound I would just think it was some clever tape tricks... It does maintain speed though, with just the pitch changing, so it would be some complicated varispeed to do with overdubbed harmonies (having to change the speed consistently on every overdub).

edit: actually just read this comment from someone under the video on youtube

"it wasnt exactly speed up, its a delay with a saw wave modulation of delay time and 4 playheads fading in and out of one another to disguise clicks caused by the saw wave. that way, the pitch goes up but the speed remains the same."

Still, if it is a delay, what kind of delay if not BBD? Some magnetically based delay with modulation of motor speed???

[Taylor]

Hmm, interesting. There were oil can delays, tapes and metal drums, but doing saw wave modulation (the same thing I'm doing with the PT2399s) seems like it would be nearly impossible with any of those media.

[Taylor]

Welp, looks like Sean Costello has us covered:

http://valhalladsp.wordpress.com/2010/05/04/pitch-shifters-pre-digital/

So the saw modulation was a misunderstanding on the youtube poster's part (but any youtube comment with no racist or homophobic epithets is ahead of the pack). This kind of processing couldn't be done in real-time, so not useful as an effect outside the studio, but it is pretty clever. It's one of those things I wouldn't expect to work that well, if reading the patent and looking at the diagram, but that song is the clear proof that it works. It can also do time-stretching without pitch change, which is pretty slick.

[Top Top]

Ah, rotary playback heads. Great link, thanks.

Now I actually feel like I have heard of this some time in the past but it got bumped out for something else, probably cat pictures 

[Jarno]

All the more reason to hold on to those old VHS recorders then? Those VCR heads were always begging for a purpose to recycle them, they are sooo nice, nice motors, great bearings.

[Top Top]

All the more reason to hold on to those old VHS recorders then? Those VCR heads were always begging for a purpose to recycle them, they are sooo nice, nice motors, great bearings.

I thought the same thing when thinking of rotary heads... I've taken apart a couple of VCRs in my day.

I wonder how involved it would be to provide a VCR head everything it needs and have it all aligned well... keep in mind that you would still need a record and erase head if you are trying to do something like that. Seems like mechanically it could get pretty complicated...

[Taylor]

A major issue I foresee would be a spinning apparatus with wires going to it: how do you keep the wires from getting wound up and then ripping out? You'd have to have some kind of electronic and mechanical linkage that isn't a wire, but a sliding groove  or something... Or else go wireless.

[Top Top]

Well VHS heads are already designed as a rotating head, so I think that part is taken care of. I would think it would take someone who understands how the head works though and a pretty customized tape path to get it lined up with other heads as well.

VHS heads look like this, FYI: