Divide by 1.333 or Multiply by 0.75 circuit

Goldsphere · Nov 30, 2007

I have been trying to find a simple logic circuit that can take a square wave frequency and output it's musical 5th. That is divide by 1.3333.
I am not experienced at all with logic circuits, but have been teaching myself, so what I'm really after is just a schematic.
From what I have gleaned, the logic path would have to be divide n by 4, then multiply by 3. But I don't know enough to be able to do this.
I found a circuit that divides by 1.5 supposedly, but when I built it, it divided by 2. By the way, it doesn't have to be 50% duty cycle either.
Any help would be greatly appreciated.

audioguru · Nov 30, 2007

In "the good old days" you could buy a top octave generator IC. It made all 12 notes in a scale. Then digital dividers gave different octaves from it.

A few places still have some of the ICs and sell them for a very high price.

Goldsphere · Nov 30, 2007

Oh yeh, sorry, forgot to mention that I want to be able to change the frequency
and not necessarily have it in the audio range and automatically have the 5th change with the new frequency.

Dean Huster · Nov 30, 2007

About all that can be said about top octave generators is that they were cute and cheap, compared to other methods of tone generation. Trouble is, they were nearly perfect and to use one resulted in chords that were very flat-sounding and on organs, stops that didn't seem to contribute to the sound at all.

If a piano, pipe organ or all the instruments in an orchestra were tuned absolutely perfectly (and every note within the instruments' range was in perfect tune with the others), it would sound very dull and strange. It's the very slight mistunings that make an instrument like a pipe organ sound so grand and beautiful. I guess it's like that with people. It takes all kinds to make for a very interesting and pleasing world.

Dean

audioguru · Nov 30, 2007

I know that the string on a piano stretches when it is hit by the hammer which changes its pitch during the playing of a note.
I don't think a pipe organ will change its pitch during the playing of a note.
I don't know if a pipe organ can be tuned or if its exact frequencies are a little random.

Then make an awful organ with twelve 555 oscillators.

Speakerguy · Nov 30, 2007

My band director had said something like that many years ago Dean. He complained about the very first generation of auto-tuners, or something like that. I very vaguely remember him saying that they sounded wrong. Hell I can't remember what he said. But it was something like that. Friggin' get old already.

Goldsphere · Nov 30, 2007

I don't really care how it sounds, it just needs to be divided by 3/4.
Examples: if I put in 24kHz, then I will get out 32kHz.
Or if it is easier to multiply, then I want to be able to put in 180kHz and get out 135kHz.

audioguru · Nov 30, 2007

A CD4046 phase-locked-loop IC plus some dividers can multiply the frequency.

Roff · Nov 30, 2007

Goldsphere said:
I don't really care how it sounds, it just needs to be divided by 3/4.
Examples: if I put in 24kHz, then I will get out 32kHz.
Or if it is easier to multiply, then I want to be able to put in 180kHz and get out 135kHz.

So apparently you can with start with either note, and you want to generate the other one.
Conceptually, you can do it either way with a PLL and a couple of digital dividers. Below are block diagrams.

theinfamousbob · Nov 30, 2007

audioguru said:
A CD4046 phase-locked-loop IC plus some dividers can multiply the frequency.

I was going to say use a 4017 and have the reset set so it would reset after 4 pulses. Probably not as elegant as a PLL, however. But cheap.

Roff · Nov 30, 2007

theinfamousbob said:
I was going to say use a 4017 and have the reset set so it would reset after 4 pulses. Probably not as elegant as a PLL, however. But cheap.

CD4017 can divide, but it can't multiply.

matc · Dec 1, 2007

Goldsphere: you need a PLL (phase locked loop).

PLL typically has a VCO (voltage controlled oscillator) and a phase detector to keep its output in sync with another signal. Point being, you divide its output by 3 and keep THAT in sync with your input. This makes the system stay at exactly 3x the input frequency. It works with reasonably steady frequency inputs only. If you need something to handle fast transients, you're into DSP territory.

With a divide by 3 in the feedback circuit (this will multiply the frequency by 3) and a dvide by 4 after it, you'll get 3/4 of your input freq.

If you can choose your input frequency though, it might be easier to generate both frequencies by division. Eg start with 100 KHz and divide down to get 1/3 and 1/4 the original.

kchriste · Dec 1, 2007

If you can choose your input frequency though, it might be easier to generate both frequencies by division. Eg start with 100 KHz and divide down to get 1/3 and 1/4 the original.

That's what I would suggest, though if the OP wants greater step resolution for more frequencies, a higher reference frequency will be necessary. If the OP wants to get really fancy, then a DDS chip would be a good choice.

Goldsphere · Dec 1, 2007

Thanks everyone for your replies, I will try the PLL and 4017. Was originally thinking of using the 4018, but 4017 is cheaper.
Wasn't too sure about the PLL, cause it is a bit tricky to understand. For instance, what values for the resistors to use if I want to divide over a large frequency range, perhaps i'll set some switches for different values.
Thanks again.

em2006 · Dec 1, 2007

audioguru said:
A CD4046 phase-locked-loop IC plus some dividers can multiply the frequency.

Below is schematic diagram.
The counter N1 plus gate G1 are the divider by n.
The CD4046 phase-locked-loop IC plus divider by m (counter N2 plus gate G2) are the multiplier (*m).

**broken link removed**

audioguru · Dec 1, 2007

Yes, real counters will work.

A 4017 is not a counter, it is a sequencer.
One of its outputs is at 1/10th of the input frequency for 1/10th duty-cycle.

em2006 · Dec 1, 2007

The same circuit where m=4 and n=3 (Fout=Fin*4/3):

**broken link removed**

Goldsphere · Dec 1, 2007

Hey that's really awesome! Thanks heaps em2006.
I will try it out as soon as I can.

Roff · Dec 1, 2007

audioguru said:
Yes, real counters will work.

A 4017 is not a counter, it is a sequencer.
One of its outputs is at 1/10th of the input frequency for 1/10th duty-cycle.

What's in a name? A 4017 can divide by any integer between 2 and 10. And it is a 5 stage Johnson counter, with all 10 stetes decoded to outputs. The datasheets I looked at call it a counter.

audioguru · Dec 1, 2007

Roff said:
What's in a name? A 4017 can divide by any integer between 2 and 10. And it is a 5 stage Johnson counter, with all 10 states decoded to outputs. The datasheets I looked at call it a counter.

Yes of course. You connect an output to the reset pin then it counts up to that pin. But each output is still only one clock pulse wide.

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