What is Phase Modulation?

[Dennis]

What exactly does it do to the sound?

Thanks
-Dennis

[commodorejohn]

I'm a little unclear on the exact underlying mathematical nitty-gritty, but Casio employed phase modulation (a.k.a. "phase distortion") as a way to get around Yamaha's patent on frequency modulation (which I gather they themselves implemented as a form of phase modulation...?) and it has similar results, distorting the waveshape of the carrier signal by stretching some sections and compressing others. (I'm not sure how the spread/amplitude of the harmonics generated is determined, as compared to FM, which is fairly simple to understand in practice. But since Casio PD limits the potential waveshapes to a select few anyway, it's not as important.) Electric Druid has a pretty good explanation of the process.

[pmh]

Phase modulation is basically the addition of an information wave to the carrier wave, by altering the phase angle between them, to give a resultant wave.

I touch on this to some extent through work, with regards to ac power transmissions, as the resultant ac power curve changes as the current changes phase with the voltage. Inductive circuits cause a lag, capacitive circuits cause a lead.

There are plenty of websites out there that explain the principles and mathematics, but it is quite complex and the reason why it isn't used for radio transmissions.

All that said it shouldn't, in theory, sound any different as it's just another method of achieving the end result.

That said I have never heard the two systems running side by side, so can't comment on the effect. Is it a bit like listening to the same music on CD and vinyl?

Kind regards,



Phil

[commodorejohn]

That said I have never heard the two systems running side by side, so can't comment on the effect. Is it a bit like listening to the same music on CD and vinyl?

There is a modest but noticeable difference between Casio PD and Yamaha FM, but there's enough differences in the overall architecture (Yamaha's multiple operator algorithms and variable carrier/modulator tuning versus Casio's single or paired modulated-carrier stacks with selectable resulting waveforms) that I couldn't say with any degree of certainty that it's not just due to that.

[Dennis]

Thx guys great answers,
The electric druid page is really great.
I appreciate the help.

-Dennis

[karmag]

Just to point out the obvious: phase modulation (technical term) is a different beast from phase distortion (marketing term used by Casio). And both are different from the term frequency modulation (technical term and a marketing term used by Yamaha).


This is how I understand it, in non-math speak: One can think of phase modulation in terms of wavetable synthesis; the modulating operator (oscillator) adds or subtracts an offset to the position (phase) of the carrier operator (oscillator) and causes a different value to be read out than what "should" have been.

If you do this with sine waves (really tables that hold sine waves, in this case) the results are similar¹ to what you would get with frequency modulation (technical term) and you could arguably call it just a variant of FM, or a clever way of implementing FM. That's how we get the use of frequency modulation (marketing term) for Yamaha's DX/TX/TG/etc synths.

If you don't use sine waves, like you can in the TX-81z for example, the result will be different from true FM, and so there is reason for people arguing angrily that "PM is not FM!". But usually people don't know and don't care, since it's equally strange and difficult to figure out either way.


The electricdruid page has a good explanation of PD. I'm not sure I buy the explanation about PD vs PM/FM though. Or fully understand it, for that matter.

This part is interesting:

PD is more limited than FM because PD always uses modulators that are at the same frequency as the carrier, or sometimes at a simple multiple of its frequency. Increasing the frequency of the triangle wave (as would be possible on an FM synth) produces a complex effect on the phase distortion.

That "complex effect" is what I'd consider to be FM. Without it, with two oscillators at the exact same frequency, I'm not sure why one would bother to call it that. (It's still a "special case of ... phase modulation" as the site says though.)


Note 1: IIRC, it might be different in phase, but there is no audible difference.

Edit: PD stuff

[Jabberwalky]

Interesting stuff Karmag. Then we have the ring mod on the CZ synths. I wonder what that is "actually" doing. I certainly doesn't sound like traditional ring mod.

[karmag]

I'd love to have a CZ synth to play around with. Shame they never made a rack version, I don't have room for a keyboard..

[megamanx]

CZ 101 is as small as it gets, won't take any room at all...

[Jabberwalky]

The CZ style of PD was really bizarre, and something I think everyone should try once. The envelopes were the key in getting great timbres. I had the Cz1000, which was really unique, but the membranes were going. The Cz101 is much smaller and has actual push buttons. I'm always tempted by those.

[karmag]

Good suggestion megamanx, I'll take one!

They don't show up very often around where I live unfortunately, being in a smaller town. On the bright side, being here means I can often afford both rent and food.

[pflosi]

I agree, everyone needs a CZ101 Pairs nicely with the DX100

[megamanx]

Good suggestion megamanx, I'll take one!

They don't show up very often around where I live unfortunately, being in a smaller town. On the bright side, being here means I can often afford both rent and food.

I have owned 2 and a cz 1000, they turn up for cheap often. try looking at swapmeets(flea markets), goodwills, salvation amy, etc...here and ebay....