"Phase Modulation" vs "Frequency Modulation" in simple terms?

[jackson881]

I have heard it said so many times that FM Synthesis (DX7 style) should really be called PM Synthesis (Phase Modulation) but I've never read a simple explanation of why?

Does that mean that the operator is not really modulating the frequency (pitch) of the carrier, but is instead modulating the phase of the carrier?

If so, that makes me think of oscillator sync, where the phase of the slave is reset by the master. Is there any similarity at all?

Is it possible to break it down in simple terms without a lot of maths?

[Chris-S]

Does that mean that the operator is not really modulating the frequency (pitch) of the carrier, but is instead modulating the phase of the carrier?

Correct.
The trick is: when using sinus waves and phase modulation, the result is: frequency modulation.

[mistmist]

in not so simple terms - fm doesnt exist in computer world all that called fm is.....variations on phase modulation, or if you really in a mood you can find out that even in hardware synths it's the same thing... or if you really wanna beimpresive on party in knock that hotlooking femlae/male you can say that 'fm was a word used in a patent from yamaha to describe phase modulation'

[mistmist]

edit: double post

[Urs]

Frequency is the derivative of phase. The faster phase of, say, a sine progresses, the higher the frequency. Phase Modulation modulates the phase, but leaves the frequency untouched. Frequency Modulation modulates frequency but leaves the phase untouched. In theory both can yield the same results, but in practice they can not.

Time for a stupid analogy: You look out of the window onto the street and a car drives by. The equivalent of FM would be hitting brake and accelerator in the car, one after the other, over and over. The equivalent of PM would be, while the car drives at constant speed, someone moves the street back and forth.

For all practical purposes, PM usually preserves pitch. FM doesn't necessarily, unless the signal meets certain conditions (i.e. the average speed of the car stays the same).

What Yamaha calls FM would be more correctly labeled PM.

[recursive one]

in not so simple terms - fm doesnt exist in computer world all that called fm is.....variations on phase modulation

Some softsynths can use an oscillator as a modulation sourse, and you can assign it to the pitch of another oscillator. Is it the "true" FM?

[Chris-S]

Yes, and there are two different kinds of FM: linear and logarithmic.

[nix808]

AFAIK-
it is not incorrect to make the distinction as:
phase modulation is plugged into phase, and modulates phase within 0-1(I think)
frequency modulation is plugged into frequency, and modulates frequency within -1 to +1

hehe

[jackson881]

Frequency is the derivative of phase. The faster phase of, say, a sine progresses, the higher the frequency. Phase Modulation modulates the phase, but leaves the frequency untouched. Frequency Modulation modulates frequency but leaves the phase untouched. In theory both can yield the same results, but in practice they can not.

Time for a stupid analogy: You look out of the window onto the street and a car drives by. The equivalent of FM would be hitting brake and accelerator in the car, one after the other, over and over. The equivalent of PM would be, while the car drives at constant speed, someone moves the street back and forth.

For all practical purposes, PM usually preserves pitch. FM doesn't necessarily, unless the signal meets certain conditions (i.e. the average speed of the car stays the same).

What Yamaha calls FM would be more correctly labeled PM.

Thank you, Urs. That analogy is actually really helpful.

So, would I be completely wrong to say that PM is related in some way to hard sync? But instead of resetting the oscillator phase like hard sync does, PM modulates phase back and forth at a certain rate (like in your street analogy?)

[Chris-S]

It's related. You can simulate sync by modulating the phase with a saw wave. But it's not exactly the same.

[aMUSEd]

So what is 'NeoFM'?

[AnX]

Sales talk

[Urs]

Frequency is the derivative of phase. The faster phase of, say, a sine progresses, the higher the frequency. Phase Modulation modulates the phase, but leaves the frequency untouched. Frequency Modulation modulates frequency but leaves the phase untouched. In theory both can yield the same results, but in practice they can not.

Time for a stupid analogy: You look out of the window onto the street and a car drives by. The equivalent of FM would be hitting brake and accelerator in the car, one after the other, over and over. The equivalent of PM would be, while the car drives at constant speed, someone moves the street back and forth.

For all practical purposes, PM usually preserves pitch. FM doesn't necessarily, unless the signal meets certain conditions (i.e. the average speed of the car stays the same).

What Yamaha calls FM would be more correctly labeled PM.

Thank you, Urs. That analogy is actually really helpful.

So, would I be completely wrong to say that PM is related in some way to hard sync? But instead of resetting the oscillator phase like hard sync does, PM modulates phase back and forth at a certain rate (like in your street analogy?)

It's related, yes, but I would use a different analogy. I wouldn't move the street, I'd teleport the car.

Let's say the the road paintings - - - - - that mark the border between two lanes represent cycles of the oscillator waveform. Let's say, white paint is the upper part of a square wave while no paint is the lower part. You have two cars, a red one (sync master) and a yellow one (sync slave). Whenever the red car starts driving by a white line, it shoots a high energy traktor beam at the yellow car which teleports it right next to the red one. So while the yellow car drives at a different speed than the red one, it isn't really faster or slower towards the street.

[aMUSEd]

Sales talk

I did wonder - not sure

[Bathrobe]

Let's say the the road paintings - - - - - that mark the border between two lanes represent cycles of the oscillator waveform. Let's say, white paint is the upper part of a square wave while no paint is the lower part. You have two cars, a red one (sync master) and a yellow one (sync slave). Whenever the red car starts driving by a white line, it shoots a high energy traktor beam at the yellow car which teleports it right next to the red one. So while the yellow car drives at a different speed than the red one, it isn't really faster or slower towards the street.

I want to be your child and for you to explain the world to me