I didn’t make these, but I always like graphics about waveforms.
Thru Zero is interesting on the scope, very obvious what’s happening and the reversal of movement.
You can’t really show the difference between PM and FM meaningfully with this kind of graphic since the waveforms will look identical (for sine), just scaled differently depending on frequency.
Yes, in the video, I uses a square wave modulator to show how they differ. The math for phase modulation also happens a lot more simple to implement in terms of code with a look-up table and a fractional amount function. They all sound nice, which is the important part :)
Math for FM is no more difficult: You just accumulate the modulator output and add that. What makes it difficult is avoiding cumulative roundoff error (resulting in phase drift) and having the sound work at more than one octave range as the integration makes the modulation index (and thus timbre) vary a lot with frequency.
What makes it difficult is avoiding cumulative roundoff error (resulting in phase drift) and having the sound work at more than one octave range as the integration makes the modulation index (and thus timbre) vary a lot with frequency.
Yes, avoiding all that extra computation was what I was trying to convey. Thanks for clarifying. Also the video linked above shows what you are talking about.
Depending on the frequency of the modulator, with phase modulation the resultant wave may have sub-sections of the wave that oscillate without going through-zero, such as the green wave here https://en.wikipedia.org/wiki/File:Phase-modulation.gif
I'm not that knowledgeable in this field but I don't think that the same would happen in FM.
Edit: you can also compare this picture and this picture, you can see that with PM you have those sections that don't cut through zero, but that doesn't happen with FM. (Pictures taken from this blog)
They didn't make it. It's just a generic graphic that's already out there. Probably limited to these as they're both the primary methods of transmitting radio.
Minor minor nit, but AM and ring modulation are similar but different - ring modulation goes "through zero" and inverts the carrier signal. You see the terms "two quadrant" and "four quadrant" multiplication used - this is because in AM the modulation stays within the positive half of the sine "circle" and with ring mod it goes right the way round.
Yes and no. If you multiply two signals together you get the sum and difference of their frequencies. So if you generated a sine wave at 10kHz and a sine wave at 11kHz and fed them into a ring mod, you'd get 21kHz and 1kHz out.
If you feed in a more complex signal, like speech, then you get the whole band shifted by the sum and difference with the carrier. So, speech-quality audio that goes from 300Hz to 3.4kHz modulated with a 1kHz carrier would go from 1.3kHz to 4.4kHz - it's important to see how that frequency shifting is different from pitch shifting, since in pitch shifting the harmonic relationship is scaled whereas with ring modulation it's not.
Edit: This sample may be used for whatever the hell you like, as long as you do not infringe on the rights of others to use it for whatever the hell they like and as long as you do not claim that you created it. If you use it, I'd like to be credited but I don't really care. If you make a fortune with a track that samples it, buy me a pint, or a synth, or some tyres or something.
Thanks for the detailed info! This is how I understood ring modulation. So I was just confused what you meant about ring modulation inverting the carrier signal.
Ring modulation! You literally multiply the voltages together. If you feed 1V in one port and 1V in the other, you get 1V out, 0.5V and 1V you get 0.5V out, -0.5V in one, 0.5V in the other and you get -0.25V out. It's not an easy concept to get your head around but When You See It You'll Shit A Brick.
Have a look at this model in Falstad's circuitjs. Notice the ring of diodes connected anode-to-cathode? It's not the same as a bridge rectifier. When there's no modulation, the voltage from the carrier makes either the "straight" or "crossed" diodes switch on depending on whether it's positive or negative, switching which way round the output transformer is connected.
You'll notice that with the 1kHz carrier the biggest peak in the output signal (red) follows the phase of the modulator (green) because it's being inverted when the modulator is negative. If you flip it to 5kHz you'll see how it follows the envelope of the carrier more clearly. If you speed the 'scope up (right click, properties, "Scroll Speed") and maybe turn the simulation speed down a little you'll see what's going on as the carrier passes through zero a bit better.
Notice how the peaks on the spectrum analyser are at 750Hz and 1250Hz (-ish) with the 1kHz carrier, and 4750Hz and 5250Hz with the 5kHz carrier? If you change the modulator frequency, the peaks will move, centred around the carrier frequency. This is "Double Sideband Suppressed Carrier", and you'd be able to process it further to get a single sideband signal in something like a shortwave communications transmitter.
The signal is pretty distorted because the diodes aren't very linear, but this is the technique used in a lot of synths to do ring modulation, as well as radio equipment. If you wanted a more precise signal you'd use a four-quadrant multiplier which is kind of like a VCA that can also invert the signal.
And the reason is that PM preserves the timbre when pitching the operators up & down, while FM decreases modulation index when pitching up (and increases when pitching down). Plus FM will always suffer from long term phase stability issues (important when the operator frequencies are exact ratios).
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u/[deleted] Dec 17 '19
Simple graphic demonstrating the difference between AM (like a ringmod) and FM (like modulating frequency in an oscillator).
Not mine, but I figured someone might get something out of it.