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Andy Leary · Posted: Fri Sep 21, 2007 10:29 pm Post subject:

Maybe I can clarify this a little. Since the PCM oscillator has no audio rate modulation inputs, it can't be considered an FM carrier. As Peter points out, it can be used as an FM modulator. It can also be mixed in with any signal or mixed into the output mixer and layered with the other oscillator outputs. The PCM oscillator can also be used as an input to a waveshaper, or as a carrier or modulator for AM or Ring Mod.

Mike Conway's example of a PCM oscillator modulating Osc 1 where Osc 1 has ratio = 0, is a special case. As Mike mentions, the output of Osc 1 does sound like the PCM waveform when Osc 1 ratio is 0 and the input level (mod. index if you like) is low. In this case, it's not really FM since Osc 1 has no frequency to modulate! What we have here is actually - ta da! - waveshaping.

The standard FM formula for a C:M pair is:

FM output = sin ( 2pi n fc + A sin ( 2 pi n fm))

For a waveshaper, if the input signal is a sine wave y = f(n) = A sin(2pi f n ) and the waveshaper is a sine lookup, z = g(y) = sin(y). The output is: z = g( f(n) ) = sin ( A sin(2pi f n))

When the carrier freq ( fc) is 0, these two cases are equivalent. When the carrier freq is 0, simple FM is the same as waveshaping the modulator with a sine wave.

As Mike pointed out, when the input level for the modulator is high, it sounds really distorted. That's because it's being waveshaped with a sine wave. When the input level is really low, the waveshaping function is still a sine wave, but around 0 it looks pretty linear. So, it's like having a waveshaper that just passes the input through.

Now, it the MOD-7 each oscillator has two modulation inputs. They add together and the modulate the sine osc. For this "waveshaper" case of ratio = 0, it's the sum of input1 and input2 that goes into the waveshaper. Since waveshaping is non-linear, what comes out is not the same as waveshaping each input separately and adding the results. It's not the same as input2 modulating input1, but it might sound similar in some cases.

What's over the top about the MOD-7 is that you can have FM, AM, Ring Mod, Waveshaping, Filtering, PCM all at the same time. And there's plenty of EGs, LFOs, Mod Mixers, etc. to go around.

Hope this helps..

Andy
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Derm · Posted: Fri Sep 21, 2007 11:26 pm Post subject:

My cat's name is Mitsy Very Happy

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Mike Conway · Posted: Fri Sep 21, 2007 11:46 pm Post subject:

I don't have a cat...

Great post, Andy!! Very Happy

silverdragonsound · Posted: Sat Sep 22, 2007 2:45 am Post subject:

Daz · Retired Joined: 01 Jan 2002 Posts: 10829

Hey Andy ... great to see you here ! We've heard that you're the brains behind a lot of this great stuff and I think your post pretty much confirmed that Wink

Thanks,

Daz.

RC-IA · Posted: Sat Sep 22, 2007 8:04 am Post subject:

hello andy, thanks for joining and your explanations Smile

curvebender · Posted: Sat Sep 22, 2007 8:32 am Post subject:

Hello Andy!

Good of you to join us and furhter explain the inner workings of the MOD-7! Very Happy

All I have to do now is make a gallon of coffee, and slowly re-read your post... Man, those physics lessons in high school seem distant now... Crying or Very sad

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domc · Posted: Sat Sep 22, 2007 8:51 am Post subject:

Thanks Andy and welcome to the forums.

Good explanation although I also may need to reread some of the maths concepts a few times. It's over a decade since I've had to think about any of that stuff. Wink

Cheers, Domc
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