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Mike Conway Approved Merchant
Joined: 28 Jan 2005 Posts: 2436 Location: Las Vegas, Nevada
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Posted: Fri Sep 21, 2007 6:20 pm Post subject: |
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domc wrote: | ski wrote: |
PCM can be used as an audio source (i.e., connected to the mixer), but it can't be modulated by a VPM or noise. So although it can be connected to the mixer directly and heard, it can't be modulated in the traditional FM sense by another oscillator (or noise). So it wouldn't be considered a "carrier". But it could be used as a modulator.
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One last thing really for Mike Conway - I see over in HC that you thought that PCM could be used as a carrier by hooking it up to a VPM and turning the ratio to 0 and then modulating it with the other input. This seemed to be different to what ski was saying so I was wondering whether by hooking it up as you were saying that it got round that limitation.
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Well, I tried it, last night. It definitely was modulating. I input a String sound into oscillator 1 and then input oscillator 2. Together, they made a different sound than either did, separately.
EDIT:
Did a little further experimenting. (This one on the SY99.) Modulation of the waveform is almost a given. But, if the sample waveform is actually a carrier it should affect oscillator 2. I put a church choir sound into a rising and falling pitch. It did affect the frequency of oscillator 2.
Next, I took the choir's envelope (but not oscillator 1's envelope) and made it into a short attack/decay. Oscillator 2 kept sounding - though not modulated. Because there are 2 inputs into oscillator 1, it is like a "summing" effect, but with no sound coming out of oscillator 1, the sample takes on some carrer characteristics. |
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Andy Leary Korg Research and Development
Joined: 21 Sep 2007 Posts: 38
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Posted: Fri Sep 21, 2007 10:29 pm Post subject: |
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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 _________________ Andy Leary
Korg Research and Development |
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Derm Senior Member
Joined: 16 Jan 2006 Posts: 276 Location: Dublin
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Mike Conway Approved Merchant
Joined: 28 Jan 2005 Posts: 2436 Location: Las Vegas, Nevada
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Posted: Fri Sep 21, 2007 11:46 pm Post subject: |
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I don't have a cat...
Great post, Andy!! |
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silverdragonsound Platinum Member
Joined: 15 Jun 2007 Posts: 512 Location: Phoenix, Arizona
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Posted: Sat Sep 22, 2007 2:45 am Post subject: |
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Andy Leary wrote: | 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 |
Well that answers that. _________________ And there was this one time at band camp...... |
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Daz Retired
Joined: 01 Jan 2002 Posts: 10829
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Posted: Sat Sep 22, 2007 2:52 am Post subject: |
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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
Thanks,
Daz. |
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RC-IA Approved Merchant
Joined: 16 Jan 2004 Posts: 971 Location: FR
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Posted: Sat Sep 22, 2007 8:04 am Post subject: |
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hello andy, thanks for joining and your explanations |
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curvebender Platinum Member
Joined: 26 Jun 2007 Posts: 784 Location: Sweden
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Posted: Sat Sep 22, 2007 8:32 am Post subject: |
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Hello Andy!
Good of you to join us and furhter explain the inner workings of the MOD-7!
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... _________________ Paul: Don't be nervous.
John: I'M NOT NERVOUS!!! |
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domc Full Member
Joined: 25 Jul 2006 Posts: 137 Location: London
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Posted: Sat Sep 22, 2007 8:51 am Post subject: |
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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.
Cheers, Domc _________________ Oasys 88
Kronos 88
Virus TI Keyboard
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