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 Forum index » Clavia Nord Modular » Nord Modular G2 Discussion
Hammond-style organ
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Falk



Joined: Mar 07, 2006
Posts: 75
Location: Sweden
G2 patch files: 29

PostPosted: Thu May 03, 2007 3:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Chet!

Sadly, it only syncs correctly down to 83 Hz (wich works fine for a B3 with foldback (or rather foldup) on the 16' drawbar).

Another drawback is that the attack of a note will always start with all oscillators starting their cycle. Hopefully key click will cover that. But wait, if I implement phase locking correctly there will be three syncing events and that will randomize the phases a bit for the attack at the cost of a longer latency from keydown to sound, hmmm....

Now if I could only understand what and where the complex tone wheels were employed and how they sounded. A phase coherent B3 will pass the 50% mark even without your stepped drawbars so I might as well go for the whole enchilada.

/Falk
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mosc
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Joined: Jan 31, 2003
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PostPosted: Thu May 03, 2007 7:40 am    Post subject: Reply with quote  Mark this post and the followings unread

On suggestion of Falk, I moved this to the General Discussion forum. Exclamation
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Falk



Joined: Mar 07, 2006
Posts: 75
Location: Sweden
G2 patch files: 29

PostPosted: Thu May 03, 2007 12:36 pm    Post subject:  Fails to hold coherency in the long run..... Reply with quote  Mark this post and the followings unread

Hello again.

Sadly the drift between the LFO's and the oscillators are large enough that it causes phasing in a 5-10 second time span in the upper range of the keyboard Crying or Very sad

Bummer.....

/Falk
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Falk



Joined: Mar 07, 2006
Posts: 75
Location: Sweden
G2 patch files: 29

PostPosted: Fri May 04, 2007 10:11 am    Post subject: Close but no cigar. Reply with quote  Mark this post and the followings unread

Better luck with another approach.

12 square oscillators in the FX area, DA converted and sent over the 1/2 bus into the VA and used to sync the oscillators continuously. Even then there is drift (and by golly, it's not cheap).

The drift causes a twitter that can almost be filtered away and it sounds really lousy in the upper octave. Foldback is your friend and it helps that I have thrown away the 1' drawbar but it is still audible.

/Falk


PhaseFreeHammond4.pch2
 Description:
A phase coherent hammond model

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 Filename:  PhaseFreeHammond4.pch2
 Filesize:  4.64 KB
 Downloaded:  709 Time(s)

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Falk



Joined: Mar 07, 2006
Posts: 75
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G2 patch files: 29

PostPosted: Sat May 05, 2007 8:29 am    Post subject:   Reply with quote  Mark this post and the followings unread

It looks like it is impossible to get a phase coherent Hammond on the G2. If you play the supplied patch it starts to twitter once you get high enough on the keyboard. And that is from just a 2 octave difference in pitch between the oscillator and the sync.

I think it is an aliasing artifact from the 96 kHz clock but why it should manifest itself in this manner is beyond me.

I hope I am worng and that someone can devise a better way.

/Falk


PhaseTest.PNG
 Description:
 Filesize:  13.8 KB
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PhaseTest.PNG



PhaseTest2.pch2
 Description:
Oscillator sync gone wrong

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 Filename:  PhaseTest2.pch2
 Filesize:  1005 Bytes
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mosc
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PostPosted: Sat May 05, 2007 10:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Nothing is impossible. Well, relatively speaking.

Anyway, philosophy aside, here's a little trick to turn off the phase sync when it becomes a problem. Will this help?


phasetest2_110.pch2
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 Filename:  phasetest2_110.pch2
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Falk



Joined: Mar 07, 2006
Posts: 75
Location: Sweden
G2 patch files: 29

PostPosted: Sun May 06, 2007 9:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry Mosc, not really.

A# is especially bad, even just below middle C and audible one octave below that.

I guess the whole idea is flawed. Thanks for the suggestion though.

What I don't understand is how the syncing can give glitches of such low frequency. Every other cycle of the audio producing oscillator is synced and if the difference is small enough to only produce syncing noise at that low frequency, how can the noise be so loud?

/Falk
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Blue Hell
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PostPosted: Sun May 06, 2007 9:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Would this give some insight ?

It's a bit over my head I'm affraid Very Happy

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Falk



Joined: Mar 07, 2006
Posts: 75
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PostPosted: Sun May 06, 2007 11:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Blue.

It might tell the story (my math is not all that hot either). Interesting paper though.

"Hard-synced sine, finally, has no Cn continuity. This approach
cannot synthesize it exactly, but only approximate it;
aliasing will fall off by 6dB/oct per derivative whose discontinuity
is made bandlimited."

The quote seems to give hope. Otherwise the paper mainly deals with hard synced saw and square wave.

/Falk

PS. Another approach wich would only work in this case (the Hammond phase coherency, that is) is to not sync on the sync signal but rather slowly (over several cycles) change the pitch so that it will be synced up after X number of cycles. I have no idea of how to achive that in th G2 DS.
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Blue Hell
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PostPosted: Sun May 06, 2007 11:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Falk wrote:

PS. Another approach wich would only work in this case (the Hammond phase coherency, that is) is to not sync on the sync signal but rather slowly (over several cycles) change the pitch so that it will be synced up after X number of cycles. I have no idea of how to achive that in th G2 DS.


Maybe with (a) phase comparator(s) and using PhaseMod oscillators.

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Falk



Joined: Mar 07, 2006
Posts: 75
Location: Sweden
G2 patch files: 29

PostPosted: Mon May 07, 2007 7:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you Mosc and Blue for not letting me give up!

Phase locked loops it is.

Play only C's across the keyboard.

/Falk


PhaseLock.pch2
 Description:
A phase locked oscillator

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 Filename:  PhaseLock.pch2
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mosc
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PostPosted: Mon May 07, 2007 10:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Wish I could help, but at this point I don't even understand the objective of sync in this case.
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Falk



Joined: Mar 07, 2006
Posts: 75
Location: Sweden
G2 patch files: 29

PostPosted: Mon May 07, 2007 12:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hello Mosc.

Lets start from the beginning.

You have a patch with an oscillator for the fundamental and another one for the octave above (called 8' and 4' on organs). Play some random keys. Now press and hold a key on the keyboard. Tap the key an octave above (the key you are holding down) a couple of times. The timbre will most likely change due to phase cancellation between the low note 4' and the high notes 8' oscillators.

This is a fundamental problem for all additive synthesis (with extremely stable oscillators that is).

Due to the construction of the Hammond organs, the low 4' and the high 8' will come from the same tone wheel and that lone wheel can not by definition be out of phase with itself Very Happy

My second attempt to fix the phase coherency had 12 mono LFO's in the VA tuned to the lowest octave that a Hammond B3 can play. Blues mod-12 building block helps to select one of these LFO's as the source of the syncing signal to keep the oscillators in phase. A sample&hold on the gate signal was used to sync the oscillators once.

This didn't work because slight inaccuracies in the fixed point math used by the DSP code causes a 0.1 to 0.3 Hz difference in pitch between the 16' and 2' oscillators on the high part of the keyboard.

So, lets instead sync the oscillators directly from the correct syncing LFO. Now we get a different issue. Aliasing (probably due to the 96 kHz clock) causes low frequency clicks in the upper region of the keyboard.

So lets use real oscillators (they must be more precise tight?) as sync source. Well, they don't have a mono button so we need to stick them into the FX area and use an DA (Digital to Analog) module to send it over to the VA on the internal buses. Better? No, not really, the twitter had lower frequency but was just as audible.

At this point I was ready to give up when Blue started to talk about phase comparators.

By sampling the output of the oscillators at the sync pulse of the LFO, scaling and inverting the value and feeding it back into the FM input of the oscillator, it will correct itself over a number of sync pulses to an integer ratio to the sync LFO's pitch. At least in theory.....

It seems to work over a quite large range of the keyboard. Hopefully I can tune it so I only need 1 set of 12 syncing LFO's.

Below is a simple patch demonstrating the 8' and 4' oscillator patch described above showing the problem with no phase coherency.

/Falk


PhaseTest3.pch2
 Description:
A patch showing the problems with additive synthesis without phase coherency

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 Filename:  PhaseTest3.pch2
 Filesize:  1.02 KB
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jamos



Joined: Jun 01, 2004
Posts: 470
Location: Eugene, Oregon, USA
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PostPosted: Fri Apr 02, 2010 4:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for this, Chet. I use it extensively in the most recent song I've recorded: http://www.myspace.com/controlincorporated (New Church).
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mosc
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PostPosted: Sun Apr 04, 2010 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Falk: A belated thanks for the explanation.

Jamos: Nice tunes on your Myspace page. Smile

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G2egory



Joined: Nov 19, 2004
Posts: 69
Location: Charlottesville VA
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PostPosted: Mon Apr 05, 2010 8:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

jamos wrote:
Thanks for this, Chet. I use it extensively in the most recent song I've recorded: http://www.myspace.com/controlincorporated (New Church).



Hey Jamos,

The musical stuff you two Control freaks are doing is great!
I adore the energy. Keep it up!
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jamos



Joined: Jun 01, 2004
Posts: 470
Location: Eugene, Oregon, USA
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PostPosted: Tue Apr 06, 2010 7:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey, thanks Mosc and G2eg!

This is a fun project.. not like anything I've done before. I didn't want to play in a duo but circumstances lead to it. I've been putting a huge amount of effort into sequencing with the G2 - everything that you hear that is not vocals or a blatantly obvious sample is G2 - so I am highly indebted to those who have created patches like Chet's B3. That thing f*cking ROCKS.
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