MOTM

Sorry, I'm a lurker (But Paul can vouch for me)

Analog FM's limitations are actually more basic then what was discussed.
why all of the points are valid the most fundamental limitation of trying to
accomplish "DX-7 style FM synthesis) with conventional analog VCOs is that
such a VCO is not capable of modulating "through zero frequency".  This
limits the modulation index to the less interesting lower range.  For
example, using a 1:1 carrier to modulator frequency ratio the maximum
modulation index with a conventional VCO is 1.  Unfortunately if any of you
remember your DX-7 experiences the most interesting sideband evolutions
start occurring above this figure.

The reason for this limitation has to do with some pretty esoteric technical
details but in a nutshell most VCOs can not "reverse on themselves".  By
this I mean that as the modulating wave drives the carrier's frequency
toward zero Hz it can not force it into "negative frequencies".  And yes
these really exist, it just a 180 degree phase reversal on the waveform.

There is a way of making a VCO that is capable of this performance and is
something I discuss if anyone is interested.  or you can just scan some old
Electronotes issues in the issues around #63 I think for a good write up.

Back into lurk mode.

Douglas Kraul

I've read about electronotes from time to time but am unable to locate any 
of them. is there anyplace online that one may download them?
Thanks,
Jim

>From: "Douglas R. Kraul" <dkraul@...>
>Reply-To: motm@onelist.com
>To: <motm@onelist.com>
>Subject: [motm] Analog FM
>Date: Sat, 21 Aug 1999 10:01:30 -0400
>
>From: "Douglas R. Kraul" <dkraul@...>
>
>Sorry, I'm a lurker (But Paul can vouch for me)
>
>Analog FM's limitations are actually more basic then what was discussed.
>why all of the points are valid the most fundamental limitation of trying 
>to
>accomplish "DX-7 style FM synthesis) with conventional analog VCOs is that
>such a VCO is not capable of modulating "through zero frequency".  This
>limits the modulation index to the less interesting lower range.  For
>example, using a 1:1 carrier to modulator frequency ratio the maximum
>modulation index with a conventional VCO is 1.  Unfortunately if any of you
>remember your DX-7 experiences the most interesting sideband evolutions
>start occurring above this figure.
>
>The reason for this limitation has to do with some pretty esoteric 
>technical
>details but in a nutshell most VCOs can not "reverse on themselves".  By
>this I mean that as the modulating wave drives the carrier's frequency
>toward zero Hz it can not force it into "negative frequencies".  And yes
>these really exist, it just a 180 degree phase reversal on the waveform.
>
>There is a way of making a VCO that is capable of this performance and is
>something I discuss if anyone is interested.  or you can just scan some old
>Electronotes issues in the issues around #63 I think for a good write up.
>
>Back into lurk mode.
>
>Douglas Kraul
>
>
>
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HI Doug,

Thanks for clearing up this FM stuff. I, for one, would appreciate any 
information about linear FM in analog and especially the through zero 
approach that you mentioned (and, for example, if someone sent me a scan of 
related material, I'd be so ecstatic as too forget to call anyone's attention 
to any possible copyright issues). On a related note, I am currently looking 
at a schematic from Juergen Habile (from his JH-3 Dual VCO board built around 
two CEM 3340s) which shows a through zero LIN FM input, so it appears to be 
possible to have through zero FM with analog.

JB

In a message dated 8/21/99 7:06:58 AM, dkraul@... writes:

>Sorry, I'm a lurker (But Paul can vouch for me)

>Analog FM's limitations are actually more basic then what was discussed.
>why all of the points are valid the most fundamental limitation of trying
>to
>accomplish "DX-7 style FM synthesis) with conventional analog VCOs is that
>such a VCO is not capable of modulating "through zero frequency".  This
>limits the modulation index to the less interesting lower range.  For
>example, using a 1:1 carrier to modulator frequency ratio the maximum
>modulation index with a conventional VCO is 1.  Unfortunately if any of
>you
>remember your DX-7 experiences the most interesting sideband evolutions
>start occurring above this figure.

>The reason for this limitation has to do with some pretty esoteric technical
>details but in a nutshell most VCOs can not "reverse on themselves".  By
>this I mean that as the modulating wave drives the carrier's frequency
>toward zero Hz it can not force it into "negative frequencies".  And yes
>these really exist, it just a 180 degree phase reversal on the waveform.

>There is a way of making a VCO that is capable of this performance and
>is
>something I discuss if anyone is interested.  or you can just scan some
>old
>Electronotes issues in the issues around #63 I think for a good write up.

> From: "Douglas R. Kraul" <dkraul@...>
> 
> Sorry, I'm a lurker (But Paul can vouch for me)

Ah ha !! One of the quiet intelligent types.  Hi Doug.  You should come out
of lurk mode more often.  I learned something and could alway use more of
that!

Larry Hendry

Here's some more details about "through zero" linear FM.

I can't speak to the circuit mentioned as I have not seen it.  There was an
approach floating around about the same time as the this other approach
(which I will describe) that used two VCOs and a balanced modulator in order
to generate a difference frequency.  This produces proper spectra for FM as
it allows the difference frequency to be negative, which is the requirement
for high modulation index FM.  So I wonder if the mentioned approach is the
same?  BTW if it is this approach it has a set of issues that make it
somewhat impractical for a precision VCO.

Anyway, the "preferred" approach IMHO is to implement it in the following
way.  Suppose for the moment that we are starting with a conventional VCO
design that produces a triangle as its base waveform, as opposed to a
sawtooth like the 300 does.  The "trick" to making a through zero VCO is
that the charging current must reverse its direction at the right moment,
the moment when the modulation has produced exactly zero Hz in the carrier
and is about to transition through zero.  Reversing the current at this
point is precisely what happens in a theoretical through-zero VCO as the
change in charging current is equivalent to a phase reversal of the
waveform, and reverse phase is equivalent to negative frequency.

Thinking about it another was, if you look at the canonical digital FM
equations and think about how the digital oscillator is implemented it is
typically done with an accumulate function.  When the modulation is large
enough it will actually be bigger then the increment that is added each time
to cause the base frequency in the carrier.  If the modulation waveform is
negative (as it will be as it completes its cycle) the result will be a
negative number being added in the accumulate.  This will momentarily cause
the accumulate to "back up".  Again this is just another way at looking at
the negative frequency thing.

The VCO modified for through zero FM in the above manner is a bit more
complex circuit-wise in that you need to detect when the linear fm input
would be going through zero and keep that from happening while at the same
time cause the charging current to reverse.  There is some hair on the
circuit to do this but really it is not much worse then any good precision
VCO design.

As for the motivation behind this, my memory of hearing this 24 years ago
was that it gave me goosebumps!  The richness of the evolving FM spectra was
a perfect counterpoint to the normal subtractive VCF idiom.  I was estatic
when the DX-7 first showed up because it was "fm in a box" but the digital
implementation never left me with quite the same first feeling.  As with
most things digital it was a bit colder.  That said the Nord Modular offers
"VCOs" with through zero FM and they sound pretty good - for digital that
is.  Unfortunately, my through zero VCO prototype has long gone to DIY
heaven otherwise I would post examples.

Douglas

For those of you newer to the list, there was a thread about analog FM about
6 months ago.  Search the archives and I am sure you will find it.  There
was even a list of all the modules you would need to pull it off.  It will
probably take me 10 years to acquire that much.

Paul, any comments on this thread???

--Shemp

> -----Original Message-----
> From: Douglas R. Kraul [mailto:dkraul@...]
> Sent: Sunday, August 22, 1999 8:14 AM
> To: motm@onelist.com
> Subject: [motm] Re: Analog FM
>
>
> From: "Douglas R. Kraul" <dkraul@...>
>
> Here's some more details about "through zero" linear FM.
>
> I can't speak to the circuit mentioned as I have not seen it.
> There was an
> approach floating around about the same time as the this other approach
> (which I will describe) that used two VCOs and a balanced
> modulator in order
> to generate a difference frequency.  This produces proper spectra
> for FM as
> it allows the difference frequency to be negative, which is the
> requirement
> for high modulation index FM.  So I wonder if the mentioned
> approach is the
> same?  BTW if it is this approach it has a set of issues that make it
> somewhat impractical for a precision VCO.
>
> Anyway, the "preferred" approach IMHO is to implement it in the following
> way.  Suppose for the moment that we are starting with a conventional VCO
> design that produces a triangle as its base waveform, as opposed to a
> sawtooth like the 300 does.  The "trick" to making a through zero VCO is
> that the charging current must reverse its direction at the right moment,
> the moment when the modulation has produced exactly zero Hz in the carrier
> and is about to transition through zero.  Reversing the current at this
> point is precisely what happens in a theoretical through-zero VCO as the
> change in charging current is equivalent to a phase reversal of the
> waveform, and reverse phase is equivalent to negative frequency.
>
> Thinking about it another was, if you look at the canonical digital FM
> equations and think about how the digital oscillator is implemented it is
> typically done with an accumulate function.  When the modulation is large
> enough it will actually be bigger then the increment that is
> added each time
> to cause the base frequency in the carrier.  If the modulation waveform is
> negative (as it will be as it completes its cycle) the result will be a
> negative number being added in the accumulate.  This will
> momentarily cause
> the accumulate to "back up".  Again this is just another way at looking at
> the negative frequency thing.
>
> The VCO modified for through zero FM in the above manner is a bit more
> complex circuit-wise in that you need to detect when the linear fm input
> would be going through zero and keep that from happening while at the same
> time cause the charging current to reverse.  There is some hair on the
> circuit to do this but really it is not much worse then any good precision
> VCO design.
>
> As for the motivation behind this, my memory of hearing this 24 years ago
> was that it gave me goosebumps!  The richness of the evolving FM
> spectra was
> a perfect counterpoint to the normal subtractive VCF idiom.  I was estatic
> when the DX-7 first showed up because it was "fm in a box" but the digital
> implementation never left me with quite the same first feeling.  As with
> most things digital it was a bit colder.  That said the Nord
> Modular offers
> "VCOs" with through zero FM and they sound pretty good - for digital that
> is.  Unfortunately, my through zero VCO prototype has long gone to DIY
> heaven otherwise I would post examples.
>
> Douglas
>
>
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Electronotes had a "plan" for a through-zero VCO (I don't think
Bernie got all the bugs out).

Mainly, you need good matched PNPs ans the only ones you can get now are the
SSM 2220s.

As Doug said, the circuitry is rather involved. It reamins to be seen
if the market can bear an even more $$$ MOTM VCO!

Paul S.

Bernie's "plan" was actually based on a design of mine that only extended
the conventional OTA VCO to include the through-zero modulation.  Bernie may
have had problems with it but I sure didn't!  His writeups were based on an
article of mine that Electronotes publish based on a research project that I
did.

However, it was never the precision VCO that the 300 has become.  But it
could be improved substantially and still use a more conventional "discrete"
architecture much like the 300 has (though different).  It probably would be
less complex in circuitry then the 300 but with a slight reduction of
absolute tracking accuracy.  Still I believe it would be superior to all but
something like the 300.

Douglas Kraul

Douglas,

It's great to have you onboard the MOTM train. Sounds like you have a very
interesting design.

So how would you describe the sound of through-zero modulation in your
oscillator? Yamaha FMish? Does it significantly increase the tonal palette
available over conventional analog FM?

If so, would you be adverse to allowing Paul to add your design to the MOTM
arsenal as an alternative VCO module?

Dave Bradley
Principal Software Engineer
Engineering Animation, Inc.
daveb@...

> -----Original Message-----
> From: Douglas R. Kraul [mailto:dkraul@...]
> Sent: Tuesday, August 24, 1999 8:28 AM
> To: motm@onelist.com
> Subject: [motm] Re: Analog FM
>
>
> From: "Douglas R. Kraul" <dkraul@...>
>
> Bernie's "plan" was actually based on a design of mine that only extended
> the conventional OTA VCO to include the through-zero modulation.
> Bernie may
> have had problems with it but I sure didn't!  His writeups were
> based on an
> article of mine that Electronotes publish based on a research
> project that I
> did.
>
> However, it was never the precision VCO that the 300 has become.  But it
> could be improved substantially and still use a more conventional
> "discrete"
> architecture much like the 300 has (though different).  It
> probably would be
> less complex in circuitry then the 300 but with a slight reduction of
> absolute tracking accuracy.  Still I believe it would be superior
> to all but
> something like the 300.
>
> Douglas Kraul
>
>
>
> --------------------------- ONElist Sponsor ----------------------------
>
> ONElist:  your connection to online communities.
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>

Thanks again Doug, for your insights and explanations concerning through zero 
linear FM in the analog domain; very interesting! And let me second the 
suggestion of a wise man (or maybe it was Larry, I can't remember) that you 
lurk less, and contribute more -- I promise if you do, I'll contribute less!

And as for a second MOTM VCO with through zero linear FM, I'd be interested 
in one (or more), but this could wait a while.

JB

Well Paul really is the one that needs to decide this as I know he has
priorities and even with using outside designs he has only so much capacity
to introduce new modules.  If not now maybe at least it can be put in the
que.

As far as I am concerned I am interested in commercializing the design but I
have design standards that are high (Paul and I are compatible in regard to
this) and would not want to introduce something into the MOTM product that
did not meet its high standards.  While doing a through zero 300 is probably
not possible (by that I mean a new VCO with near 300 performance) we may be
able to get close and certainly high enough accuracy over a practical
frequency range with comparable drift specifications.  But this needs to be
tested.  Finally, cost is an issue I am sure.  Such a product may not be as
expensive as the 300 but it wouldn't be far behind as the critical elements
(especially the expo gen and charging circuits) will be at least as
expensive.  My guess is that the cost would be the same, less any reduction
due to reduced panel controls if any.

As far as being a non-lurker I will chime in when I have something to add or
the conversation is interesting.  Now that the ice is broken you may wish I
would just shut up!

Douglas Kraul