MOTM

Scope? You don't need no steenkin' scope (or freq counter)<g>!

There are lots of good reasons to buy a scope, and I plan on doing so also,
but it's not really necessary for setting the tracking on a VCO. It will be
more useful to verify waveform amplitude and purity, DC offsets, etc.
Believe it or not, the most sensitive instrument for tuning and scaling is
the human ear.

I've calibrated dozens of VCOs over the years, all by ear. Use the same
CV->Midi convertor or V/Oct keyboard you plan to normally use to calibrate
it. If you are driving it from more than one source, you have to make sure
the sources track identically first, but that's another story.

The easiest thing to do is to take 2 oscillators, unhook 1 from pitch
control, hook the other through its 1V/Oct jack, play a note, and tune them
in strict unison (no beats) around 100 Hz or so. Now play 1 octave higher
and tweak the scaling depending on whether the interval is sharp or flat of
a true octave. Now play the lower note and tune them back together again,
and repeat. When you get that sounding in tune, play 2 octaves apart, then
increase the interval. Eventually you get to where they will not beat when
they are many octaves apart. Then you set the scaling on the first
oscillator the same way, except that you can allow them both to track the
keyboard voltage now.

It used to be that many oscillator designs had a separate adjustment for
tracking when you get into high frequencies. If so, it gets more complicated
since adjusting the hi freq tracking can affect the low freq tracking
somewhat. I'm not sure what current state of the art is.

Paul, does your design use a separate high frequency tracking adjustment?

Caveat 1 - you need a decent sense of pitch to do this, as in being able to
tell if one pitch is sharp or flat relative to another. If a tracking
oscillator plays beat free octaves with a fixed pitch oscillator, you're
there!

Caveat 2 - Paul will probably have his own preferred procedure, and will
tell you to ignore all of this.

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

> -----Original Message-----
> From: Gur Milstein [mailto:gur-m@...]
> Sent: Thursday, March 25, 1999 4:32 PM
> To: motm@onelist.com
> Subject: [motm] vco preparation
>
>
> From: Gur Milstein <gur-m@...>
>
> hi list.
> well its about time for me to prepare for the vco.
> i need to buy a scope but my question is how the tune caliberation process
> is gone be,is the scope is the best way to tune your vco,
> or do i need to go for a freq counter ?
>
> allso can you guys give some tips on what is importent when going
> to buy a second hand scope,and what is a good price for 50Mhz ,
> and a good manufecture name.....  ?
>
> its gone be hard for me to find where
> i leave the old TEK scope as Paul once suggest.
>
> thanx
> Gur Milstein
>
>
>
> ------------------------------------------------------------------------
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>

> From: Gur Milstein <gur-m@...>
> 
> hi list.
> well its about time for me to prepare for the vco.
> i need to buy a scope but my question is how the tune caliberation
process
> is gone be,is the scope is the best way to tune your vco,
> or do i need to go for a freq counter ?
> 
> allso can you guys give some tips on what is importent when going
> to buy a second hand scope,and what is a good price for 50Mhz ,
> and a good manufecture name.....  ?
> 
> its gone be hard for me to find where
> i leave the old TEK scope as Paul once suggest.
> 
> thanx
> Gur Milstein

Here is a post from a while back from Paul in case you didn't happen to see
it or save it.
----------------------------------------------------------------

From: "Paul Schreiber" <synth1@...>

Since the VCO kit really requires a scope to really check it out, I went to
www.ebay.com and searched the current scope offerings. There were over 75
available,
95% would work for analog synth poking around.

Here is a quick guide:

1) 50Mhz is all you need. 40 is OK, 20 is too slow, even for audio. Buy Tek
if you can (Tektronics).
2) Avoid the following:

HP: the HP17xx scopes are easily the worse product line in the history of
HP. *trust me*
military: requires 4 people to lift it.
Dumont: tubes
Heathkit or Eico: crappy & more crappy

3) Good buys: Philips (vastly underrated scopes), B&K Precision (had one
for
8 years)

Note that 99% of these DO NOT come with probes. Meduim quality, 50Mhz
probes
are about $35 ea.
and you will need 2.

4) Best solution if you have LOTS of room: Tek 7000 series. In the mid-80s
this was TEK's "flagship" line.
You get a 'mainframe' (power supply and CRT) with room for 'plug-ins'
(usually 4-wide). A dual-channel,
150Mhz delayed-sweep 7000 was $12,500 in 1984. Today, you can get the
*exact* same configuration for $400!!

What's the catch??? Well, they are *BIG*. About the size of 1 drawer in a
filing cabinet. In other words, they
are *DEEP*. I mean like 28 inches deep. But, if you have the space
(depth-wise) these puppies rock the scope world.

I use a TEK TAS250 (2 ch, 50Mhz, no delayed sweep) for all MOTM check-out.
This is a smaller, lightweight portable
scope. I bought a "demo" unit off the web from a liquidator for $400. It
was
*brand new in the box*! This was a $1300 new
scope. I also have a TEK TDS380 2Ghz , 450Mhz sampling scope. I won't say
how much THAT cost: let's say that
*replacement* probes are $395 each!!!

I you find something, and want me to go look at it, send me the URL of the
page (ebay has a cute feature that does this).

Paul Schreiber

hi list.
well its about time for me to prepare for the vco.
i need to buy a scope but my question is how the tune caliberation process
is gone be,is the scope is the best way to tune your vco,
or do i need to go for a freq counter ?

allso can you guys give some tips on what is importent when going
to buy a second hand scope,and what is a good price for 50Mhz ,
and a good manufecture name.....  ?

its gone be hard for me to find where
i leave the old TEK scope as Paul once suggest.

thanx
Gur Milstein

First, thanks to Larry for the URL for the AHMW gathering pix; it looked like
great fun to me (I've seen Forbidden Planet a whole bunch of times, and I DO
hear Theremin type sounds, though not the more cliche sci fi sounds -- what do
you guys think?).

Secondly, I fully agree with Dave's proposition about scaling VCOs. But I do
want to add a couple of thoughts. It is a VERY tedious process, and somewhat
maddening to have one pitch droning on for such a long time. And I've often
found it very difficult to "hit that right spot" on the trimmer, and this
increases the maddening effect (I suspect the MOTM trimmers will be MUCH
EASIER to adjust to the right spec., just comparing the previous MOTM modules
to their counterparts in non MOTM equipment). I also seem to remember Paul
saying that these VCOs will track over something like 13 octaves(!!!!!!); well
I'll have to HEAR it to believe it. But all seriousness aside folks, you would
need some extra audio calibration. I BTW find it hard enough to get VCOs to
track over the five octave range, but I never seem to think it is not close
enough.

Synth Peon
John B.