[sdiy] A440 -- almost

[John L Marshall]

The color burst xtal probably could be bent downward 585 Hz with a small or
better a variable capacitor in parallel.

In the good/bad old days Novice Hams were required to xtal control
transmitters. They would parallel xtals with a variable capacitor to make a
"VXO".


Take care,
John
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Pacific Northwest DIY Synthesizer meeting, January 18, 2003
See: www.sound-photo.com
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----- Original Message -----
From: "Michael Baxter" <mab at cruzio.com>
To: "synthDIY" <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, October 19, 2002 1:01 PM
Subject: [sdiy] A440 -- almost


> Hi all synthDIYer friends,
>
> I'm designing and building a modular system, and I'm very excited to see
> all of the activity and innovative designs in this community! As I get
> circuits designed and built, I'll post them somewhere. I did of lot of
> this kind of design in the 1970's, so I'm hoping to contribute to "module
> diversity," if you will. More later.
>
> In this first post, thought I'd share some quick notes on the A-440
> reference frequency or calibrator thread that came up recently.
>
> I discovered that the NTSC color burst reference frequency is very close
> to a necessary superharmonic. 3.579545 MHz / 8134 yields 440.07192 Hz.
>
> Compared to a true 440.0 Hz, this is still pretty close. The "correct"
> reference frequency would need to be 585 Hz lower, and this is probably
> out of the the crystal adjustment range, esp. as the NTSC frequency is
> supposed to be within +/- 10 Hz. However, quantitatively, the actual
> difference seems small compared to a +/- 1-cent interval above A4.
>
> The hack to "adjust" crystal mechanically is very interesting, I had not
> seen that before. It definitely has the feel of amateur radio hints
> and kinks lore, and probably works. Starting with a cheap crystal, one
> could try several times, to eventually get it right. Not so different than
> Vbe matching transistors, :-).
>
> I did see the later posts on the 4.40 MHz crystals, which would be of
> course "spot-on." But I was very surprised at how close one could get with
> the garden variety NTSC crystal, the kind laying around in your junkbox.
>
> Kindly yours,
> Michael Baxter
> California, USA
>