[sdiy] A440 -- almost

[Scott Gravenhorst]

Michael Baxter <mab at cruzio.com> wrote:
>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.

The prime factors of 8134 are 2, 7, & 83 with 7 used twice.  Ugh.

>
>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
>

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