[sdiy] A440 -- almost

[Michael Baxter]

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