[sdiy] A440 -- almost

[Tim Ressel]

Hiya,

Of course, the ultimate COOL way to do this is with a
DDS chip a la AD9830 from Analog Devices. This
solution gets you to within a few PPM of the actual
frequency and is very adjustable. The down side? Well,
as you know, 'cool' is Greek for 'expensive'.

--TR

--- 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.
> 
> 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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