[time-nuts] ANFSCD - Synchronizing time in home video recorders
azelio.boriani at screen.it
Thu Feb 2 22:38:01 UTC 2012
No doubt, the correct way to generate accurate clocks from an accurate
10MHz is by PLLs. There are DDS too, then there is a strange method that
uses a sort of dual (triple? Quadruple? ...) modulus. The advantage is that
you don't need another oscillator (the PLL needs a VCO) or the (co)sine
lookup and DAC combination: just divide (oddly, of course,
inserting/removing cycles every here and there). Maybe the sigma at short
tau isn't so good but in the long run it gets better. The derived clock can
be used to synchronize equipment that is not so sensitive at short taus.
This method can show how can a clock be not so good in the short term but
very good in the long term.
On Thu, Feb 2, 2012 at 11:10 PM, Clint Turner <turner at ussc.com> wrote:
> Years ago (in the 80's) I needed to lock a homebrew DDS to an accurate,
> stable 10 MHz reference (a good TCXO in this case) that was set to WWV/H.
> Considering that the DDS was clocked at 2^24 Hz (16.777216 MHz) this was
> slightly awkward, but I did it using standard HC and 4000 logic.
> The convoluted path was:
> 10 MHz / 625 = 16 kHz (HC40103 as a div-by-125 and an HC4017 as a div-by-5
> would work...)
> 16 kHz * 32 = 512 kHz (using a 4046 and 4040)
> 512 kHz /125 = 4096 Hz (using 40103 or similar)
> From there, it was a no-brainer to compare this with the 16.777216 MHz /
> 4096 with another 4046/integrator - but the same 'HC4040 that did this also
> had a tap with 32768 kHz on it.
> With a fairly slow loop and a low-noise 2^24 Hz VCXO, the DDS's clock was
> both clean and stable - and tuned in 1 Hz steps! A cheap and more-common
> 4.194304 MHz crystal would work and I suppose that a similar scheme could
> be used to lock a 32768 Hz VCXO but I've never tried to 'VCXO a tuning-fork
> crystal before:-)
> * * *
> I, too, have an older (Philips) DVR that has lost its time sync since the
> analogs went dark. For a while, I used the XDS time code that happened to
> be in the vertical interval of one of its standard definition DTV PBS
> station's sub-channels (received on a set-top box and modulated onto a TV
> channel to which the DVR would "look" for its time code) but this has code
> since been dropped.
> Before I discovered this, I dug up the line 21 (IIRC) code specifications
> and noted that even a PIC could probably generate the proper code,
> synchronized either from a GPS or a WWVB receiver. I'd thought about
> putting it on multiple lines and then RF modulating it for the DVR to see,
> but lost enthusiasm after I discovered the time code on the sub-channel.
> Since that went away (about a year ago) I've just remembered to set the
> clock once a month, not being able to quickly find the specs for the time
> code again online...
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