[time-nuts] ANFSCD - Synchronizing time in home video recorders

Clint Turner turner at ussc.com
Thu Feb 2 22:10:24 UTC 2012

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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