[time-nuts] webcam app to watch for and time stamp changes

Peter Gottlieb nerd at verizon.net
Sat Mar 2 19:00:43 EST 2013

Perhaps you can detect EMI from the device especially if you put it it a 
shielded metal box with pickup antenna.  You might be able to get the clock 
right from that.

On 3/2/2013 6:50 PM, Jim Lux wrote:
> On 3/2/13 2:52 PM, Tom Van Baak wrote:
>> Hi  Jim,
>> I had a similar challenge a while ago. I ended up capturing a
>> 4-digit, 7-segment display with a USB/LAN webcam, converting the JPG
>> to BMP, analyzing pixel gradients, matching the image with heuristic
>> masks, and appending an ascii log file with the 4 decimal digit
>> result once a minute. It worked amazingly well. I can send you the C
>> source code (contact me off-line). See also these legacy links:
>> http://leapsecond.com/webcam/
>> http://tycho.usno.navy.mil/ptti/ptti2003/paper35.pdf
>> More recently I used the same webcam to capture AC mains wall clock
>> time vs. a cesium clock:
>> http://leapsecond.com/pages/tec/mains-clock-ani.gif
>> Now, it may seem very old-fashion and crude to use a web cam to
>> capture data.
> I've used that scheme more than once.. as well as similar approaches of 
> spinning wheels or counters driving LEDs to measure camera shutter timing.
>> But, if I understand correctly, in your case, you are more interested
>> using the external LCD display as a *proxy* for the internal crystal
>> oscillator.
> exactly
> Thus you are not concerned with the actual numeric value
>> that the LCD segments are displaying; you are simply interested in
>> measuring the time at which the LCD digits and segments change --
>> because that corresponds to the phase/frequency of the internal
>> oscillator.
> yes
>> Measuring clock drift and tempco would be very easy and would produce
>> interesting results. The higher the rate of web cam capture the finer
>> the resolution of your TIC detection. Note that multiplexed displays
>> are not a problem; in fact, if you're clever it can actually improve
>> your resolution by observing the segment transition times. That is,
>> instead of *polling* the display, you use *changes* in the display as
>> your timing trigger. Edge detection. You move from a world of
>> periodic/gated frequency counters to a world of a reciprocal period
>> counters, or even time-stamping counters.
>> In other words, since you are interested in the underlying clock
>> performance rather than the RSA algorithm itself, just focus a few
>> photo-transistor(s) on the LCD segment(s).
> That's what I was thinking.. the segments are pretty small, though, so some 
> optics and fixturing would need to be fabricated. The idea of "point webcam at 
> RSA fob and run software" was pretty appealing.
>  The transition from
>> light/dark/light/dark will exactly correspond to some internal
>> crystal clock phase/frequency, from which you can gather precise
>> long-term phase, frequency, and stability information. Edges are
>> always better than polling.
> Yes indeed..
> I can't think of a good reason why they would decouple the segment mux from 
> the underlying clock (unless there's some built in RC oscillator in the chip 
> doing the mux, separate from the timekeeping).
> Actually, there's probably some information around on the internal design of 
> the fob.. After all, the security is in the algorithm, not the hardware.
>> The idea is that segment *transition* instants rather than periodic
>> observation of segment *state* provides much greater accuracy. What
>> you want to do is measure the time of transition of individual
>> segments. I would use time-stamping rather than polling or time
>> interval or frequency measurement. That is, use a CNT-91, or picPET
>> (www.leapsecond.com/pic).
>> If you look at a 7-segment digit encoding table
>> (http://en.wikipedia.org/wiki/Seven-segment_display) you can see that
>> focusing on segment 'e' gives you 8 transitions out of every 10, and
>> using two opto-detectors on segments 'a' and 'e' will allow you to
>> generate a timing event for every change in LCD digit. This will be
>> several orders of magnitude more precise than using a web cam. You
>> don't need photo-transistors on all segments.
> Even better, there's a little segment that blinks on and off every second, as 
> well as a stack of 6 that go away, one every 10 seconds, and then, finally, 
> the 6 digits.  If I rig up an optical system, the 1 pps segment is the logical 
> one to look at.
> Time to look for some suitable optics.
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