[time-nuts] GPS->audio interface
Magnus Danielson
magnus at rubidium.dyndns.org
Mon May 11 08:20:45 UTC 2009
Lux, James P skrev:
>
>
> On 5/10/09 4:26 PM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
>
>> Lux, James P skrev:
>>>
>>> On 5/10/09 12:15 PM, "Brooke Clarke" <brooke at pacific.net> wrote:
>>>
>>>> Hi James:
>>>>
>>>> You might want to get a KIWI-OSD, Video overlay of GPS precision time
>>>> stamp. It adds HH:MM:SS EEEE OOOO FFFFFF at the bottom of the image and
>>>> so can be seen in every field. They have also developed a way to
>>>> calibrate the camera shutter in relation to the frame time by using a
>>>> number of LEDs. The main use if for star occulation timing.
>>>> http://www.geocities.com/kiwi_36_nz/kiwi_osd/kiwi_osd.htm
>>>>
>>>> Have Fun,
>>>>
>>>> Brooke Clarke
>>> Yes, but that requires getting access to the video stream, which isn't easy
>>> in a "camcorder" environment. Separate video recorders actually cost more
>>> than camcorders.
>> What precision of anything do you need?
>>
>> Expect 50-100 ppm oscillators in there.
>>
>
> Yes, but probably fairly good in the short run, and if you were recording
> almost any sync signal (e.g. The 1kHz sine) you could calibrate that out.
Certainly, this is what I expect.
A very simple approach is to pull the audio-track down and post-process
it with say Matlab/Octave to pull out the frequency, if it is 1 ppm low
then the sampling rate was 1 ppm high. That's about all the things you
need to know from that I think.
>> Recorded material is compressed. Consider how that affects your readout.
>> Recorded material is said progressive/interlaced. This may not reflect
>> how the CCD is snapshot and readout. You may not have the expected 1125
>> or 750 lines per frame, and exactly what frame rate do you have?
>
> I think that's something they'll need to experiment with...
Indeed. I think a pair of (fast) diodes blinking at some adjustable rate
and some motor rotating a pointer or something at a rate of 1 or a few
turns a second should be interesting objects to test on. They should
both be able to illustrate interlaced/progressive scanning and rate of
line-scanning/snapshot. The rotating object is easier to understand for
students while the blinking diodes can require a little more involved
discussions. Maybe three or four in a row would even more illustrative
since you can make the rate such that every other should be dark if you
adjust the frequency just right.
I think grey-scale contrast should be at about the maximum for best
result, since both vertical and horizontal sub-rate sampling can exist.
Keeping the picture free of other distractions can also allow the
bit-budget to be directed to the object of interest.
For your purpose I think progressive scan would be the best, if available.
Cheers,
Magnus
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