[time-nuts] GPS->audio interface

Lux, James P james.p.lux at jpl.nasa.gov
Sun May 10 18:57:05 UTC 2009




On 5/10/09 11:21 AM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:

> Lux, James P skrev:
>> I'm looking for a way to take GPS time and generate a signal that can
>> be recorded on the audio track of a video recording to time stamp it.
>> This is so it can be aligned with other data that's collected with GPS
>> based time.  It needs to be portable/small (i.e. Something you could
>> attach to a small camcorder, or such).
> 
> Traditionally SMPTE 12M LTC is being used for this purpose. For most
> cases it is only known as LTC but SMPTE/EBU time code is among other
> names fairly common name.
> 
Yes.. But most consumer recorders (e.g. The cheap AIPtek) don't do timecode
(or genlock, or other useful stuff)..

So what they're really looking for is someway to time align after the fact.

> I actually have a GPS with IRIG-B output in about the same size of a
> Thunderbolt.

Is that an off the shelf, not hideously expensive, widget?  (There are other
applications...)
> 
>> Another alternate is if something like a iPhone records accurate time
>> with the video stream. (another of the data sources is an iPhone
>> recording something else)
>> 
>> I think the basic requirement is accuracy to some few milliseconds
>> (e.g. Frame rate of the video)..
> 
> SMPTE LTC encodes the frame numbers.
> 
>> (It's for a high school science project, where they want to record
>> various things, and line them up.. I think they could deal with
>> looking at the timestamps over many frames to do interpolation)
> 
> SMPTE LTC should be preferred in that case, as it is something that
> editing equipment understands.

I don't think "real" editing gear is in the cards.  Probably more like
iMovie or something on a PC.

As a model for what they're trying to do, say you were going to measure the
acceleration due to gravity by videotaping a falling object against a scale
in the background. Except that the motion is more complex.. Maybe imagine
putting an accelerometer in the "payload" of a trebuchet... And you want to
time align the position of the trebuchet components (from the video) with
the forces on the payload. That's not what they're doing, but now that I
describe it, that WOULD be a cool science project.  And get away from the
"here, I built a trebuchet and launched a projectile X meters" which is kind
of tired.  (get one of those analog devices 3 axis accelerometers, hook it
to a suitable datalogger..)


> 
>> Anyway, cheap and cheerful consumer gear is what is sought. (so no
>> suggestions of synthesizing SMPTE from the output of my Z3801 and
>> feeding it to a RED camera.. We're talking AIPtek and iPhone here..)
> 
> Locking the camera with a black burst GENLOCK signal with VITC does need
> to be done, but the LTC may very well be practical for the purpose.
> 
> BTW. Cameras lacking GENLOCK is highly annoying, especially when they
> have el cheapo crystals... el cheapo crystals is fine, as long as they
> lock up to a GENLOCK when needed.


That pretty well describes just about everything they'll have available.


> 
> Cheers,
> Magnus
> 
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