[time-nuts] GPS->audio interface

[Magnus Danielson]

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.

> Seems that a GPS->IRIG B interface would work, but I was wondering if
> someone has done this already (e.g. Someone must have made a IRIG
> encoder in a PIC or similar)..

I actually have a GPS with IRIG-B output in about the same size of a 
Thunderbolt. If you have your own encoder in the form of a PIC/AVR then 
maybe producing SMPTE LTC should be better or at least considered as an 
alternative output.

> 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.

> 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.

Cheers,
Magnus