turns out that Robin Gareus who wrote the libltc had already written an arduino LTC code reader that works from audio input ! code here
I'm just finding out if he ever wrote a generator too !!
The idea now would be to edit the code from using #include <FrequencyTimer2.h>
and somehow make the arduino use a LTXCO as the timer. or have FrequencyTimer2.h be conditioned by the TXXO oscillator + GPS ( UTC time ) if neccessery.
as I thought 8bits audio is good enough quality to read the 80bit LTC frames.
Robin used 8bit 16khz sample rate to recieve the samples.
If he never wrote an encoder then I'll have to write one from scratch I think using the decoder as a guide. Robin thinks the ltclib is too heavy to run on the arduino or at least it would'nt leave much room for anything else.