[time-nuts] TCVCXO Adjustment

Magnus Danielson magnus at rubidium.dyndns.org
Sat Apr 14 07:43:15 EDT 2018


Wayne,

On 04/11/2018 01:10 AM, Wayne Holder wrote:
> I'm designing a small, portable, SMPTE LTC Timecode Generator
> <https://en.wikipedia.org/wiki/SMPTE_timecode> as an open source/hardware
> project for amateur filmmakers and videographers.  LTC Timecode is
> typically recorded on the audio tracks of cameras and sound recorders so
> the video and sound comments can be automatically sync'd later.  I'm
> planing on using a small, SMD TCVCXO such as the LFTVXO075806Cutt
> <https://www.mouser.com/ProductDetail/IQD/LFTVXO075806Cutt?qs=sGAEpiMZZMscy%2f6qMaFHY0htnjNN0iZ6XRXaS1jehPSKkjjKOKqbkg%3d%3d>,
> which is spec'd at a frequency tolerance or +/- 1 PPM and a frequency
> stability of 0.28 PPM and a yearly aging of +/- 1 PPM max/year which, to
> me, seems pretty impressive for a part that costs about $8.
> 
> Since the TCVCXO includes a voltage control input, my plan is to also add
> a 12-Bit Digital-to-Analog Converter with EEPROM Memory, such as the
> Microchip MCP4725
> <https://www.mouser.com/ProductDetail/Microchip-Technology/MCP4725A0T-E-CH?qs=sGAEpiMZZMvfFCidbTccA97L6UsE6%2fky>
> to
> provide a way to initially check and calibrate the frequency after surface
> mount soldering and also later to compensate for aging.  But since this is
> intended as an open source/hardware project rather than a commercially
> manufactured one, I've been pondering how someone building the device would
> be able to easily and reliably calibrate it.
> 
> I'm basing the design around the Arduino, so the device could, in theory,
> use the USB Serial connection as a way to connect to a calibration program
> running on a PC.  I have a few idea on how to attempt to do this, but this
> is new territory for me, so I'm asking for advice and/or thoughts on how
> feasible this might be.  Is this a crazy, impractical idea given that all
> the builder will probably have available to perform the calibration is a
> regular PC and an Internet connection, or is there a way to make it work?

When recording things like this, if you manage to synchronize everything
to the same source locally, you don't care too much about the absolute
frequency of that source. The unsteered oscillator would suffice, as
within 10 ppm is what is modern specifications.

Consider if you can produce black-burst video reference and word-clock
signals as well, as that helps to actually synchronize video and audio
sample rates. Some cameras may not take "genlock" video but have video
out, then you could take that to be the common clock and produce the
rest from that as reference.

A trick that has been used especially under battery operation has been
to have stable clocks, that is OCXOs, and before the shoot synchronize
them to each other and then during the shoot have them free-wheel. Since
they are stable they do not loose frequency and phase too much and that
saves effort later in the post-processing. That is where you would use
steering to make them agree before hold-over. Then the relative
frequency difference is what matters.

You should also look at ViTC, for the video time code form of SMPTE 12M
time code standard.

Cheers,
Magnus



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