Thursday, 11 March 2010

The modification to the DirecTV SUP2400 went well.
I can now receive on 436 MHz. The frequency the STB
needs tuning to is 2400 - 436 MHz. The SUP2400 is
a lot smaller than I thought but with my OptiVisor and
some daylight it took about 30 mins.

I am beginning to fathom out the MPEGII stream
formats now. I pretended I was a STB and thought
what information do I need to display a picture.
Looking at it that way the specification does not
seem so bad. Making a list of the necessary
packets and which page they appear on makes a
huge difference.

Should get my used colour camera soon.
I put a bid on some
yesterday. They are good to 7 GHz and should
have around 0.5 dB NF. The seller seems to have
loads of them. It should be possible to make a
simple board using some double sided PCB and a
sharp knife. The App note has a reference circuit
for use just above 24 cms. Looking at the S-params
it should be fairly easy to get it to go on 24.
With some component changes it should be fine
on 70 cms as well.

The modulator program I wrote (using MMX opcodes)
is now working. There was a slight bug in the
convolutional interleaver but the Reed-Solomon
encoder and the convolutional FEC encoder work.

I have not played with RS codes before and Galois
fields took me a while to understand. With the
help of a BBC white paper on RS codes which
conveniently gave the required polynomials in
the expanded form required to do the encoder all
is well.

The only slight problem is the shape of the transmitted
signal is not as good as it could be. Either I have made a
mistake in the code or more likely I need to use a few
more filter coefficients. The filter is a combined RRC filter
and x5 interpolator. The x5 interpolator is needed to get
the sample rate to something that divides into 100 M.
Unfortunately the fractional interpolator in GNU Radio
uses too many CPU cycles for the job.

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