Above is a photo of the inside of one of the Comtech modules I bought.
I plan using this module as a 1.3 GHz down converter so that I can
receive 1.3 GHz DVB-T on a domestic digital TV. Rather than using a PIC
controller to program the synthesiser I am considering using a USB chip
in its parallel port mode. I have a couple of DLP2232M-G development
modules. These can either be programmed as 2 serial ports or as two 8 bit
parallel ports. The spare lines I can use to control RF relays etc. I have used
these devices in the Windows environment but never under Linux.
Talking about DVB-T. I have now managed to get a stable transmission.
To do this I had to increase the size of the Kernel buffers that the UHD module
uses to communicate with the USRP2. I can now send up to 19 Mbits.
The TV reports a BER of zero. You can have too much of a good thing
though, if I increase the kernel buffers too much the whole program locks up
because all the internal queues empty and my program tries to fill them
with NULL transport packets.
I have 2 STBs, a Fortec Star Lifetime Classic and a
Fortec Star Ultimate PVR. I have been using the
Classic but I swapped to the Ultimate today. The
received picture quality is much better and I am
getting less motion glitches on the picture. You
would think that two STBs made by the same
company and bought about the same time would
have similar performance. It appears not!
I corrected an issue with the FEC configuration
in the DVB-T mode. The actual FEC and that
being signalled in the TP information were not
the same. Consequently the TV would not lock to
anything other than 1/2. Long story simple fix.
I have added the bit-rate lookup table from the
DVB-T spec to my code so when you change DVB-T
modulation parameters a suitable video bit-rate
is chosen automatically. The audio bit-rate remains
constant for all modes.
I ordered a couple of Comtech analogue STB modules
from a retailer on eBay. The plan is to use them
as a cheap 24 cms down-converter as they have
an IF of 479.5 MHz. I will have to open them
up and put a vampire tap inside to pull out the IF.
Still I should be able to receive 24 cms DVB-T on
my TV when that is done.
I think the next job is to go through the
SI descriptors for DVB, as I am still not getting a
sensible EPG on the STB. I would also like to add
Teletext when I figure out the format. Just simple
stuff like QRA locator and a station description.
Those of you that follow the GNURADIO forum will
know I appear to be getting only about 25% of the
power I should be getting. It seems there is a
software bug somewhere. Hopefully this will be
sorted out soon but is not under my control.
That is the problem with all this bleeding
edge technology :-).
