Tuesday, 19 March 2013

Yet more, you guessed it express!

Yes another photo, this time of my HP spectrum analyser. What you are seeing is
a 2MS/s 32APSK 3/4 FEC DVB-S2 transmission with a 95 tap compensated root
raised cosine response will 0.35 rolloff. The filter takes the DVB-S2 symbols and
interpolates them by a factor of 8 to put the aliases outside the LC Nyquist filter

Believe it or not 6 Mbits/s of video is crammed into that 2 MHz piece of spectrum

DATV-Express will operate at symbol rates below 2MS/s but the alias responses start
to creep in. That is what comes of having a fixed analogue filter. Still if people need
lower symbol rates a new FPGA file can be created with x16 interpolation. It has taken
me a few days to get this to work but I have also re-arranged the code to make it more
readable and easier to change in the future.

The blip on the left hand side at 1.24 GHz is probably a multiple of 20 MHz reference
clock signal, it remains stationary when I change the operating frequency.

I am now going to take a step back and think a bit more as to where I go next with the
FPGA code. I am modularising the code as much as possible so I can use the same
modules for different variants. They all have some parts that are common and those
are the bits I am concentrating on first.


  1. All Looking good so far Charles, Looking forward to this in the future.
    I wounder how much it will cost in the end when project goes on sale.

  2. That is a difficult to answer. We are negotiating with a non profit organisation to distribute the boards and they will ultimately set the price, as yet I can't give a figure but it will be sold as cheaply as we can. Hopefully for about the same price as it costs to assemble a Digilite. The boards will be available when I finish the first version of the software. I am hoping to have a saleable product ready by October at the latest. The distributor wants to make it available as a package so you can buy it, turn it on and it works. However it will also be possible to buy just the assembled board and use my Linux code but that will have to be community supported and should appeal to the inner computer Geek. Sorry for being so vague but each person on the project has their own ideas as to how we should manufacture / sell it. I just stick to software.