Saturday, 27 August 2016

Small steps in DVB-S2 decoder

Firstly I finally found a suitable enclosure for the Jetson TK1 I bought a while
ago as you can see in the photo above. The red box is a Hammond enclosure.
I mounted the Jetson on a piece of 1.6 mm Aluminium sheet and below that I
added an SSD.

The Jetson has a 192 Core GPU as well as USB3 and Gigabit Ethernet so may
well make a nice basis for a portable computer to use with one of the LimeSDRs
I have on order. Last time I played with it I had difficulty in getting the expected
USB3 performance out of it. By default it sets the USB3 port to USB2 only (daft).

I have done some work on the DVB-S2 receiver software since I last posted.
I managed to write a BCH hard decision decoder. So far I have only tested it with
one of the S2 Normal frame formats but it should work on all the various formats.
The Short frames use slightly different generator polynomials so some extra work
will have to be done to support that. I based it on the RS decoder I wrote a while

I have also moved the syndrome calculation code to run on the CUDA device, in my
case that is a GTX980 TI.

The reason for doing that is the syndrome calculation has to run on every received
frame to check to see if there are any errors. If no errors are detected further
processing is not required. The Host communicates with the Device (GPU)
via PCI and that is very time consuming so it is best to make as few memory
transfers between the GPU and host as possible. The syndromes are quite small
compared to the data frame so the overhead is not very great.
The BCH decoder is implement as a predictor so is not easy to implement using
parallelism, well at least not for me. So it makes more sense to do it on the host.

One thing I learned today is that if the GPU code takes more than a couple of
seconds to run then Windows times out and resets the graphics card. This can be
avoided by either changing a registry entry or by breaking the code up into smaller
tasks. There is a lot for me to learn. The different types of memory on the GPU
and how you use them is also something that needs careful attention to when
writing code.

I am not sure how much time I am going to be able to spend on this little project in
the next few weeks as I have been asked to create a video to go with my CAT16
talk at the end of September and that is going to be time consuming no doubt.

That is it for now.......

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