Sunday 23 September 2012

DVB-S2

In case anyone was wondering I am currently debugging the
DVB-S2 module in the Express software. So far I have
managed to get my SatLink box to lock to the signal and
report that it is DVB-S2 and the FEC rate, modulation and
roll off. Unfortunately there is something not quite right.
It only works if I invert the 26 bit sync sequence in the
physical header. The physical header is transmitted using
BPSK with a 90 deg offset to prevent the usual 180 degree shift
with BPSK. The information is sent using a bi-orthogonally
coded (64,7) Reed Muller based code. The 7 bits being the
code rate, modulation, frame type and presence of pilots
information. The physical header is 90 symbols in duration.

I spent most of this morning making sure the CRC8 code which is
used to check the baseband header and the transport packets
was correct. I also checked the LDPC (Low Density Parity Check)
lookup tables were correct. I still have the BCH encoders to
check. Unfortunately there is little help in the ETSI EN 302 307
spec with any test vectors so I just have to double check everything
and keep my fingers crossed. As I now have a couple of ways to
receive DVB-S2 I will know when it is all working.

The QPSK FEC=1/4 mode looks interesting as that will operate
at below the noise level. So MPEG4, 70 cms, 1 MS/s and DVB-S2
look like a pretty powerful combination.

I am sure I will return to this later in my blog.

2 comments:

  1. Hi Charles - this all sounds really. While I really don't understand too much of the software behind this it sounds like you are on to a winner.
    My gut feeling is the MPEG4 DVB S2 encoding is the way forward for ATV (rather than MPEG 2) - could be especially interesting if proposals to widen 2m ever come to fruition.
    Keep up the good work - always intersted to read your blog.
    Look forward to seeing your demo snd your project at the BATC convention.

    73 Ian

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  2. Hi Ian, thanks for the feedback. Hopefully this blog will get people not traditionally interested in TV or Amateur Radio curious. With 1MS/s we could almost use 10m or 6M. With HVEC (H.265) which uses half the data rate of H.264 coming along things are getting quite exciting. I managed to transmit/receive DVB-S2 pictures this morning but there still appears to be a problem as I am getting very high error rates even with the antennas 10 cm apart. Back to the code!

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