[Discuss-gnuradio] Reading/processing off-air DVB-T transport stream

[Tom Crane]

I am attempting to read samples of an off-air DVB-T transport stream into 
an IQ file for offline analysis but can't get it to work at all.

I can run the dvbt_rx_8k.grc example which reads the adv16.cfile IQ TS 
file produced by Ron Economos, displays the FFT and 16 QAM constellation 
and produces a valid .ts file which can be played with a video player..

I adapted that example flow-graph for a local off-air MUX with parameters; 
64QAM, 2/3 rate, 1/4 guard interval, 8K FFT.  The FFT displays OK but the 
constellation graph does not work and show the expected 8 x 8 matrix -- 
instead it shows just a single dot in the middle.

This 
https://www.mklab.rhul.ac.uk/~tom/tvdx/gr/dvbt_rx_8k_64QAM_chD44-live.grc 
cut-down demo attempts to sample channel channel D44 live using an 
RTL2832/E4000 USB stick.  I have also tried using the rtl_sdr 
dump-to-IQ-file utility and converting the format from 8-bit uint into 
32-bit float (complex) using the sox utility, eg.

rtl_sdr -f 658000000 -s 2800000 -n $[280000*60] chD44.raw
sox -t raw -r 2800000 -b 8 -c 1 -e unsigned-integer chD44.raw -t raw -r 2800000 
-c 1 -e float chD44-sox.raw

using a sample rate of 2.8M here, and processing the resultant IQ file 
with grc but the result is the same.

I also tried with a 3.2M sample rate, the maximum permitted value with 
RT2832 USBs sticks but again without success.

Am I doing something wrong in grc or it is just that these sample rates 
are nowhere near sufficient for the whole-MUX type of data I am attempting 
to capture?

I was able to adapt the dvbt_rx_8k.grc demo for a 64QAM file (adv64.cfile 
also from Ron Economos) and process that OK in grc.

Many thanks
Tom Crane

--
Tom Crane, Dept. Physics, Royal Holloway, University of London, Egham Hill,
Egham, Surrey, TW20 0EX, England.
Email:  address@hidden
Fax:    +44 (0) 1784 472794