The bandwidth of DVB-T is 8 MHz (or 7 MHz for VHF). You can't capture
that much bandwidth with an RTL-SDR. The required sample rate is
(8000000.0 * 8) / 7 = 9.14286 Msps for an 8 MHz channel, or (8000000.0 *
7) / 7 = 8 Msps for a 7 MHz channel.
You need an SDR that can handle these sample rates such as bladeRF,
hackRF, USRP, etc.
Ron
On 10/15/19 18:11, Tom Crane wrote:
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
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