"RTL SDR dongle not working as expected" - SDR noob questions.

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laurentp
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Joined: Tue Mar 24, 2020 12:26 pm

"RTL SDR dongle not working as expected" - SDR noob questions.

Post by laurentp » Mon Mar 30, 2020 3:01 am

Hi there,
just got my RTL-SDR USB dongle, the blue one,
System Linux Fedora 24 32 bit, on [email protected]/4GB RAM,
lsusb reports:
Bus 002 Device 003: ID 0bda:2838 Realtek Semiconductor Corp. RTL2838 DVB-T
installed SDR packages from network repo:
[[email protected] Net-MQTT-Simple-1.24]# rpm -qa | grep sdr
rtl-sdr-0.5.3-6.fc24.i686
rtlsdr-scanner-1.0.22497.10311-1.fc24.noarch
python2-pyrtlsdr-0.2.2-6.fc24.noarch
now trying to receive my favourite FM station @91MHz:

rtl_fm -f 91000000 | aplay -f S16_LE -t raw -c 1

Found 1 device(s):
0: Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM
Detached kernel driver
Found Rafael Micro R820T tuner
Tuner gain set to automatic.
Tuned to 91252000 Hz.
Oversampling input by: 42x.
Oversampling output by: 1x.
Buffer size: 8.13ms
Exact sample rate is: 1008000.009613 Hz
Sampling at 1008000 S/s.
Output at 24000 Hz.
Playing raw data 'stdin' : Signed 16 bit Little Endian, Rate 8000 Hz, Mono

Hear only noise, but... freq is incorrect. What is going on, how to correct it?

rtlsdr_scan starts w/o problem, but how to interpret output?

rtlsdrblog
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Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by rtlsdrblog » Thu Apr 02, 2020 5:12 am

If you're just starting out, then it's best to start with a program with a GUI like GQRX or CubicSDR.

If you want to persist with the command line, then there is a good rtl_fm guide here http://kmkeen.com/rtl-demod-guide/

laurentp
Posts: 8
Joined: Tue Mar 24, 2020 12:26 pm

Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by laurentp » Sun Apr 05, 2020 7:57 pm

Will read, thx for pointing me at.
In mean time I started rtlsdr_scan and it shows (expected) peak in red @91MHz (and about dozen others, set max gain), so what is wrong with rtl_fm?

rtlsdrblog
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Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by rtlsdrblog » Mon Apr 13, 2020 9:24 am

laurentp wrote:
Sun Apr 05, 2020 7:57 pm
Will read, thx for pointing me at.
In mean time I started rtlsdr_scan and it shows (expected) peak in red @91MHz (and about dozen others, set max gain), so what is wrong with rtl_fm?
Check the guide. You'll want to use a line like

rtl_fm -M wbfm -f 89.1M | play -r 32k -t raw -e s -b 16 -c 1 -V1 -

laurentp
Posts: 8
Joined: Tue Mar 24, 2020 12:26 pm

Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by laurentp » Fri Apr 17, 2020 3:24 pm

Same result, only noise and DIFFERENT freq from what is in command line:
[[email protected] ~]$ rtl_fm -M wbfm -f 91.0M | play -r 32k -t raw -e s -b 16 -c 1 -V1 -
Found 1 device(s):
0: Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM

-: (raw)

File Size: 0
Encoding: Signed PCM
Channels: 1 @ 16-bit
Samplerate: 32000Hz
Replaygain: off
Duration: unknown

In:0.00% 00:00:00.00 [00:00:00.00] Out:0 [ | ] Clip:0 Detached kernel driver
Found Rafael Micro R820T tuner
Tuner gain set to automatic.
Tuned to 91271000 Hz.
Oversampling input by: 6x.
Oversampling output by: 1x.
Buffer size: 8.03ms
Exact sample rate is: 1020000.026345 Hz
Sampling at 1020000 S/s.
Output at 170000 Hz.
In:0.00% 00:00:36.61 [00:00:00.00] Out:1.17M [ -===|===- ] Clip:0 ^CSignal caught, exiting!

User cancel, exiting...
In:0.00% 00:00:36.86 [00:00:00.00] Out:1.18M [ -===|===- ] Clip:0
Aborted.
Reattached kernel driver
Signal caught, exiting!

Attached rtlsdr_scan

w_scan also produces some output:
[[email protected] ~]$ w_scan
w_scan
w_scan version 20161022 (compiled for DVB API 5.10)
guessing country 'PL', use -c <country> to override
using settings for POLAND
DVB aerial
DVB-T Europe
scan type TERRESTRIAL, channellist 4
output format vdr-2.0
output charset 'UTF-8', use -C <charset> to override
Info: using DVB adapter auto detection.
/dev/dvb/adapter0/frontend0 -> TERRESTRIAL "Realtek RTL2832 (DVB-T)": good :-)
Using TERRESTRIAL frontend (adapter /dev/dvb/adapter0/frontend0)
-_-_-_-_ Getting frontend capabilities-_-_-_-_
Using DVB API 5.10
frontend 'Realtek RTL2832 (DVB-T)' supports
INVERSION_AUTO
QAM_AUTO
TRANSMISSION_MODE_AUTO
GUARD_INTERVAL_AUTO
HIERARCHY_AUTO
FEC_AUTO
BANDWIDTH_AUTO not supported, trying 6/7/8 MHz.
FREQ (174.00MHz ... 862.00MHz)
-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_
Scanning DVB-T...
Scanning 8MHz frequencies...
474000: (time: 00:05.064)
482000: (time: 00:07.424)
490000: (time: 00:09.664)
498000: (time: 00:11.903)
506000: (time: 00:14.142)
514000: (time: 00:16.384)
522000: (time: 00:18.623)
530000: (time: 00:20.870)
538000: (time: 00:23.103)
546000: (time: 00:25.344)
554000: (time: 00:27.590)
562000: (time: 00:29.831)
570000: (time: 00:32.062)
578000: (time: 00:34.301)
586000: (time: 00:36.527)
594000: (time: 00:38.782) c
602000: (time: 00:41.023) ^C
ERROR: interrupted by SIGINT, dumping partial result...
(time: 00:43.077) dumping lists (0 services)
..
Done, scan time: 00:43.078

But trying to use VLC DVB-T with these freqs gives nothing.
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rtlsdrblog
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Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by rtlsdrblog » Wed Apr 22, 2020 9:31 am

rtl_fm is using offset tuning, so it's not going to tune the LO to exactly the freq you specify which is normal. But it will still receive the 91 MHz frequency you enter.

Your scan shows weak/almost no signals in the broadcast FM band. What antenna are you using?

laurentp
Posts: 8
Joined: Tue Mar 24, 2020 12:26 pm

Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by laurentp » Sat Apr 25, 2020 8:39 pm

Small one shipped with dongle. On monday I'll get cable to convert F-type connector to MCX one. Then I'll try with my neighbours antenna (known to good reception of at least TV stations).
First with Win$ software shipped with dongle.
FM signals are at least sufficient to feed car radio with antenna on the roof. (at least at 91MHz, not tried other).

laurentp
Posts: 8
Joined: Tue Mar 24, 2020 12:26 pm

Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by laurentp » Tue Apr 28, 2020 8:55 pm

Ok, got the MCX -> F-type conversion cable, maybe tomorrow I'll ask my neighbour to try again to receive anything.
If I need antenna, for (mainly) FM / DAB reception, what type should I look for? Grid or Yagi? And (after scan plot) would be an wideband amplifier needed?

tim_rtd
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Joined: Fri Jun 30, 2017 11:34 am

Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by tim_rtd » Wed Apr 29, 2020 6:38 pm

Just FYI, Linux has a very good TV app called kaffeine. It has a built in channel scanner and I have used it, but not on a rtl2832 unit.

laurentp
Posts: 8
Joined: Tue Mar 24, 2020 12:26 pm

Re: "RTL SDR dongle not working as expected" - SDR noob questions.

Post by laurentp » Tue May 05, 2020 9:03 pm

I know Kaffeine, but I disliked it in "DVD era". For multimedia I usually use VLC, it has settings for DVB, don't know if it can scan automatically (as for now my dongle is at my neighbor, to test it with big grid antenna).

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