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rs1729 / RS

radiosonde decoding
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Commandline for direct decode with rtl_fm #31

Closed JsBergbau closed 3 years ago

JsBergbau commented 3 years ago

Hello,

trying to decode RS41 data as described in https://github.com/rs1729/RS/tree/master/rs41

However I'm missing an option to directly pipe into from rtl_fm. How is this possible to follow live data ?

JsBergbau commented 3 years ago

It works with 

rtl_fm -g 60 -A lut -s 11025 -f 402300000 -E dc -d 1  - | sox -t s16 -r 11025 -c1 - -t wav - lowpass 2800 | ./rs41ptu --ecc --crc --ptu -vx
sox WARN wav: Length in output .wav header will be wrong since can't seek to fix it
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 49.60 dB.
Tuned to 402550818 Hz.
Oversampling input by: 91x.
Oversampling output by: 1x.
Buffer size: 8.17ms
Exact sample rate is: 1003275.031911 Hz
Allocating 15 zero-copy buffers
Sampling at 1003275 S/s.
Output at 11025 Hz.
sample_rate: 11025
bits       : 16
channels   : 1
samples/bit: 2.30
So 2021-01-10 11:48:55.001  # [10011]
[ 5352] (S2251569)  # [01111]
  lat: 50.23186  lon: 10.84883  alt: 18976.15   vH: 40.9  D:  95.0  vV: 4.1  # [11110]
So 2021-01-10 11:49:08.001 # [11011]
[ 5366] (S2251569)  # [01111]
  lat: 50.23102  lon: 10.86065  alt: 19084.26   vH: 38.2  D:  96.6  vV: 4.2  # [11110]
[ 5381] (S2251569) So 2021-01-10 11:49:28.001  # [00011]
So 2021-01-10 11:49:33.001 # [11011]

using Nooelec SDR stick. However I don't know if there are better settings to make decoding more robust. I have the impression that there are a lot of packets lost when 5352 5366 and 8381 are the packet numbers. SNR was 17 dB and for that low modulation it should be sufficient, shouldn't it?

rs1729 commented 3 years ago

I guess you could do something like (ppm 0, frequency 402.0 MHz)

rtl_fm -p 0 -M fm -s 12k -f 402.0M | sox -t raw -r 12k -e s -b 16 -c 1 - -r 48000 -t wav - 2>/dev/null | ./rs41mod -v --ptu2

I recommend using rs41mod https://github.com/rs1729/RS/tree/master/demod/mod it can also read IQ-data (rtl_fm -M raw or rtl_sdr), no need to resample, and for IQ data, input can be read as raw data:

rtl_fm -p 0 -M raw -s 48k -f 402.0M | ./rs41mod -v --ptu2 --IQ 0.0 --lp - 48000 16

Here the sample rate is 48kHz, rtl_fm has 16bit samples. Then you could also use rtl_sdr (8 bit samples) baseband data.

Here are some examples for baseband data (see description) https://www.youtube.com/watch?v=5YXP9LYUgLs Instead of rs_multi you can use a single decoder (e.g. rs41mod) with the --IQ option.

rs1729 commented 3 years ago

With rtl_rm/rtl_sdr you need to know the ppm and the exact frequency of the signal. For IQ-decoding (rs41mod --IQ <freq> --lp) there is also the --dc option that can help a little bit removing dc-offset.

rs41mod decodes a lot better than the old rs41ptu. See also the readme on https://github.com/rs1729/RS/tree/master/demod/mod

JsBergbau commented 3 years ago

Thanks for the detailed answer.

It works now much better

 rtl_fm  -M raw -s 48k -f 402.3M | ./rs41mod -v --ptu2 --IQ 0.0 --lp - 48000 16
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM
IF: 48000
dec: 1
Detached kernel driver
Found Rafael Micro R820T tuner
Tuner gain set to automatic.
Tuned to 402552000 Hz.
Oversampling input by: 21x.
Oversampling output by: 1x.
Buffer size: 8.13ms
Exact sample rate is: 1008000.009613 Hz
Allocating 15 zero-copy buffers
Sampling at 1008000 S/s.
Output at 48000 Hz.
[ 6693] (S2251569)  Sun 2021-01-10 12:11:20.001  lat: 50.21614  lon: 11.75591  alt: 26018.01   vH: 60.2  D:  96.4  vV: 4.9   (6)
[ 6694] (S2251569)  Sun 2021-01-10 12:11:21.001  lat: 50.21608  lon: 11.75676  alt: 26022.97   vH: 61.0  D:  95.6  vV: 4.8   (6)
[ 6695] (S2251569)  Sun 2021-01-10 12:11:22.001  lat: 50.21603  lon: 11.75761  alt: 26028.50   vH: 61.5  D:  95.5  vV: 6.2   (15)
[ 6698] (S2251569)  Sun 2021-01-10 12:11:25.001  lat: 50.21586  lon: 11.76016  alt: 26045.32   vH: 60.9  D:  95.9  vV: 5.6   (18)
[ 6699] (S2251569)  Sun 2021-01-10 12:11:26.001  lat: 50.21581  lon: 11.76100  alt: 26051.41   vH: 60.3  D:  95.0  vV: 6.4   (4)
[ 6700] (S2251569)  Sun 2021-01-10 12:11:27.001  lat: 50.21577  lon: 11.76184  alt: 26057.58   vH: 59.7  D:  94.1  vV: 5.6   (11)
[ 6701] (S2251569)  Sun 2021-01-10 12:11:28.001  lat: 50.21573  lon: 11.76266  alt: 26063.31   vH: 58.9  D:  94.0  vV: 5.3   (5)
[ 6702] (S2251569)  Sun 2021-01-10 12:11:29.001  lat: 50.21569  lon: 11.76347  alt: 26068.94   vH: 58.0  D:  94.3  vV: 5.9   (5)
[ 6703] (S2251569)  Sun 2021-01-10 12:11:30.001  lat: 50.21565  lon: 11.76427  alt: 26075.29   vH: 57.0  D:  95.6  vV: 7.0   (9)
[ 6705] (S2251569)  Sun 2021-01-10 12:11:32.001  lat: 50.21553  lon: 11.76585  alt: 26086.89   vH: 56.7  D:  97.3  vV: 5.4   (23)
[ 6707] (S2251569)  Sun 2021-01-10 12:11:34.001  lat: 50.21540  lon: 11.76744  alt: 26099.58   vH: 59.0  D:  96.5  vV: 7.2   (19)
[ 6708] (S2251569)  Sun 2021-01-10 12:11:35.001  lat: 50.21534  lon: 11.76827  alt: 26105.99   vH: 60.3  D:  95.9  vV: 5.4
[ 6709] (S2251569)  Sun 2021-01-10 12:11:36.001  lat: 50.21529  lon: 11.76912  alt: 26111.43   vH: 62.0  D:  95.7  vV: 5.2   (13)
[ 6710] (S2251569)  Sun 2021-01-10 12:11:37.001  lat: 50.21523  lon: 11.76999  alt: 26117.28   vH: 63.4  D:  96.2  vV: 6.4   (15)
[ 6711] (S2251569)  Sun 2021-01-10 12:11:38.001  lat: 50.21517  lon: 11.77088  alt: 26123.61   vH: 64.3  D:  97.0  vV: 5.8   (17)
[ 6712] (S2251569)  Sun 2021-01-10 12:11:39.001  lat: 50.21509  lon: 11.77177  alt: 26129.25   vH: 64.4  D:  97.7  vV: 5.3   (13)
[ 6713] (S2251569)  Sun 2021-01-10 12:11:40.001  lat: 50.21501  lon: 11.77266  alt: 26134.86   vH: 64.3  D:  97.7  vV: 6.0   (21)
[ 6714] (S2251569)  Sun 2021-01-10 12:11:41.001  lat: 50.21494  lon: 11.77354  alt: 26141.09   vH: 63.7  D:  97.0  vV: 6.8   (9)
[ 6719] (S2251569)  Sun 2021-01-10 12:11:46.001  lat: 50.21467  lon: 11.77772  alt: 26171.36   vH: 55.8  D:  96.6  vV: 6.0   (10)
[ 6720] (S2251569)  Sun 2021-01-10 12:11:47.001  lat: 50.21461  lon: 11.77849  alt: 26176.88   vH: 55.4  D:  97.2  vV: 4.9   (12)
[ 6722] (S2251569)  Sun 2021-01-10 12:11:49.001  lat: 50.21448  lon: 11.78006  alt: 26189.03   vH: 58.4  D:  96.8  vV: 7.5   (5)
[ 6723] (S2251569)  Sun 2021-01-10 12:11:50.001  lat: 50.21442  lon: 11.78088  alt: 26196.17   vH: 60.4  D:  96.1  vV: 6.5   (10)
[ 6724] (S2251569)  Sun 2021-01-10 12:11:51.001  lat: 50.21436  lon: 11.78173  alt: 26201.93   vH: 62.3  D:  95.5  vV: 5.0   (17)
[ 6725] (S2251569)  Sun 2021-01-10 12:11:52.001  lat: 50.21431  lon: 11.78261  alt: 26207.12   vH: 64.0  D:  95.2  vV: 5.6   (9)
[ 6726] (S2251569)  Sun 2021-01-10 12:11:53.001  lat: 50.21426  lon: 11.78350  alt: 26213.19   vH: 64.6  D:  95.4  vV: 7.0   (9)
[ 6727] (S2251569)  Sun 2021-01-10 12:11:54.001  lat: 50.21420  lon: 11.78439  alt: 26219.77   vH: 64.1  D:  95.6  vV: 6.4   (14)
[ 6728] (S2251569)  Sun 2021-01-10 12:11:55.001  lat: 50.21415  lon: 11.78528  alt: 26226.29   vH: 63.1  D:  95.3  vV: 6.5   (14)
[ 6729] (S2251569)  Sun 2021-01-10 12:11:56.001  lat: 50.21410  lon: 11.78615  alt: 26233.13   vH: 61.9  D:  94.4  vV: 6.9   (8)
[ 6730] (S2251569)  Sun 2021-01-10 12:11:57.001  lat: 50.21406  lon: 11.78700  alt: 26240.11   vH: 60.7  D:  93.3  vV: 6.8   (15)
 lat: 50.21404  lon: 11.78784  alt: 26246.13   vH: 59.5  D:  92.7  vV: 5.1  [11110] (-+)
[ 6737] (S2251569)  Sun 2021-01-10 12:12:04.001  lat: 50.21380  lon: 11.79262  alt: 26283.52   vH: 60.1  D:  95.3  vV: 7.7   (14)
 lat: 50.21375  lon: 11.79347  alt: 26290.05   vH: 61.7  D:  94.6  vV: 5.3  [11110] (-+)
[ 6739] (S2251569)  Sun 2021-01-10 12:12:06.001  lat: 50.21371  lon: 11.79434  alt: 26295.54   vH: 63.3  D:  94.2  vV: 5.4   (3)
[ 6740] (S2251569)  Sun 2021-01-10 12:12:07.001  lat: 50.21366  lon: 11.79522  alt: 26301.78   vH: 64.0  D:  94.1  vV: 6.7   (11)
[ 6741] (S2251569)  Sun 2021-01-10 12:12:08.001  lat: 50.21362  lon: 11.79611  alt: 26308.68   vH: 63.6  D:  94.2  vV: 6.8   (11)
[ 6742] (S2251569)  Sun 2021-01-10 12:12:09.001  lat: 50.21358  lon: 11.79699  alt: 26314.93   vH: 62.7  D:  93.7  vV: 5.8   (10)
[ 6743] (S2251569)  Sun 2021-01-10 12:12:10.001  lat: 50.21355  lon: 11.79786  alt: 26320.99   vH: 61.4  D:  92.9  vV: 6.4   (10)
[ 6744] (S2251569)  Sun 2021-01-10 12:12:11.001  lat: 50.21353  lon: 11.79871  alt: 26327.63   vH: 60.5  D:  91.6  vV: 7.0   (13)
[ 6745] (S2251569)  Sun 2021-01-10 12:12:12.001  lat: 50.21352  lon: 11.79954  alt: 26334.18   vH: 59.8  D:  90.8  vV: 5.9   (7)
[ 6746] (S2251569)  Sun 2021-01-10 12:12:13.001  lat: 50.21351  lon: 11.80037  alt: 26340.47   vH: 59.0  D:  90.9  vV: 6.4   (6)
[ 6747] (S2251569)  Sun 2021-01-10 12:12:14.001  lat: 50.21350  lon: 11.80119  alt: 26347.37   vH: 58.2  D:  92.0  vV: 7.6   (7)
[ 6748] (S2251569)  Sun 2021-01-10 12:12:15.001  lat: 50.21347  lon: 11.80200  alt: 26354.39   vH: 57.8  D:  93.5  vV: 6.7   (14)

Whats that number (14) at the end of line?

When next RS41 passes I'll try if -E dc in rtl_fm option gives even better results.

I look for frequeny via SDRSharp and Nooelec RTL Dongle has a TCXO so luckly it is exact enough so frequencies don't be corrected.

rs1729 commented 3 years ago

The numbers in parantheses are the number of errors the error correction code was able to correct. It can correct up to 2x12=24 errors. If there is no number, then there were no errors in the frame, i.e. you know you have a good signal. In your example

[ 6730] (S2251569)  Sun 2021-01-10 12:11:57.001  lat: 50.21406  lon: 11.78700  alt: 26240.11   vH: 60.7  D:  93.3  vV: 6.8   (15)
 lat: 50.21404  lon: 11.78784  alt: 26246.13   vH: 59.5  D:  92.7  vV: 5.1  [11110] (-+)
[ 6737] (S2251569)  Sun 2021-01-10 12:12:04.001  lat: 50.21380  lon: 11.79262  alt: 26283.52   vH: 60.1  D:  95.3  vV: 7.7   (14)
 lat: 50.21375  lon: 11.79347  alt: 26290.05   vH: 61.7  D:  94.6  vV: 5.3  [11110] (-+)
[ 6739] (S2251569)  Sun 2021-01-10 12:12:06.001  lat: 50.21371  lon: 11.79434  alt: 26295.54   vH: 63.3  D:  94.2  vV: 5.4   (3)

If there are too many errors, it is possible that sub-packets still have have valid CRC, this is indicated by "0" (in the square brackets "[11110]") and the packet (e.g. GPS3 in your example) is displayed. "1" means CRC error for that packet. (-+) means that the second ECC word could be corrected (+), but not the first (-). Each frame has 2 Reed-Solomon codewords (interleaved) for error correction. (Note: If Reed-Solomon (255, 231) was able to correct errors, the probability that the codeword is the one that was transmitted is very very high. However if error correction was not possible, a valid CRC for a sub-packet is not as reliable. Once I received a frame with errors where the sub-packet with the serial number was obviously wrong but still had valid CRC, just enough bits flipped.) The signal in your example seems to be right on the edge for decoding.

If you are sure the signal is strong, maybe there is still some dc/frequency offset. Try if the additional option "--dc" for the decoder makes a difference. Although with TCXO the frequency offset shouldn't be such a problem.

rs1729 commented 3 years ago

If you are unsure about the signal quality, you can save the rtl_fm IQ data into a file and add a wav header or convert to float32, such that sdrsharp or gqrx resp. can replay the file. If rtl_fm uses only a very simple lowpass filter before decimation/downsampling (try option -F9), there could be other parts in the baseband that disturb the signal. Start with e.g.

rtl_fm -p 0 -M raw -s 48k -f 402.0M -F9 data_402000kHz_48k_s16_iq.raw

Then prepare the IQ data for viewing in gqrx or sdr#,

gqrx (raw s16 -> raw f32):

sox -r 48k -e s -b 16 -c 2 data_402000kHz_48k_s16_iq.raw -e f -b 32 data_402000kHz_48k_f32_iq.raw

sdr# (raw s16 -> wav s16):

sox -r 48k -e s -b 16 -c 2 data_402000kHz_48k_s16_iq.raw data_402000kHz_48k_s16_iq.wav
JsBergbau commented 3 years ago

Hi rs1729,

thanks for all your support. Finally I've managed to receive some RS41 again. Now I have a RTL-SDR v3 dongle I've tested with, also with TCXO.

Just tried with and without dc Option. Without:

rtl_fm  -M raw -s 48k -f 402.3M -g 60 | ./rs41mod -v --ptu2 --IQ 0.0 --lp - 48000 16
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM
IF: 48000
dec: 1
Detached kernel driver
Found Rafael Micro R820T tuner
Tuner gain set to 49.60 dB.
Tuned to 402552000 Hz.
Oversampling input by: 21x.
Oversampling output by: 1x.
Buffer size: 8.13ms
Exact sample rate is: 1008000.009613 Hz
Allocating 15 zero-copy buffers
Sampling at 1008000 S/s.
Output at 48000 Hz.
[ 6183] (S2251325)  Sun 2021-01-17 00:02:18.001  lat: 49.95574  lon: 11.15281  alt: 22838.23   vH: 30.2  D: 135.3  vV: 6.4   (14)
[ 6184] (S2251325)  Sun 2021-01-17 00:02:19.001  lat: 49.95555  lon: 11.15310  alt: 22844.18   vH: 29.2  D: 135.8  vV: 5.8   (3)
[ 6185] (S2251325)  Sun 2021-01-17 00:02:20.001  lat: 49.95536  lon: 11.15337  alt: 22849.31   vH: 29.0  D: 137.1  vV: 4.4   (9)
[ 6186] (S2251325)  Sun 2021-01-17 00:02:21.001  lat: 49.95517  lon: 11.15365  alt: 22854.21   vH: 29.3  D: 138.4  vV: 5.4   (4)
[ 6187] (S2251325)  Sun 2021-01-17 00:02:22.001  lat: 49.95497  lon: 11.15392  alt: 22860.11   vH: 30.0  D: 138.9  vV: 6.2   (17)
[ 6188] (S2251325)  Sun 2021-01-17 00:02:23.001  lat: 49.95476  lon: 11.15419  alt: 22865.62   vH: 30.5  D: 138.9  vV: 4.9   (19)
[ 6189] (S2251325)  Sun 2021-01-17 00:02:24.001  lat: 49.95456  lon: 11.15447  alt: 22870.58   vH: 30.7  D: 139.0  vV: 5.0   (4)
[ 6190] (S2251325)  Sun 2021-01-17 00:02:25.001  lat: 49.95435  lon: 11.15475  alt: 22876.04   vH: 29.8  D: 139.2  vV: 6.1   (2)
[ 6191] (S2251325)  Sun 2021-01-17 00:02:26.001  lat: 49.95415  lon: 11.15501  alt: 22881.58   vH: 29.1  D: 139.9  vV: 5.4   (7)
[ 6192] (S2251325)  Sun 2021-01-17 00:02:27.001  lat: 49.95395  lon: 11.15527  alt: 22887.21   vH: 28.8  D: 140.3  vV: 6.1   (13)
Sun 2021-01-17 00:02:28.001  lat: 49.95375  lon: 11.15553  alt: 22893.39   vH: 29.2  D: 140.5  vV: 6.4  [11010] (+-)
[ 6194] (S2251325)  Sun 2021-01-17 00:02:29.001  lat: 49.95355  lon: 11.15579  alt: 22898.78   vH: 29.7  D: 140.3  vV: 4.2   (9)
[ 6195] (S2251325)  Sun 2021-01-17 00:02:30.001  lat: 49.95335  lon: 11.15605  alt: 22903.42   vH: 29.7  D: 140.0  vV: 5.0   (4)
[ 6196] (S2251325)  Sun 2021-01-17 00:02:31.001  lat: 49.95315  lon: 11.15632  alt: 22908.75   vH: 29.0  D: 139.6  vV: 5.6   (5)
[ 6197] (S2251325)  Sun 2021-01-17 00:02:32.001  lat: 49.95295  lon: 11.15657  alt: 22913.83   vH: 28.2  D: 139.1  vV: 4.5   (1)
[ 6198] (S2251325)  Sun 2021-01-17 00:02:33.001  lat: 49.95276  lon: 11.15683  alt: 22918.26   vH: 27.8  D: 137.9  vV: 4.5   (8)
[ 6199] (S2251325)  Sun 2021-01-17 00:02:34.001  lat: 49.95258  lon: 11.15710  alt: 22923.00   vH: 28.0  D: 136.2  vV: 5.3   (2)
[ 6200] (S2251325)  Sun 2021-01-17 00:02:35.001  lat: 49.95240  lon: 11.15737  alt: 22928.48   vH: 28.8  D: 134.6  vV: 5.9   (4)
[ 6201] (S2251325)  Sun 2021-01-17 00:02:36.001  lat: 49.95222  lon: 11.15766  alt: 22934.06   vH: 29.4  D: 134.0  vV: 5.2   (1)
[ 6202] (S2251325)  Sun 2021-01-17 00:02:37.001  lat: 49.95203  lon: 11.15796  alt: 22939.39   vH: 29.6  D: 134.3  vV: 5.4   (6)
[ 6203] (S2251325)  Sun 2021-01-17 00:02:38.001  lat: 49.95185  lon: 11.15824  alt: 22945.44   vH: 29.1  D: 135.6  vV: 6.7   (6)

With -E dc option

rtl_fm  -M raw -s 48k -f 402.3M -g 60 -E dc | ./rs41mod -v --ptu2 --IQ 0.0 --lp - 48000 16
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM
IF: 48000
dec: 1
Detached kernel driver
Found Rafael Micro R820T tuner
Tuner gain set to 49.60 dB.
Tuned to 402552000 Hz.
Oversampling input by: 21x.
Oversampling output by: 1x.
Buffer size: 8.13ms
Exact sample rate is: 1008000.009613 Hz
Allocating 15 zero-copy buffers
Sampling at 1008000 S/s.
Output at 48000 Hz.
[ 6216] (S2251325)  Sun 2021-01-17 00:02:51.001  lat: 49.94953  lon: 11.16192  alt: 23013.55   vH: 28.8  D: 128.8  vV: 5.3   (4)
[ 6217] (S2251325)  Sun 2021-01-17 00:02:52.001  lat: 49.94937  lon: 11.16222  alt: 23018.91   vH: 28.6  D: 130.4  vV: 5.7   (3)
[ 6218] (S2251325)  Sun 2021-01-17 00:02:53.001  lat: 49.94920  lon: 11.16252  alt: 23024.24   vH: 28.7  D: 131.6  vV: 5.0   (12)
[ 6219] (S2251325)  Sun 2021-01-17 00:02:54.001  lat: 49.94903  lon: 11.16282  alt: 23029.58   vH: 29.0  D: 132.3  vV: 5.6   (9)
[ 6220] (S2251325)  Sun 2021-01-17 00:02:55.001  lat: 49.94885  lon: 11.16312  alt: 23035.10   vH: 29.5  D: 132.0  vV: 5.7   (3)
[ 6221] (S2251325)  Sun 2021-01-17 00:02:56.001  lat: 49.94867  lon: 11.16343  alt: 23040.12   vH: 29.7  D: 131.7  vV: 4.1   (4)
[ 6222] (S2251325)  Sun 2021-01-17 00:02:57.001  lat: 49.94849  lon: 11.16374  alt: 23044.72   vH: 29.9  D: 132.6  vV: 5.0   (5)
[ 6223] (S2251325)  Sun 2021-01-17 00:02:58.001  lat: 49.94831  lon: 11.16404  alt: 23050.30   vH: 29.5  D: 134.0  vV: 6.2  : fq 402300  (1)
[ 6224] (S2251325)  Sun 2021-01-17 00:02:59.001  lat: 49.94813  lon: 11.16432  alt: 23055.68   vH: 28.9  D: 135.2  vV: 4.9  : fw 0x4ef7  (2)
[ 6227] (S2251325)  Sun 2021-01-17 00:03:02.001  lat: 49.94761  lon: 11.16518  alt: 23071.68   vH: 28.2  D: 130.0  vV: 4.9   (10)
[ 6228] (S2251325)  Sun 2021-01-17 00:03:03.001  lat: 49.94745  lon: 11.16549  alt: 23076.62   vH: 28.3  D: 128.1  vV: 4.9   (4)
[ 6229] (S2251325)  Sun 2021-01-17 00:03:04.001  lat: 49.94729  lon: 11.16580  alt: 23082.33   vH: 28.2  D: 127.2  vV: 6.3   (3)
[ 6230] (S2251325)  Sun 2021-01-17 00:03:05.001  lat: 49.94714  lon: 11.16611  alt: 23088.10   vH: 27.8  D: 127.3  vV: 5.1   (23)
[ 6231] (S2251325)  Sun 2021-01-17 00:03:06.001  lat: 49.94699  lon: 11.16641  alt: 23093.04   vH: 27.7  D: 127.3  vV: 4.5   (22)
[ 6232] (S2251325)  Sun 2021-01-17 00:03:07.001  lat: 49.94684  lon: 11.16672  alt: 23098.03   vH: 27.9  D: 126.5  vV: 5.6   (9)
[ 6233] (S2251325)  Sun 2021-01-17 00:03:08.001  lat: 49.94670  lon: 11.16704  alt: 23103.65   vH: 28.5  D: 124.9  vV: 6.1   (4)
 lat: 49.94655  lon: 11.16737  alt: 23108.96   vH: 29.1  D: 123.8  vV: 4.5  [11110] (-+)
[ 6235] (S2251325)  Sun 2021-01-17 00:03:10.001  lat: 49.94641  lon: 11.16771  alt: 23113.54   vH: 29.2  D: 123.4  vV: 4.5   (14)
[ 6236] (S2251325)  Sun 2021-01-17 00:03:11.001  lat: 49.94626  lon: 11.16804  alt: 23118.70   vH: 28.8  D: 124.1  vV: 5.5
[ 6237] (S2251325)  Sun 2021-01-17 00:03:12.001  lat: 49.94611  lon: 11.16837  alt: 23123.89   vH: 28.4  D: 125.8  vV: 4.8   (13)
[ 6239] (S2251325)  Sun 2021-01-17 00:03:14.001  lat: 49.94581  lon: 11.16900  alt: 23134.77   vH: 28.6  D: 127.9  vV: 6.6   (9)
[ 6240] (S2251325)  Sun 2021-01-17 00:03:15.001  lat: 49.94565  lon: 11.16931  alt: 23140.43   vH: 28.9  D: 127.5  vV: 4.9   (6)
[ 6241] (S2251325)  Sun 2021-01-17 00:03:16.001  lat: 49.94549  lon: 11.16963  alt: 23145.20   vH: 29.0  D: 127.7  vV: 4.4   (7)
Sun 2021-01-17 00:03:17.001  [11001] (-+)
[ 6243] (S2251325)  Sun 2021-01-17 00:03:18.001  lat: 49.94516  lon: 11.17026  alt: 23156.77   vH: 29.4  D: 131.5  vV: 5.9   (3)
[ 6244] (S2251325)  Sun 2021-01-17 00:03:19.001  lat: 49.94498  lon: 11.17056  alt: 23161.98   vH: 29.9  D: 133.3  vV: 4.6   (5)
Sun 2021-01-17 00:03:20.001  [11011] (+-)
Sun 2021-01-17 00:03:21.001  [11011] (--)
^CSignal caught, exiting!

So I'd say makes not so much difference.

Viewing spectrum in SDR Sharp via rtl_tcp shows about 16 dB SNR.

Tried a few minutes later with about 18 dB SNR, which changes quite fast

rtl_fm  -M raw -s 48k -f 402.3M -g 60 -E dc | ./rs41mod -v --ptu2 --IQ 0.0 --lp - 48000 16
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM
IF: 48000
dec: 1
Detached kernel driver
Found Rafael Micro R820T tuner
Tuner gain set to 49.60 dB.
Tuned to 402552000 Hz.
Oversampling input by: 21x.
Oversampling output by: 1x.
Buffer size: 8.13ms
Exact sample rate is: 1008000.009613 Hz
Allocating 15 zero-copy buffers
Sampling at 1008000 S/s.
Output at 48000 Hz.
[ 6575] (S2251325)  Sun 2021-01-17 00:08:50.001  lat: 49.88081  lon: 11.28280  alt: 24786.00   vH: 39.8  D: 122.9  vV: 4.9
[ 6576] (S2251325)  Sun 2021-01-17 00:08:51.001  lat: 49.88062  lon: 11.28327  alt: 24791.01   vH: 40.2  D: 122.8  vV: 5.2   (1)
[ 6577] (S2251325)  Sun 2021-01-17 00:08:52.001  lat: 49.88042  lon: 11.28373  alt: 24796.50   vH: 40.0  D: 123.3  vV: 5.8
[ 6578] (S2251325)  Sun 2021-01-17 00:08:53.001  lat: 49.88023  lon: 11.28419  alt: 24802.31   vH: 39.5  D: 124.1  vV: 5.8
[ 6579] (S2251325)  Sun 2021-01-17 00:08:54.001  lat: 49.88003  lon: 11.28464  alt: 24807.63   vH: 39.3  D: 124.5  vV: 4.8   (1)
[ 6580] (S2251325)  Sun 2021-01-17 00:08:55.001  lat: 49.87983  lon: 11.28509  alt: 24812.90   vH: 39.5  D: 124.8  vV: 5.7  : fq 402300  (2)
[ 6581] (S2251325)  Sun 2021-01-17 00:08:56.000  lat: 49.87962  lon: 11.28555  alt: 24818.83   vH: 40.3  D: 124.9  vV: 6.0  : fw 0x4ef7
[ 6582] (S2251325)  Sun 2021-01-17 00:08:57.000  lat: 49.87941  lon: 11.28601  alt: 24824.48   vH: 41.1  D: 125.3  vV: 4.8  : BK 00
[ 6583] (S2251325)  Sun 2021-01-17 00:08:58.000  lat: 49.87920  lon: 11.28647  alt: 24829.40   vH: 41.3  D: 126.1  vV: 5.0
[ 6584] (S2251325)  Sun 2021-01-17 00:08:59.000  lat: 49.87898  lon: 11.28693  alt: 24834.76   vH: 40.8  D: 127.5  vV: 6.1   (1)
[ 6585] (S2251325)  Sun 2021-01-17 00:09:00.000  lat: 49.87875  lon: 11.28737  alt: 24840.58   vH: 40.3  D: 129.0  vV: 5.8   (5)
[ 6586] (S2251325)  Sun 2021-01-17 00:09:01.000  lat: 49.87853  lon: 11.28780  alt: 24845.71   vH: 39.8  D: 129.6  vV: 4.6   T=-67.0C  (2)
[ 6587] (S2251325)  Sun 2021-01-17 00:09:02.000  lat: 49.87830  lon: 11.28823  alt: 24850.50   vH: 39.5  D: 129.1  vV: 5.1   T=-67.0C  (1)
[ 6588] (S2251325)  Sun 2021-01-17 00:09:03.000  lat: 49.87808  lon: 11.28865  alt: 24856.05   vH: 39.3  D: 127.5  vV: 6.2   T=-66.9C  (1)
[ 6589] (S2251325)  Sun 2021-01-17 00:09:04.000  lat: 49.87787  lon: 11.28909  alt: 24861.68   vH: 39.2  D: 125.9  vV: 4.8   T=-66.9C  (2)
[ 6590] (S2251325)  Sun 2021-01-17 00:09:05.000  lat: 49.87767  lon: 11.28954  alt: 24866.78   vH: 39.4  D: 124.6  vV: 5.2   T=-66.9C  (2)
[ 6591] (S2251325)  Sun 2021-01-17 00:09:06.000  lat: 49.87747  lon: 11.28999  alt: 24872.59   vH: 38.9  D: 123.8  vV: 6.4   T=-67.0C  (1)
[ 6592] (S2251325)  Sun 2021-01-17 00:09:07.000  lat: 49.87728  lon: 11.29043  alt: 24878.25   vH: 38.8  D: 124.3  vV: 5.3   T=-67.0C
[ 6593] (S2251325)  Sun 2021-01-17 00:09:08.000  lat: 49.87708  lon: 11.29087  alt: 24883.09   vH: 38.8  D: 124.7  vV: 4.7   T=-67.0C  (1)
[ 6594] (S2251325)  Sun 2021-01-17 00:09:09.000  lat: 49.87688  lon: 11.29132  alt: 24888.13   vH: 39.1  D: 124.9  vV: 5.4   T=-67.1C  (3)
[ 6595] (S2251325)  Sun 2021-01-17 00:09:10.000  lat: 49.87668  lon: 11.29177  alt: 24893.57   vH: 40.0  D: 125.0  vV: 5.7   T=-67.1C  (2)
[ 6596] (S2251325)  Sun 2021-01-17 00:09:11.000  lat: 49.87647  lon: 11.29223  alt: 24898.86   vH: 41.0  D: 125.6  vV: 4.9   T=-67.1C
[ 6597] (S2251325)  Sun 2021-01-17 00:09:12.000  lat: 49.87625  lon: 11.29269  alt: 24904.31   vH: 41.7  D: 127.0  vV: 6.0   T=-67.2C
[ 6598] (S2251325)  Sun 2021-01-17 00:09:13.000  lat: 49.87602  lon: 11.29314  alt: 24910.30   vH: 42.0  D: 129.0  vV: 6.1   T=-67.2C  (2)
[ 6599] (S2251325)  Sun 2021-01-17 00:09:14.000  lat: 49.87578  lon: 11.29359  alt: 24915.53   vH: 41.9  D: 130.3  vV: 4.3   T=-67.2C  RH2=2%
[ 6600] (S2251325)  Sun 2021-01-17 00:09:15.000  lat: 49.87554  lon: 11.29403  alt: 24919.82   vH: 41.4  D: 130.5  vV: 4.6   T=-67.3C  RH2=2%
[ 6601] (S2251325)  Sun 2021-01-17 00:09:16.000  lat: 49.87530  lon: 11.29447  alt: 24924.86   vH: 40.9  D: 129.5  vV: 5.9   T=-67.3C  RH2=2%  (1)
[ 6602] (S2251325)  Sun 2021-01-17 00:09:17.000  lat: 49.87507  lon: 11.29491  alt: 24930.48   vH: 40.6  D: 127.6  vV: 5.6   T=-67.3C  RH2=2%
[ 6603] (S2251325)  Sun 2021-01-17 00:09:18.000  lat: 49.87486  lon: 11.29536  alt: 24935.59   vH: 40.5  D: 126.3  vV: 4.8   T=-67.3C  RH2=2%
[ 6604] (S2251325)  Sun 2021-01-17 00:09:19.000  lat: 49.87464  lon: 11.29581  alt: 24940.75   vH: 40.3  D: 125.3  vV: 5.5   T=-67.3C  RH2=2%
[ 6605] (S2251325)  Sun 2021-01-17 00:09:20.000  lat: 49.87444  lon: 11.29626  alt: 24946.72   vH: 39.7  D: 125.6  vV: 6.1   T=-67.3C  RH2=2%
[ 6606] (S2251325)  Sun 2021-01-17 00:09:21.000  lat: 49.87423  lon: 11.29671  alt: 24952.24   vH: 39.6  D: 126.2  vV: 4.7   T=-67.3C  RH2=2%
[ 6607] (S2251325)  Sun 2021-01-17 00:09:22.000  lat: 49.87402  lon: 11.29715  alt: 24956.91   vH: 39.7  D: 126.7  vV: 4.4   T=-67.3C  RH2=2%  (1)
[ 6608] (S2251325)  Sun 2021-01-17 00:09:23.000  lat: 49.87380  lon: 11.29760  alt: 24961.67   vH: 40.2  D: 126.5  vV: 5.5   T=-67.3C  RH2=2%  (2)
[ 6609] (S2251325)  Sun 2021-01-17 00:09:24.000  lat: 49.87359  lon: 11.29805  alt: 24967.33   vH: 40.6  D: 125.9  vV: 6.0   T=-67.4C  RH2=2%  (1)
[ 6610] (S2251325)  Sun 2021-01-17 00:09:25.000  lat: 49.87338  lon: 11.29850  alt: 24972.65   vH: 41.0  D: 125.7  vV: 4.8   T=-67.4C  RH2=2%
[ 6611] (S2251325)  Sun 2021-01-17 00:09:26.000  lat: 49.87316  lon: 11.29896  alt: 24977.96   vH: 41.2  D: 126.6  vV: 5.6   T=-67.4C  RH2=2%
[ 6612] (S2251325)  Sun 2021-01-17 00:09:27.000  lat: 49.87293  lon: 11.29942  alt: 24983.91   vH: 41.5  D: 128.3  vV: 6.2   T=-67.5C  RH2=2%  (2)
[ 6613] (S2251325)  Sun 2021-01-17 00:09:28.000  lat: 49.87270  lon: 11.29986  alt: 24989.32   vH: 41.6  D: 129.8  vV: 4.3   T=-67.5C  RH2=2% : RS41-SGP
[ 6614] (S2251325)  Sun 2021-01-17 00:09:29.000  lat: 49.87246  lon: 11.30031  alt: 24993.60   vH: 41.9  D: 130.6  vV: 4.2   T=-67.6C  RH2=2%
[ 6615] (S2251325)  Sun 2021-01-17 00:09:30.000  lat: 49.87221  lon: 11.30075  alt: 24998.52   vH: 42.2  D: 130.5  vV: 5.7   T=-67.6C  RH2=2%
[ 6616] (S2251325)  Sun 2021-01-17 00:09:31.000  lat: 49.87197  lon: 11.30120  alt: 25004.21   vH: 42.2  D: 129.2  vV: 5.9   T=-67.6C  RH2=2%  (1)
[ 6617] (S2251325)  Sun 2021-01-17 00:09:32.000  lat: 49.87173  lon: 11.30165  alt: 25009.52   vH: 42.2  D: 128.5  vV: 4.9   T=-67.6C  RH2=2%  (1)
[ 6618] (S2251325)  Sun 2021-01-17 00:09:33.000  lat: 49.87150  lon: 11.30211  alt: 25014.52   vH: 42.2  D: 128.1  vV: 5.2   T=-67.6C  RH2=2%  (2)
[ 6619] (S2251325)  Sun 2021-01-17 00:09:34.000  lat: 49.87127  lon: 11.30257  alt: 25020.25   vH: 42.1  D: 128.6  vV: 6.1   T=-67.6C  RH2=2%
[ 6620] (S2251325)  Sun 2021-01-17 00:09:35.000  lat: 49.87103  lon: 11.30302  alt: 25025.87   vH: 41.8  D: 129.3  vV: 4.9   T=-67.6C  RH2=2%
[ 6621] (S2251325)  Sun 2021-01-17 00:09:36.000  lat: 49.87079  lon: 11.30347  alt: 25030.83   vH: 41.3  D: 129.5  vV: 4.8   T=-67.6C  RH2=2%
[ 6622] (S2251325)  Sun 2021-01-17 00:09:37.000  lat: 49.87056  lon: 11.30391  alt: 25036.13   vH: 40.6  D: 128.9  vV: 5.6   T=-67.7C  P=22.15hPa  RH2=2%  (                                                 1)
[ 6623] (S2251325)  Sun 2021-01-17 00:09:38.000  lat: 49.87033  lon: 11.30435  alt: 25041.37   vH: 40.5  D: 128.1  vV: 4.9   T=-67.7C  P=22.12hPa  RH2=2%
[ 6624] (S2251325)  Sun 2021-01-17 00:09:39.000  lat: 49.87011  lon: 11.30479  alt: 25045.97   vH: 40.7  D: 127.7  vV: 4.5   T=-67.7C  P=22.11hPa  RH2=2%
[ 6625] (S2251325)  Sun 2021-01-17 00:09:40.000  lat: 49.86989  lon: 11.30524  alt: 25051.16   vH: 40.9  D: 128.1  vV: 6.3   T=-67.7C  P=22.08hPa  RH2=2%
[ 6626] (S2251325)  Sun 2021-01-17 00:09:41.000  lat: 49.86966  lon: 11.30568  alt: 25057.06   vH: 41.0  D: 129.4  vV: 5.8   T=-67.7C  P=22.06hPa  RH2=2%  (                                                 13)
[ 6627] (S2251325)  Sun 2021-01-17 00:09:42.000  lat: 49.86942  lon: 11.30612  alt: 25061.89   vH: 41.1  D: 130.4  vV: 3.9   T=-67.7C  P=22.05hPa  RH2=2%  (                                                 1)
[ 6628] (S2251325)  Sun 2021-01-17 00:09:43.000  lat: 49.86918  lon: 11.30655  alt: 25066.13   vH: 41.1  D: 130.8  vV: 4.5   T=-67.7C  P=22.02hPa  RH2=2%
[ 6629] (S2251325)  Sun 2021-01-17 00:09:44.000  lat: 49.86894  lon: 11.30698  alt: 25071.07   vH: 41.0  D: 130.4  vV: 5.5   T=-67.7C  P=22.01hPa  RH2=2%  (                                                 1)
[ 6630] (S2251325)  Sun 2021-01-17 00:09:45.000  lat: 49.86871  lon: 11.30741  alt: 25076.19   vH: 40.7  D: 129.7  vV: 4.8   T=-67.8C  P=21.99hPa  RH2=2%  (                                                 1)
[ 6631] (S2251325)  Sun 2021-01-17 00:09:46.000  lat: 49.86848  lon: 11.30784  alt: 25080.99   vH: 40.3  D: 129.4  vV: 4.9   T=-67.8C  P=21.97hPa  RH2=2% :                                                  fq 402300
[ 6632] (S2251325)  Sun 2021-01-17 00:09:47.000  lat: 49.86825  lon: 11.30827  alt: 25086.55   vH: 40.0  D: 130.0  vV: 6.1   T=-67.8C  P=21.95hPa  RH2=2% :                                                  fw 0x4ef7
[ 6633] (S2251325)  Sun 2021-01-17 00:09:48.000  lat: 49.86801  lon: 11.30869  alt: 25092.23   vH: 40.2  D: 131.2  vV: 5.3   T=-67.9C  P=21.92hPa  RH2=2% :                                                  BK 00
[ 6634] (S2251325)  Sun 2021-01-17 00:09:49.000  lat: 49.86778  lon: 11.30911  alt: 25097.23   vH: 40.1  D: 131.9  vV: 4.6   T=-67.9C  P=21.91hPa  RH2=2%
[ 6635] (S2251325)  Sun 2021-01-17 00:09:50.000  lat: 49.86754  lon: 11.30952  alt: 25102.08   vH: 40.2  D: 131.9  vV: 5.0   T=-67.9C  P=21.89hPa  RH2=2%  (                                                 4)
[ 6636] (S2251325)  Sun 2021-01-17 00:09:51.000  lat: 49.86730  lon: 11.30994  alt: 25107.31   vH: 40.2  D: 131.0  vV: 5.6   T=-67.8C  P=21.87hPa  RH2=2%  (                                                 3)
[ 6637] (S2251325)  Sun 2021-01-17 00:09:52.000  lat: 49.86706  lon: 11.31036  alt: 25112.45   vH: 40.3  D: 130.6  vV: 4.7   T=-67.8C  P=21.84hPa  RH2=2%  (                                                 1)
[ 6638] (S2251325)  Sun 2021-01-17 00:09:53.000  lat: 49.86683  lon: 11.31079  alt: 25117.36   vH: 40.6  D: 130.5  vV: 5.2   T=-67.8C  P=21.83hPa  RH2=2%
[ 6639] (S2251325)  Sun 2021-01-17 00:09:54.000  lat: 49.86659  lon: 11.31122  alt: 25123.03   vH: 41.0  D: 131.1  vV: 6.1   T=-67.8C  P=21.82hPa  RH2=2%  (                                                 1)
[ 6640] (S2251325)  Sun 2021-01-17 00:09:55.000  lat: 49.86634  lon: 11.31164  alt: 25128.63   vH: 41.2  D: 132.0  vV: 4.8   T=-67.8C  P=21.79hPa  RH2=2%  (                                                 1)
[ 6641] (S2251325)  Sun 2021-01-17 00:09:56.000  lat: 49.86610  lon: 11.31207  alt: 25133.47   vH: 40.9  D: 132.1  vV: 4.8   T=-67.8C  P=21.78hPa  RH2=2%  (                                                 4)
[ 6642] (S2251325)  Sun 2021-01-17 00:09:57.000  lat: 49.86586  lon: 11.31248  alt: 25138.75   vH: 40.3  D: 131.6  vV: 5.5   T=-67.8C  P=21.76hPa  RH2=2%
[ 6643] (S2251325)  Sun 2021-01-17 00:09:58.000  lat: 49.86562  lon: 11.31290  alt: 25144.01   vH: 39.6  D: 130.5  vV: 5.3   T=-67.8C  P=21.74hPa  RH2=2%
[ 6644] (S2251325)  Sun 2021-01-17 00:09:59.000  lat: 49.86539  lon: 11.31332  alt: 25148.82   vH: 39.2  D: 129.8  vV: 4.5   T=-67.8C  P=21.73hPa  RH2=2%  (2)
[ 6645] (S2251325)  Sun 2021-01-17 00:10:00.000  lat: 49.86517  lon: 11.31373  alt: 25153.82   vH: 39.0  D: 130.4  vV: 5.8   T=-67.8C  P=21.71hPa  RH2=2%
[ 6646] (S2251325)  Sun 2021-01-17 00:10:01.000  lat: 49.86494  lon: 11.31414  alt: 25159.68   vH: 39.0  D: 131.6  vV: 6.2   T=-67.8C  P=21.69hPa  RH2=2%
[ 6647] (S2251325)  Sun 2021-01-17 00:10:02.000  lat: 49.86471  lon: 11.31454  alt: 25165.01   vH: 38.8  D: 132.8  vV: 4.7   T=-67.8C  P=21.67hPa  RH2=2%
[ 6648] (S2251325)  Sun 2021-01-17 00:10:03.000  lat: 49.86447  lon: 11.31493  alt: 25169.73   vH: 38.4  D: 133.3  vV: 4.4   T=-67.8C  P=21.66hPa  RH2=2%
[ 6649] (S2251325)  Sun 2021-01-17 00:10:04.000  lat: 49.86424  lon: 11.31532  alt: 25174.63   vH: 37.8  D: 132.9  vV: 5.3   T=-67.8C  P=21.64hPa  RH2=2%
[ 6650] (S2251325)  Sun 2021-01-17 00:10:05.000  lat: 49.86401  lon: 11.31570  alt: 25179.89   vH: 37.3  D: 132.1  vV: 5.4   T=-67.8C  P=21.61hPa  RH2=2%
[ 6651] (S2251325)  Sun 2021-01-17 00:10:06.000  lat: 49.86379  lon: 11.31609  alt: 25184.85   vH: 37.3  D: 131.4  vV: 4.8   T=-67.8C  P=21.59hPa  RH2=2%
[ 6652] (S2251325)  Sun 2021-01-17 00:10:07.000  lat: 49.86357  lon: 11.31648  alt: 25190.10   vH: 37.6  D: 131.5  vV: 5.9   T=-67.8C  P=21.59hPa  RH2=2%  (1)
[ 6653] (S2251325)  Sun 2021-01-17 00:10:08.000  lat: 49.86334  lon: 11.31687  alt: 25195.89   vH: 38.3  D: 132.1  vV: 5.9   T=-67.9C  P=21.55hPa  RH2=2%  (3)
[ 6654] (S2251325)  Sun 2021-01-17 00:10:09.000  lat: 49.86311  lon: 11.31727  alt: 25201.01   vH: 38.8  D: 132.2  vV: 4.5   T=-67.9C  P=21.55hPa  RH2=2%

So now I even get temperature, pressure and humidity. Receive quality is better now, but why is there now temperature , pressure and humidity in addition? Just by coincidence that RS41 is transmitting this data now? Or just because of the better signal quality?

JsBergbau commented 3 years ago

On https://radiosondy.info/sonde.php?sondenumber=S2251325 there is current RH reported about 5 % whereas here is always 2% shown. Am I missing some parameters?

EDIT: Just quitted and restarted rs41mod and now Temperature and humidity data is gone. However it can't be cached data displayed because Temperature and pressure changed often between frames.

rs1729 commented 3 years ago

PTU data is only displayed when the required coefficients are received. They are distributed over 51 frames, you can see the config/calib data with option -vv. Temperature and pressure don't have as many coefficients as humidity, the temperature coefficients come just after the frequency information. Hence it can take a minute or sometimes more (if some frames are missing), until the PTU data is calculated.

Above about 12-14km when the radiosonde reaches the stratosphere, the rel. humidity usually drops down to 0-1%. At the poles this region is lower at approx. 8km altitude, near the equator rather 17km. Though the rel. humidity is difficult to measure, sometimes the sensors show higher values, errors up to 5% RH are not unusual. And it is not that easy to calculate the rel. humidity from the data, the sensors are temperature dependent and are not simple to model.

Example T_RH_alt

You can compare the data here http://weather.uwyo.edu/upperair/bufrraob.shtml See also https://github.com/rs1729/RS/issues/15#issuecomment-723487991

rs1729 commented 3 years ago

Viewing spectrum in SDR Sharp via rtl_tcp shows about 16 dB SNR. Tried a few minutes later with about 18 dB SNR, which changes quite fast

It looks like in sdr# the calculated dB is not only depending on the chosen filter bandwidth but also settings like sample rate or if you zoom in. So maybe it is the displayed sample rate or FFT length. With sample rates above 1MHz (and directly from the 8bit rtl-sdr) and zoom=0 it shows me 10-12 dB for a signal that would show about 20-22 dB when I play the 48kHz 16bit IQ recording, what seems a bit high. (Even if you could benefit from decimation a little, the signal is definitely not 10dB better.) Below that there are more and more decoding errors. So with 16-18dB in sdr# for rtl_fm 48k you have probably a rather weak signal.