Bug #6032


Poor APRS demodulation using RTL_FM on RaspberryPI or on old PC

Added by IU1IPB 16 days ago.

Target version:
Start date:
Due date:
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Spec Reference:



I'm finding unexpected issues in decoding APRS data streams using RaspberryPI (6.1.21+ #1642 Mo armv6l) or not young PC (5.10.0-20-rt-amd64 #Debian 5.10.158-2 (2022-12-13) x86_64).
Generally speaking, RTL_FM is able to decode strong signals (e.g. broadcast WBFM on GQRX) both on PI and on PC, but fails with quite strong APRS signals (no decoding using both Direwolf, Multimon or GQRX). Decoding Narrow band FM happens correctly, at least with strong signals.
RTL-SDR is installed from Debian repository, latest version.
The same hardware (same dongle and antenna) allows easily APRS decoding using a different Debian PC with the same setup (RTL-SDR same version, Direwolf or GQRX), than it's should not be a problem of RF side.
Tying to monitor the channel using waterfall, I can still see APRS signals decoded by the Raspberry, nut apparently received very weekly.
Here below the details.

Do someone has any suggestion?
Best Regards

Reception command :

rtl_fm -M fm -f 144.8M -p 25 -s 24000 -g 42 - | direwolf -c /home/ugo/rtl-sdr/direwolf.conf -r 24000 -D 1 -B 1200 -l /home/ugo/rtl-sdr

p 25


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

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
[R82XX] PLL not locked!
Sampling at 2048000 S/s.
No E4000 tuner found, aborting.

PC Capabilities

product: Celeron(R) Dual-Core CPU T3000 @ 1.80GHz
vendor: Intel Corp.
physical id: 1
bus info: cpu@0
width: 64 bits
capabilities: fpu fpu_exception wp vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ht tm pbe sy scall nx x86-64 constant_tsc arch_perfmon pebs bts rep_good nopl cpuid aperfmper f pni dtes64 monitor ds_cpl tm2 ssse3 cx16 xtpr pdcm xsave lahf_lm pti dtherm

Raspberry PI capabilities :

description: CPU
product: cpu
physical id: 0
bus info: cpu@0
size: 700MHz
capacity: 700MHz
capabilities: half thumb fastmult vfp edsp java tls cpufreq

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