Able to receive FM but not UHF

[Mike Pate]

I've got a head scratcher...

I can run this command and am able to hear clear audio:

rtl_fm -f 106.9M -M wbfm -s 200000 -r 48000 | aplay -r 48k -f S16_LE

But when I run this command I only get noise:

rtl_fm -M fm -f 453.125M -s 12k -g 50 -l 0 -F 0 | aplay -r 12k

I've tried to figure this out for hours without success... anyone know what needs to be done to resolve my issue?

Regards,

Mike

[Gaston Picard]

If you look closely, you are not really tuning 453.125MHz...

gaston@gaston-MM061:~$ rtl_fm -M fm -f 453125k -s 12k -g 50 -l 0 -F 0 | aplay -r 12k
Found 1 device(s):
  0:  Generic, RTL2832U, SN: 77771111153705700

Using device 0: Generic RTL2832U
Found Rafael Micro R820T tuner
Tuner gain set to 49.60 dB.
Tuned to 453509000 Hz.
Oversampling input by: 128x.
Oversampling output by: 1x.
Buffer size: 5.33ms
Sampling at 1536000 S/s.
Output at 12000 Hz.
Playing raw data 'stdin' : Unsigned 8 bit, Rate 12000 Hz, Mono

I have tried different settings and I can't get it to tune to 453.125 MHz.

Gaston

[Adam Nielsen]

> Tuned to 453509000 Hz.

>
> I have tried different settings and I can't get it to tune to 453.125
> MHz.

I believe rtl_fm will never tune to exactly the frequency you specify,
because the RTL dongles tend to have a big DC spike at the tuned
frequency. If they were tuned exactly then the signal would likely be
swamped and thus too weak to decode. So rtl_fm tunes slightly higher or
lower than the desired frequency, then decodes at the corresponding
offset.

The frequency you specify with -f is where the decoding will take
place, even though the dongle doesn't tune to exactly that frequency
(but it will always be close.)

Cheers,
Adam.

[Mike Pate]

Adam,

My understanding of how the decoding occurs was what you stated but appreciate the confirmation.  Do you have any idea why I'm not able to hear any audio on that frequency?  Do I have both commands correctly defined?

Mike

[Adam Nielsen]

> My understanding of how the decoding occurs was what you stated but
> appreciate the confirmation. Do you have any idea why I'm not able
> to hear any audio on that frequency? Do I have both commands
> correctly defined?

I'm a little confused - in your first post you said you got audio but
it was only noise, but do you mean you aren't getting any audio at all?

If you're only getting noise then the most obvious issue is there is no
signal making it to the dongle, but if you're not getting any audio at
all then it could be a command line problem.

I would suggest using -r on both rtl_fm and aplay to ensure the audio
sample rate is at 48kHz. My sound card only supports this sample rate
so I either get no audio at other sample rates or ALSA upsamples it to
48kHz anyway.

Other than that maybe you can try some other frequencies to work out
what signals are and aren't making it to the dongle. Maybe also leave
the AGC on instead of overriding it and setting it to 50.

Cheers,
Adam.

[Mike Pate]

After re-reading my previous post, it was as clear as mud :)

What I should have said was on 106.9M, I am able to hear clearly the local FM radio station.  When I run either of the following commands, I only hear static even though I have a HT tuned to the same freq and can hear my local fire/EMS clearly:

rtl_fm -M fm -f 453.125M -s 48k  -F 0 | aplay -r 48k

rtl_fm -M fm -f 146.76M -s 48k | aplay -r 48k

[Adam Nielsen]

> What I should have said was on 106.9M, I am able to hear clearly the
> local FM radio station. When I run either of the following commands,
> I only hear static even though I have a HT tuned to the same freq and
> can hear my local fire/EMS clearly:

It's difficult to say - if you're hearing static then everything is
working fine, and it all points to the signal being too weak (perhaps
caused by a problem with the RTL device, or a nearby station being too
strong.)

You'll probably have to fire up gqrx or equivalent to see a waterfall
plot of the whole tuned spectrum, otherwise you'll be stumbling around
in the dark, unable to see any other signals in the tuned spectrum that
could be causing problems.

Cheers,
Adam.

[jdow]

He's attempting narrow FM. I don't see a ppm correction on his command. And the
odds are very high that he will need it.

{o.o} Joanne

[Mike Pate]

Joanne,

I'm not familiar with ppm nor have I seen it used by others in their setup.  Can you advise how I should use it?

Mike

[David J Taylor]

Joanne,

I'm not familiar with ppm nor have I seen it used by others in their setup.
Can you advise how I should use it?

Mike

==================================

Mike,

"ppm" is parts per million, the amount by which your dongle is in error when
setting the frequency. Typical values can range up to 100 ppm, i.e. 100 Hz
at 1 MHz, 1 kHz at 10 MHz, or 10 kHz at 100 MHz. So if you are using a
narrow-band mode (e.g. AM, NBFM, AIS) you need to calibrate out the error in
your dongle's oscillator.

For the RTL DVB-T dongle, you can use the "kalibrate" program to measure,
see:

http://rtl-sdr.sceners.org/?p=193

Linux source: https://github.com/steve-m/kalibrate-rtl

Windows : http://rtlsdr.org/files/kalibrate-win-release.zip

and then apply the correction to the software you are using.

Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-...@blueyonder.co.uk

[jdow]

First you need to figure out the clock error on your dongle. Being a PC-oid
(Linux is for servers not desktops) I use SDRSharp and estimate calibration
using NOAA transmitters (2ppm). Other tools exist that calibrate against GSM
signals or the like.

Given that number plug it in. I have dongles as low as +9 ppm to +80 ppm. At 450
MHz 1ppm is 450 Hz. 10 ppm is 4.5 kHz. 20 ppm is outside the NFM bandwidth. So
you do need to figure out the error and apply -p 40 or whatever your error is to
your command line.

{^_^} Joanne

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[David J Taylor]

First you need to figure out the clock error on your dongle. Being a PC-oid
(Linux is for servers not desktops) I use SDRSharp and estimate calibration
using NOAA transmitters (2ppm). Other tools exist that calibrate against GSM
signals or the like.

[]
{^_^} Joanne
=======================================

Joanne,

There is a Windows version of Kalibrate available, which works well:

http://rtlsdr.org/files/kalibrate-win-release.zip

It takes the guesswork out of using SDR# or whatever.

73,
David GM8ARV

[Mike Pate]

I very much appreciate everyone's reply... I have made good progress using 'kal'.  Based on its results, my ppm = 18.686.  I then ran these commands:

rtl_fm -M fm -f 144.39M -s 48k -p19 -l 96 | aplay -r 48k

rtl_fm -M fm -f 453.125M -s 48k -p 19 -l 89 | aplay -r 48k

My next issue on the first list, I am able to receive audio but I have to set the squelch to 96 before I eliminate the static when there is no audio.  On the 2nd line, I have the set the squelch to 89 to eliminate the static.  These two values seem awful high to me or am I concerned about nothing?

Mike

On Friday, April 24, 2015 at 10:25:43 PM UTC-5, Mike Pate wrote:

[Adam Nielsen]

> I have to set the squelch to 96 before I eliminate the static when
> there is no audio. On the 2nd line, I have the set the squelch to 89
> to eliminate the static. These two values seem awful high to me or
> am I concerned about nothing?

This might be related to AGC. Probably as soon as the transmission
stops, the gain increases to maximum, requiring the squelch values to
be quite high. Now you have an audible signal you could experiment with
manual gain values which may improve the situation.

Cheers,
Adam.