Clipped audio recording

[Jem Church]

Hi all,

I've been using GQRX for a long time now, on various laptops. I've just installed it on one I've just repaired the screen on ( it all works perfectly now ) and there's some really odd audio recording behaviour. For example, when recording a NOAA pass, the recording was maxed out and is completely clipped.

With Narrow FM, APT (25KHz) Tau Off and no antenna, the audio waveform bottom right displays around -30dB ( as I remember it used to ) but the recording show by Audacity is severely clipped at both the 0dB end and at the -ve end. I've not seen anything like this before. I've just tried the same on my main machine and the same happens.

A recording of a local FM station is perfect, as expected.

Main machine is MINT17.1 and GQRX 2.6.1. trusty. New lappy is MINT 18 and 2.6.1 xenial.

I'll try to dig out the lappy I used a couple of years ago for this and try the same again.

In the meantime does anyone have any ideas ? Not knowing how the demodulated audio is recorded ( I have a look at pavucontrol and alsamixer and all I could find was inputs set to the lowest level possible.

Cheers,

Jem.

[Jem Church]

Hi again,

The old lappy fired up and had no clipping under the same conditions. There were occasional peaks at 0dB but definitely no clipping.

It has version 2.2.0.74.d97bd7-2 (?) on it .

Jem.

[James Clark - KC3CDV]

How do you know its GQRX and not the new computers soundcard, sounds to me like the gain on the new computer is to hot.

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[Jem Church]

Hi James,

It does show the audio level is too hot, which is why I a) tried to look for recording level settings in alsamixer and pavucontrol ( all were set to minimum ) and then tried it on my big PC, which is completely different hardware and does exactly the same.

I don't remember having to ever set a recording level manually before. I don't know if it's possible, or if GQRX has an 'auto record level' type thing built in.

FM radio ( broadcast band WFM ) recording is fine.

Does anyone know how / where the recording level is set during a GQRX recording on this version ( 2.6.1 ) of the software ?

Can anyone replicate the same problem with the settings set to what I used above ?

Thanks,

Jem.

[Jem Church]

Hi all,

I didn't say what the receiver is ... DOH ! I'm using a cheapy DVB-T RTL2832U stick. I bought two specifically for this about four years ago, and they're the ones I've been using ever since.

Jem.

[Alexander Fasching]

alsamixer or pavucontrol don't influence the recording level. This is
all done in the GNU Radio flowgraph. At the moment, it is not possible
for the user to change the recording level.

This issue is also briefly mentioned here:
https://github.com/csete/gqrx/issues/459

Maybe someone will find some time to fix it.

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[Jem Church]

Hello Alexander f.,

I've literally just found the https://github.com/csete/gqrx/issues/459 and come back here to post the link.

It seems that it's a Tau bug thang .....

I might try 2.5 as it seems to be introduced in 2.6

Thanks,

Jem.

[Alexander Fasching]

I see the same issue with v2.5

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[Alexandru Csete]

Hi,

What you experience is a characteristic of the FM demodulator type
used in gqrx. When you only have noise on the channel it will
overload; however, the audio level should come at least 3 dB below
full scale when there is a strong enough FM signal. It is more or less
the same on analog radios, that's why we have squelch.

I could reduce the audio level artificially but that would also
decrease the audio level when there is a signal.

In any case, the level should be below full scale when you have a
signal -- unless your channel filter width is too narrow compared to
the signal width. For APT you should use filter around 50 kHz.

Alex

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[James Clark]

Hello, yes there is an automatic game controller in GQ RX, also whatever system you're on go look on her control panel if you're on windows and see if you can lower the volume. If you're on Mac you want to go to system preferences and lower the recording volume output.

Sent from my iPhone

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[Alexandru Csete]

Hi James,

The automatic gain controller does not have any effect in FM mode. It
only works with AM and SSB.

Alex

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[James Clark - KC3CDV]

Oh, I don’t mess with it much, I was just trying to figure why the laptop would respond like that, I still think its a soundcard being to hot.

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[Jem Church]

Hi Alexandru !

Your GQRX software is awesome ! It has opened so many windows ( not the Micro$oft ones ... ) for me. Thank you ! And GPredict is just a joy to use too.

Thanks for your information and answer. When I read https://github.com/csete/gqrx/issues/459 I thought WOW that's my problem too !

I'll try the suggestions of non-zero de-emphasis and / or WFM recording, but I ( very ) respectfully reiterate that the behaviour is different than the last time I got NOAA pictures. It just worked last time, and for lots of passes, but that was two or three years ago.

Thanks,

Jem

[Alexandru Csete]

Hi Jem,

I don't question that things have changed and work differently but
it's quite impossible to simply revert when the point of reference is
so long ago. If you look at the end of issue 459 you will see that
there was actually a bug in 2.6 that is fixed now in git. In another
issue I have posted an APT image captured using gqrx, see at the end
of issue 500
https://github.com/csete/gqrx/issues/500

Feel free to add your experiences to issue 459. All info is useful.

Alex

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[Jem Church]

Hi Alex,

Will do and thank you for the info.

After a couple of years away from sdr, it's again a strong interest. So much has happened in the last three years. Meteor M2 is my next target ...

Thanks again,

Jem.