CTCSS and DCS codes

[David Chatterton]

Gqrx is excellent in providing RDS data for FM broadcast reception.

I wonder whether or not it is possible to include CTCSS and DCS data (codes) for NFM channels. Many transceivers provide the capability to open the squelch based on receipt of the correct code. Many scanners will display the accompanying codes when receiving such signals.

This capability will contribute to Gqrx becoming a truly universal communications receiver.

[Alexandru Csete]

On Wed, Apr 13, 2016 at 1:59 PM, David Chatterton
<dmchat...@gmail.com> wrote:
> Gqrx is excellent in providing RDS data for FM broadcast reception.
>
> I wonder whether or not it is possible to include CTCSS and DCS data (codes)
> for NFM channels.

Dave,

Anything is possible, the real question is whether there is somebody
who has the time and interest to work on it. I work on other projects
at the moment so it's not going to be me for a while.

I would strongly recommend adding such feature requests to the issue
tracker on github:
https://github.com/csete/gqrx/issues

There it can serve as input for both future gqrx development and for
other projects. If it only exists on this mailing list, it will
probably be forgotten in time.

Alex

[Brad Ramsey]

Alexandru,

I have integrated a filter into gqrx to reject the CTCSS tones while using the NFM demod.  There's a checkbox in the option dialog to enable or disable it.  This solves half of the CTCSS problem: removing the hum from the audio.  I'm planning to extend the squelch functionality to include CTCSS decode after this.

How do I contribute my changes?

Thanks,

Brad

[Alexandru Csete]

On Sat, Apr 16, 2016 at 11:46 PM, Brad Ramsey <bradley...@gmail.com> wrote:
> Alexandru,
>
> I have integrated a filter into gqrx to reject the CTCSS tones while using
> the NFM demod. There's a checkbox in the option dialog to enable or disable
> it. This solves half of the CTCSS problem: removing the hum from the audio.
> I'm planning to extend the squelch functionality to include CTCSS decode
> after this.

Hi Brad,

That's nice, but I think this is a different topic and not related to squelch.

> How do I contribute my changes?

You can submit a pull request on github and I will take a look at it
whenever I can. It may take several weeks or even months before I can
attend to it as I am very busy.

It would be nice if it not only contained a high-pass filter for
eliminating low frequency tines, but also option to select low pass or
band pass filters.

Alex

[Brad Ramsey]

You're not technically wrong, of course, but I am making mods to address a specific feature, based on two issues:

1. Repeaters typically employ a low-frequency tone to discriminate from non-intentional use.  These tones

   are noticeable on the audio unless they are filtered out.  Most receivers include a high pass filter to eliminate

   the tones.  Gqrx does not.  My mod add this feature.

2. Most receivers also use the tones to gate the squelch so that audio is passed only when the pre-programmed tone

    is present on the audio. I plan to add a new control to select a gating tone frequency and the ability to display the tone on the currently-tuned audio.

    This plan is directly and uniquely related to squelch.

I don't know of any other reasonable use-case for a post-detection audio filter.  But that may be do to my own ignorance.  If you really do have a use-case for a band-pass or (really?) low-pass filter

then I'm willing to add them to the implementation.  My changes are general,  they can accommodate any filter that you wish.

My sincere wish is to work with you, and I completely understand that you may have higher priorities.

Brad

[Alexandru Csete]

On Sun, Apr 17, 2016 at 10:41 AM, Brad Ramsey <bradley...@gmail.com> wrote:
> You're not technically wrong, of course, but I am making mods to address a
> specific feature, based on two issues:
>
> 1. Repeaters typically employ a low-frequency tone to discriminate from
> non-intentional use. These tones are noticeable on the audio unless they are filtered out. Most receivers
> include a high pass filter to eliminate the tones. Gqrx does not. My mod add this feature.
>
> 2. Most receivers also use the tones to gate the squelch so that audio is
> passed only when the pre-programmed tone is present on the audio. I plan to add a new control to select a gating
> tone frequency and the ability to display the tone on the currently-tuned
> audio. This plan is directly and uniquely related to squelch.

I am fully aware of this, all I meant was that these two items are in
my opinion different topics / tasks in the sense that one does not
depend on the other.

> I don't know of any other reasonable use-case for a post-detection audio
> filter. But that may be do to my own ignorance. If you really do have a
> use-case for a band-pass or (really?) low-pass filter then I'm willing to add
> them to the implementation. My changes are general, they can accommodate
> any filter that you wish.

In SDR it is often necessary with a post-demod low pass filter to
remove high frequency noise. The audio sample rate in gqrx in 48 kHz
but the voice bandwidth is usually only 3 kHz. In SSB the lack of
audio filter is not a problem because signal is already band limited
by the channel filter; however, the output from FM demodulator will be
the full audio spectrum, in this case 24 kHz.

In fact, the only case when you don't want a low pass filter following
an FM demodulator is if you are receiving data or if the audio sample
rate is already low, e.g 8 kHz.

But this was just an idea and you don't have to worry about it.

Alex

[budg...@gmail.com]

Hi Brad,

Im interested in your CTCSS filter patch could you send it to me by any chance.

Thanks Anthony 

[Ira Fuchs]

Does anyone have a working CTCSS or DCS squelch? If not for gqrx, what about SDR#? I presume it could be built using GNU radio but I have no experience with GR.

[Adrian Musceac]

CTCSS yes, DCS no. GNU radio already has a good CTCSS filter. It's
trivial to add the GNU radio block, however the UI widget is more
problematic (there's not much space left as it is).

Regards,
Adrian

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