[Hackrf-dev] FM Modulating external MPX Signal

[Karl Koscher]

GNU Radio supports JACK as an audio source and sink. You can set the
default audio interface with a config file (see
http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/master/entry/gr-audio/lib/gr-audio.conf
for an example).

You probably don't want to use the WBFM block since it will apply
preemphasis, which stereotool should already do, and may also apply a low
pass filter, which would effectively destroy the encoding that stereotool
does.

Instead, you should just use the standard FM Modulation block. It will ask
for a "sensitivity" parameter, but looking at the WBFM block source, it
seems like you should just set it to 2 * math.pi * max_dev / quad_rate,
where max_dev is 75 kHz and quad_rate is the output symbol rate. Note that
the output symbol rate of the block is the same as the input symbol rate,
so you'll probably want a rational resampler before the FM modulator to
take the signal from 192 kHz to whatever sample rate you feed in to the
HackRF (you'll probably want to use something like 2112 ksps, which is the
smallest integer multiple of 192 ksps that's above the 2 msps minimum of
the HackRF... 1920 ksps may also work?)

On Fri, Sep 4, 2015 at 10:17 AM, Mitja kocjančič <veso266 at gmail.com> wrote:

> Hi anyone know How I can FM Modulate External MPX SIgnal?
>
> I am using MPY Signal Generator called Stereo Tool:
> http://www.stereotool.com/
>
> the program uses two Jack Audio Cables (left and right) and it outputs
> 192khz MPX Generated Audio over it. and there where problem starts I know
> that I need 288k sample rate and WBFM Block needs 288k Quadrature rate and
> 192 Audio rate but I don't know how to link GNU Radoi with jack
> Is there any JackAudio to Linux standart Pipe sender or anything like that
> So I can feed MPX Data from Stereo Tool to it?
>
> PS: How would GNU Radio flowgraph look like (I would probably need some
> Rational Resampler or something like that.
>
> I know there is gr-rds Block Package but I would like to use Stereo Tool
> for RDS because It has even RT+ and it can read RT details from external
> file
>
>
>
> Thanks for Anwsering and Best Regards
>
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>
>
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