[FFmpeg-devel] [RFC] SDR

Tomas Härdin git at haerdin.se
Sat Jun 24 13:12:50 EEST 2023

fre 2023-06-23 klockan 23:36 +0200 skrev Michael Niedermayer:
> On Fri, Jun 23, 2023 at 10:19:37PM +0200, Tomas Härdin wrote:
> > fre 2023-06-23 klockan 22:18 +0200 skrev Tomas Härdin:
> > > tor 2023-06-22 klockan 16:57 +0200 skrev Michael Niedermayer:
> > > > If we support just audio and video (de)modulation it fits
> > > > nicely in
> > > > FFmpeg
> > > 
> > > You have no idea what you are getting into. Are we going to
> > > implement
> > > a
> > > DVB-T demuxer in this project? For what purpose? You can just as
> > > well
> > > implement a modem externally that pipes bits into the mpegts
> > > demuxer.
> > 
> > Herp, I obviously mean a DVB-T *modem*
> as said previously i did a AM demodulator, one classical
> implementation
> and one that demodulates it synchronously mapping in phase and
> quadrature
> to mid and side channels or just mono
> The idea of the mid and side channels is that if theres 1 transmitter
> its
> mono, if there are 2 transmitters they will appear to come from
> different
> directions as one listens

There are multiple (quasi-)standards for stereo AM. Then there are
things like DRM.

> And i plan to do FM demodulation.

Drop in a WFM demod box in GNU Radio Companion -> done. Or use fmtools.

> after that i likely have many other non SDR things i want to work on
> and would very much be happy to then find someone else working on
> but if noone does and i find time i might look at DAB because the
> existing DAB
> software thats on my ubuntu box works very poorly with the hw i had
> in fact it even has difficulty finding the frequency of the DAB
> signal
> alot of the time. (which is ridiculous)

There are undoubtedly better DVB implementations around because it is
used in DATV QSOs with the ISS. Indeed this is why you want to have a
separate program just for demodulation so that you get a bitstream that
can then be piped to various other programs. A good DVB demod would
output an MPEG-TS stream that can then be consumed by various programs.

ATV de/mod is another thing that technically could be done in FFmpeg
but shouldn't.

This stuff is also already a part of V4L though I haven't looked into
it particularly deep. Also for comparison, in the FreeDV project we're
looking at writing a TUN/TAP device for packet radio over VHF using
cheap hardware.


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