Added a frequency error calibration mode to rtl_fm

[sm0svx]

The rtl_fm utility may now be used to calibrate the frequency error of a
DVB-T dongle using the "-c" command line switch:

  [-c] Do frequency error calibration
      Frequency error calibration will only work for FM.
      Use a higher sample rate, like -s170k, to handle
      large frequency errors. A strong and noise free
      signal is needed for good results. For example, use a
      broadcast FM station for calibration. Let the
      calibration run for some minutes until the values
      have stabilized. Rerun with -p option using the
      suggested ppm_error. Try this on a couple of stations.
      Repeat until the error is stable at about zero.
      Example: rtl_fm -f99.3M -s170k -r48k -c - | aplay -r48k -fS16_LE -

[RSully]

Copying a comment I just left on #10:

[...] like I commented on pull-request #7, it looks like this project doesn't use pull-requests for contributions :(.

[sm0svx]

Yes, I noticed that after creating this pull request so I have also sent my patches to the osmocom-sdr mailing list.

[Romeo-Golf]

Did anything come from the mailing list submission ?

[sm0svx]

I don't remember how this panned out in the end but you can find my modifications in my fork (https://github.com/sm0svx/librtlsdr) in the sm0svx branch.