Re: [Discuss-gnuradio] Problem converting RTL 8 bit IQ samples to gnuradio native format.

[Nick Foster]

On 12/19/2012 12:02 PM, Salvatore Sanfilippo wrote:
> On Wed, Dec 19, 2012 at 8:48 PM, Nick Foster <address@hidden> wrote:
> > I believe Salvatore's message was hung up in the listserv as I've answered
> > him already on Github. ;)
> >
> > For posterity, modes_rx with the -F option is looking for the usual Gnuradio
> > complex sample format -- 32-bit floating point I,Q,I,Q....
> Hello Nick,
>
> actually I posted here because after your reply on Github I'm now sure
> that the problemi is not about modes_rx -F, but the fact I do the
> conversion in the wrong way or something like that, as I believe I'm
> outputting 32 bit floating point I,Q,I,Q samples.
> So the Github message was to check if the format was what I believed
> it was (and it is).
> This message was a request for help, to understand what I'm doing wrong.
>
> The code I posted is a trivial transposition of the Gnuradio block I
> see everywhere on the internet to convert from RTL raw IQ samples to
> Gnuradio format, but maybe writing the C code I did some mistake.
Sorry about that, I didn't read closely enough.

Does rtl_sdr really output 4.0Msps? I thought it was confined to 2.8Msps 
or 3.2Msps. If it's operating at one or the other of those two native 
sample rates, you will have to resample to 4Msps for use with modes_rx, 
or modify modes_rx so it resamples when using the file sink, similar to 
how it resamples for rtl_sdr sources now.

If this is a common use case it might be worthwhile to make modes_rx 
automatically resample to an integer number of samples per symbol for 
any file source sample rate, or to make the guts operate on a 
non-integer-number of samples per symbol; the latter would be quite a 
bit of work.

--n

> Regards,
> Salvatore
>
> > --n
> >
> >
> > On 12/19/2012 11:42 AM, Salvatore Sanfilippo wrote:
> > > Hello, I'm using modes_rx in order to decode ADS-B messages.
> > > Usually I use it with an RTL-based SDR simply as:
> > >
> > > modes_rx -d
> > >
> > > In this way it works perfectly.
> > >
> > > However I need to record raw IQ samples using rtl_sdr and later
> > > process them with modes_rx -F option (that reads data from file, in
> > > the gnuradio format).
> > >
> > > So I tried the following:
> > >
> > > rtl_sdr -f 1090000000 -s 4000000 /tmp/modes.bin
> > >
> > > And alter I converted modes.bin into modes.cfile using the following C
> > > program:
> > >
> > > int main(void) {
> > >       unsigned char in[2];
> > >       float out[2];
> > >
> > >       while(fread(in,1,2,stdin) == 2) {
> > >           out[0] = in[0];
> > >           out[1] = in[1];
> > >           out[0] = (out[0] - 127)*(1.0/128);
> > >           out[1] = (out[1] - 127)*(1.0/128);
> > >           fwrite(out,sizeof(float),2,stdout);
> > >       }
> > >       return 0;
> > > }
> > >
> > > I'm using a little endian computer so in theory I'm converting the 8
> > > bit I,Q samples into 4 byte floats I,Q samples in little endian
> > > format, that should be the native format of Gnuradio from what I read
> > > on the internet.
> > >
> > > Unfortunately after this conversion modes_rx -F does not work as
> > > expected, and instead outputs garbage.
> > >
> > > I guess I'm doing something wrong in the recording or in the
> > > conversion step, but I can't find what the problem is. If you can
> > > point me to the right direction I'll appreciate your help.
> > >
> > > Regards,
> > > Salvatore
> > >
> > > --
> > > Salvatore 'antirez' Sanfilippo
> > > open source developer - VMware
> > > http://invece.org
> > >
> > > Beauty is more important in computing than anywhere else in technology
> > > because software is so complicated. Beauty is the ultimate defence
> > > against complexity.
> > >          — David Gelernter
> > >
> > > _______________________________________________
> > > Discuss-gnuradio mailing list
> > > address@hidden
> > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> >
> > _______________________________________________
> > Discuss-gnuradio mailing list
> > address@hidden
> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
>
> --
> Salvatore 'antirez' Sanfilippo
> open source developer - VMware
> http://invece.org
>
> Beauty is more important in computing than anywhere else in technology
> because software is so complicated. Beauty is the ultimate defence
> against complexity.
>         — David Gelernter