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Re: [Discuss-gnuradio] Problem converting RTL 8 bit IQ samples to gnurad

From: Nick Foster
Subject: Re: [Discuss-gnuradio] Problem converting RTL 8 bit IQ samples to gnuradio native format.
Date: Wed, 19 Dec 2012 11:48:17 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0

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....


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);
     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.


Salvatore 'antirez' Sanfilippo
open source developer - VMware

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

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