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Re: [Discuss-gnuradio] working with WFM sample data

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] working with WFM sample data
Date: Wed, 15 Feb 2012 14:12:36 -0500
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On 02/15/2012 02:05 PM, emilio gonzalez wrote:
hi Marcus,

thanks for this tip, it pushed me the the right direction. i guess this capture file isn't the standard "complex" datatype, and a conversion was the answer. i found the answer on the gnuradio.org Octave page that said the magic word, short. i use this graph to render on an FFT scope:

[ File Source (short) ]
  -> [ Throttle (complex, 25M) ]
    -> [ IShort to Complex ]
      -> [ FFT Scope (complex, rate 25M) ]

using IShort to Complex was just a result of experimentation, as Short to Complex seemed to output only half the captured band. now i get to delve in to the world of filters, oof!

i'm surprised this isn't listed on the gnuradio.org Sample Data page ( http://gnuradio.org/redmine/projects/gnuradio/wiki/SampleData ), as i've come across messages from other folks confused about the data types and semantics of these USRP capture files. i suggest an addition to that page that mentions the necessity of an IShort conversion.

You can capture *any* of the native sample formats in Gnu Radio, so it's not the case that disk-resident samples are necessarily recorded in "complex short" format. It depends on what the program that recorded them asked for, and you have to understand what the
  program that recorded them intended.

Gnu Radio doesn't use file headers or any other such technique--the file sinks records raw samples in whatever format they're presented

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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