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Re: [Discuss-gnuradio] The format of I/Q sample

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] The format of I/Q sample
Date: Mon, 15 Aug 2011 06:30:44 -0400

On Mon, Aug 15, 2011 at 12:48 AM, xi yang <address@hidden> wrote:
> Hi, all
> I want to use fpga to do FFT before the data are transmitted to usb.
> In FFT module, I used float point numbers to be its input. According to the
> IEEE Floating-Point Standard, a float point number is represented by 32 bits
> (bits 0 through 22 form the mantissa, bits 23 through 30 form the exponent,
> and bit 31 is the sign bit). However, in GNU Radio, the UHD drive sends
> 16bits I, and 16bits Q.
> Are I and Q float point numbers? How do the 16bits represent a float number?
> Any suggestion about the I/Q's format is appreciated!
> Thanks,
> Yooxi

The interface to the USRP is defined to take floating point numbers.
It will perform the conversion to the 16-bit shorts that are sent over
the wire for you. Also remember that the output of the FFT is a
vector, so you'll want to use the gr_vector_to_stream block to convert
it back to a stream of floating point samples.


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