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

[Marcus D. Leech]

On 08/15/2011 01:22 PM, xi yang wrote:
> Hi, Tom,
>
> Thanks a lot for your prompt reply!
> Do you mean on the Transmitting side?
> I want to know how 32-bit floating point number is converted to 16bits 
> or from 16bits to 32bits.
> Where can I find the file that performs this conversion?
>
> We are actually working on the Receiving side.
> Before we move I/Q samples from rx_buffer to USB, we want to do FFT 
> and deliver the FFT results instead of the raw I/Q samples to the host.
> For my FPGA FFT module, I use 32bits floating point numbers as input.
> So I want to know how the I/Q samples are represented by 16bits so 
> that I can convert them to 32bits.
> In 32bits representation, bits 0 through 22 form the mantissa, bits 23 
> through 30 form the exponent, and bit 31 is the sign bit.
> In 16bits, do we simply cut 16bits of the mantissa?
> Then bits 0 through 6 form the mantissa, bits7 through 14 form the 
> exponent,and bit 15 is the sign bit. Is that right?
> It's better if we can have a look at the conversion file.
> Thanks,
> Yooxi
The host interface uses 16-bit integers for I and Q.  They're only 
represented as floating-point numbers within a Gnu Radio flow-graph.

Internally in the FPGA, everything is integers, starting with 12-bit 
integers from the ADC (on the USRP1), then through the CIC decimators
  and DDC, they become larger, but what is finally injected into the 
host is 16-bit (integer) I and 16-bit (integer) Q.
  Standard twos-complement representation for the integers, I don't 
happen to recall off the top of my head whether they're "big endian"
  or "little endian".






--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org