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

From: yooxi
Subject: Re: [Discuss-gnuradio] The format of I/Q sample
Date: Mon, 15 Aug 2011 10:38:44 -0700 (PDT)

Hi, Marcus,

Thanks a lot for your quick reply!
Then start from where, the integers become floating point numbers?
I'd like to see how they are converted.


Marcus D. Leech wrote:
> 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
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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