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nic.dc

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11-17-2015 01:15 AM

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11-17-2015

Hi, I'm starting to use the FFT v.7 but then I would want to know the format of the bits for the input and output for unscaled method with 8 bit input and 12 bit output. It says nothing about the format in the data sheet. This format being the no. of bits for the integer part and fraction part of the input and output.

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Effectively at the input of system you can assume the format anything, even 1.x (ie 1 sign and the rest fractional). Once you start filtering, you will need to decide how the whole part grows and/or how many bits you need to keep from the fractional part (adding multiple numbers grows the dynamic range and multiplying them increases the fractional part too) so you need to adjust the format as you go.

muzaffer

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11-22-2015 09:15 PM

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03-31-2012

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10 Replies

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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nic.dc

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11-17-2015 01:35 AM

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11-17-2015

By the way, I also used Truncation rounding and used Natural order

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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ds260 page 2 says this: "For fixed-point inputs, the input data is a vector of N complex values represented as dual bx-bit two’s-complement numbers, that is, bx bits for each of the real and imaginary components of the data sample, where bx is in the range

8 to 34 bits inclusive."

which is all you need. Any fractional number has an "imaginary" binary point which is used to decide which bit has the weight 2^0. Other than that, the bit order is decided by two's complement behavior

muzaffer

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11-17-2015 02:15 AM

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03-31-2012

8 to 34 bits inclusive."

which is all you need. Any fractional number has an "imaginary" binary point which is used to decide which bit has the weight 2^0. Other than that, the bit order is decided by two's complement behavior

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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Hi muzaffer! Thanks for the insight! but, how would I know where the binary point is placed for the input and output?

Thanks!

nic.dc

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11-17-2015 02:37 AM

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11-17-2015

Thanks!

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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bwiec

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11-17-2015 10:01 AM

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08-02-2011

Hello,

The doc also says:

Note that the core does not have a specific location for the binary point. The output maintains the same binary point location as the input.

www.xilinx.com

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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nic.dc

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11-19-2015 07:53 AM

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11-17-2015

Thanks for that bwiec! How would I know then where the binary point from the input is?

Waiting for a reply

Thanks!

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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Input format is something you decide/control and depends on the filtering/processing you do before hand. You can always assume all numbers are in 1.x format with a single bit for sign and (x=n-1) bits for fraction.

muzaffer

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11-19-2015 07:57 AM

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03-31-2012

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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Oh, so I can also assume that the format is 1x.y where: There's a single bit for sign, x no. of bits for the integer, and y no. of bits for the fraction?

Waiting for a reply

Thanks!

nic.dc

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11-22-2015 04:43 PM

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11-17-2015

Waiting for a reply

Thanks!

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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Yes. That's reasonable.
- Please mark the Answer as "Accept as solution" if information provided is helpful.

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muzaffer

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11-22-2015 05:10 PM

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03-31-2012

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Re: How to know format (# of bits for fraction and integer part) of the input and output for an unscaled, pipelined/streaming io in FFT v7.1

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So, initially, I can always assume my desired format (such as the previous format 1x.y) even without filtering/processing?

And, is 1.x assumed to be the default format?

Waiting for a reply

Thanks!

nic.dc

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11-22-2015 06:04 PM

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Registered:
11-17-2015

And, is 1.x assumed to be the default format?

Waiting for a reply

Thanks!

Highlighted
Effectively at the input of system you can assume the format anything, even 1.x (ie 1 sign and the rest fractional). Once you start filtering, you will need to decide how the whole part grows and/or how many bits you need to keep from the fractional part (adding multiple numbers grows the dynamic range and multiplying them increases the fractional part too) so you need to adjust the format as you go.
- Please mark the Answer as "Accept as solution" if information provided is helpful.

Give Kudos to a post which you think is helpful and reply oriented.

muzaffer

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11-22-2015 09:15 PM

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03-31-2012

Give Kudos to a post which you think is helpful and reply oriented.