# Forum: FPGA, VHDL & Verilog fixed point precision

 Author: Abdallah      (Company: Home) (my_name) Posted on: 2014-04-10 23:35

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Peace!
I want ask about the number of bits shoud I use in an fixed_point number
to obtein the good precision, precision of distance eclidean.
some detail:
I want to determine an vector of foor element VECT[MOY1,MOY2,MOY3,MOY4],
each element is the moyen of one quart of matrix of (64*64), the matrix'
element value varies (0-255)'''it's pixel valor'''
--> the result of division (MOY1=SUM/Nbits) has 8 bits in the integer
part;
Question: how many bits of fractional part shout I use to get the good
result (result of euclidean distance between two vector
VECT1[MOY11,MOY12,MOY13,MOY14] and VECT[MOY21,MOY22,MOY23,MOY24])..
thank you....peace:

 Author: Duke Scarring (Guest) Posted on: 2014-04-18 10:22

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Abdallah      wrote:
> about the number of bits shoud I use in an fixed_point number
> to obtein the good precision
You have to define the precision. Independed if you use fixed point or
integer number format.

The number of bits depends on your dynamic range:
http://en.wikipedia.org/wiki/Dynamic_range

Roughly: NoBs = SNR / 6

eg.: 16 Bit = 96 dB / 6

Duke

 Author: Markus W. (elektrowagi78) Posted on: 2014-04-19 15:11

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Why would you consider to be the DR at 16 Bits not to be 96dB but
96dB/6?

 Author: ... (Guest) Posted on: 2014-04-19 15:19

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You wanted a GOOD precission in the result, right? then divide it by 6
:-)

 Author: Duke Scarring (Guest) Posted on: 2014-04-20 00:15

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Markus Wagner wrote:
> Why would you consider to be the DR at 16 Bits not to be 96dB but
> 96dB/6?
number of bits * 6 dB = dynamic range

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