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Contributor
Contributor
6,987 Views
Registered: ‎10-29-2007

Very strange auto-inferred quantizing in accelDSP

I have a vector that is auto-quantized to fixed wrap [20 6] format. Let us call the vector X. So far so good.
 
But then I take two elements in the vector and subtract. Y = X(1)-X(5);
Auto inferred precision is then fixed wrap [14 6] for Y!!!!!!
 
Can anyone explain how the fixed point generator can greate a 14 bit value for the difference between two signed 20-bit values???
 
Edit: I use AccelDSP 9.2.01 and Matlab R2007a
 


Message Edited by andax on 03-13-2008 09:27 AM
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3 Replies
Xilinx Employee
Xilinx Employee
6,956 Views
Registered: ‎08-07-2007

Re: Very strange auto-inferred quantizing in accelDSP

are the any elements in the X vector constants?
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Contributor
Contributor
6,945 Views
Registered: ‎10-29-2007

Re: Very strange auto-inferred quantizing in accelDSP

No, X is a result vector based on the input stream of samples and thus not constant in any sense.
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Xilinx Employee
Xilinx Employee
6,925 Views
Registered: ‎08-07-2007

Re: Very strange auto-inferred quantizing in accelDSP

Check if there are any quantize directives on 'Y' forcing  the type.  You can search through the .add file for lines containing "SetDirective -quantize". 

Any quantize directives will override any implied data types.  You can also have an explicit quantization coded into your MATLAB. 

If you don't think either of these is the case please open a WebCase with Xilinx support so we can investigate this further.
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