Altera FIR Compiler User Manual
Page 34
3–10
Chapter 3: Parameter Settings
Specify the Input and Output Specifications
© May 2011
Altera Corporation
The FIR Compiler calculates how many bits your filter requires for full resolution
using two methods: actual coefficients or the coefficient bit widths. These parameters
define the maximum positive and negative output values. Select either Bit Width
Only
or Actual Coefficients in the Output Specification drop-down box. The FIR
Compiler will extrapolate the number of bits required to represent that range of
values. For full precision, you must use this number of bits in your system.
1
If your filter has coefficient reloading or multiple sets of coefficients, you must select
Bit Width Only
.
You can use full or limited precision for the filtered output (
out
). To use full precision,
leave the Output Number System set to Full Resolution (default). To limit the
precision, select Custom Resolution or Signed Binary Fractional from the drop down
box.
When the Output Number System is set to Custom Resolution, you can choose to
truncate or saturate the most significant bit (MSB) and to truncate or round the least
significant bit (LSB). Saturation, truncation, and rounding are non-linear operations.
shows the options for limiting the precision of your filter.
shows an example of removing bits from the MSB and LSB.
Alternatively, you can select Signed Binary Fractional notation and specify the
number of bits to keep. The FIR Compiler displays how many bits are removed.
Table 3–2. Options for Limiting Precision
Bit Range
Option
Result
MSB
Truncate
In truncation, the filter disregards specified bits. (
Saturate
In saturation, if the filtered output is greater than the maximum positive
or negative value that can be represented, the output is forced (or
saturated) to the maximum positive or negative value.
LSB
Truncate
Same process as for MSB.
Round
The output is rounded away from zero.
Figure 3–8. Removing Bits from the MSB and LSB
D15
D14
D13
D12
D11
D10
D9
D8
.
.
D0
D9
D8
.
.
D0
Bits Removed from MSB
Full
Precision
Limited
Precision
D15
D14
.
.
.
.
D4
D3
D2
D1
D0
D11
D10
.
.
.
D1
D0
Bits Removed from LSB
Full
Precision
Limited
Precision
D15
D14
D13
D12
.
.
.
D3
D2
D1
D0
D10
D9
.
.
.
D1
D0
Bits Removed from both MSB & LSB
Full
Precision
Limited
Precision