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15.3.13. General Multitwiddle and General Twiddle (GeneralMultiTwiddle, GeneralMultVTwiddle, GeneralTwiddle, GeneralVTwiddle)
The GeneralTwiddle block generates its twiddle factors using the TwiddleRom block; the GeneralMultTwiddle block uses the TwiddleMultRom block. The GeneralMultTwiddle uses approximately twice as many DSP blocks as the GeneralTwiddle block, but (for large FFTs) uses far fewer memory blocks.
Each data sample in the input stream has a unique address. The address consists of the timeslot in which it arrived tbits concatenated with the number of wires on which it arrived sbits. The sbits is forms the least significant part of the address; the tbits forms the most significant part.
Each data sample is multiplied by a twiddle factor. For an FFT, the twiddle factor is:
twiddle = exp(-2*pi*i*angle/K)
For an IFFT, the twiddle factor is:
twiddle = exp(2*pi*i*angle/K)
where K items exist in each block of data.
For each data sample, the twiddle angle is calculated as:
angle = X*Y
where X and Y depend on the position of that data sample in the input stream.
Obtain the value of X (or Y) by extracting user-specified bits from the address of the data sample, and concatenating them.
The GeneralVTwiddle block (and the memory-optimized GeneralMultVTwiddle offer variable size and include an additional size input. They both use a General Twiddle Counter rather than a Counter block.
Parameter | Description |
---|---|
iFFT | Generate twiddle factors for an FFT or an IFFT. |
sbits | The number of spatial address bits i.e. log2(N) where there are N wires. |
xbits | The vector of bit positions for X. |
ybits | The vector of bit positions for Y. |
Input type | The type of the input before the twiddle. |
Twiddle type | The type of the twiddle factor. |
Output type | The type of the output after the twiddle. |
Use faithful rounding | Use faithful rather than correct rounding. Only for floating-point twiddle types. |
Parameter | Description |
---|---|
iFFT | Generate twiddle factors for an FFT or an IFFT. |
Number of spatial bits | The number of spatial address bits i.e. log2(N) where there are N wires. |
maxsize | Maximum FFT size is 2^maxsize. |
Size of the parallel section | The number of radix-2 stages assigned to the parallel section in the surrounding HybridVFFT (the GeneralVTwiddle links between the serial and parallel sections of the HybridVFFT) |
Input type | The type of the input before the twiddle. |
Twiddle type | The type of the twiddle factor. |
Output type | The type of the output after the twiddle. |
Use faithful rounding | Use faithful rather than correct rounding. Only for floating-point twiddle types. |
Signal | Direction | Type | Description |
---|---|---|---|
v | Input | Boolean. | Input valid signal. |
d | Input | Compatible with user-specified input type. | Vector of N data inputs. |
drop | Input | Unsigned integer | The number of stages to drop. GeneralMultiVTwiddle only. |
qv | Output | Boolean. | Output valid signal. |
q | Output | User-specified output type. | Vector of N data outputs. |
qdrop | Output | Unisgned integer | GeneralMultiVTwiddle only. |