Library
AI Engine/DSP/Stream IO
Description
Stream-based FFT implementation targeted for AI Engines. The FFT block has two stream ports per subframe processor
to maximize performance. The specified value for SSR parameter should be of the form
2^N, where N is a positive integer.
Parameters
- Main
- Input/Output Data Type:
- Describes the type of individual data samples input/output of the
stream FFT. It can be
cint16,
cint32, cfloat types.
- FFT Size:
- This is an unsigned integer which describes the point size of the
transformation. This must be 2^N, where N is in the range
4 to 11 inclusive.
- Input Frame Size (Number of Samples):
- Specifies the number of samples for a particular frame. The value
must be in the range
8 to 1024 and
the default value is 64. The FFT operation will not
begin until this number of samples has been input.
- Scale Output Down by 2^:
- Describes the power of 2 shift down applied before output. The following
table shows the range of valid values of this parameter for
different data types.
Table 1. Valid Parameters
| Data Type |
Scale output down by 2^ |
| cint16 |
[0, log2(FFT Size) + 15] |
| cint32 |
[0, log2(FFT Size) + 31] |
| cfloat |
0 |
- SSR:
- This parameter is intended to improve performance and support FFT sizes
beyond the limitations of a single tile. For an SSR value of 'n'
(which must be of the form 2^N, where N is a positive integer), the
FFT operation is performed in parallel and the actual FFT size is
divided by 'n'. For example, a 16384 point FFT with SSR value of 8
creates 8 stream inputs and there will be 8 subframe FFTs each of
point size 2048. The specified FFT size and SSR values should be
such that (2 * FFT size / SSR) is
in the range of 16 and 4096.
- Advanced
- Target Output Throughput (MSPS):
- Specifies the output sampling rate of the FFT function in Mega
Samples per Second (MSPS). The value must be in the range
1 to 1000 and the default
value is 200.
- Specify the Number of Cascade Stages:
- When this option is not enabled, the tool will determine the FFT
configuration that best achieves the specified input sampling rate.
When the option is enabled, the Number of
cascade stages can be specified (which
describes the number of AI Engine processors to
split the operation over). This allows resources to be traded for
higher performance, but the specified input sampling rate constraint
may not be achieved. The value must be in the range of
1 to 9.
