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126 lines
3.7 KiB
Markdown
126 lines
3.7 KiB
Markdown
# Example 2
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Please refer to the [simple example](../simple/README.md) to have an overview of how to define a graph and it nodes and how to generate the C++ code for the static scheduler.
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In this example. we are just analyzing a much more complex example to see some new features:
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- Delay
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- CMSIS-DSP function
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- Constant node
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- SlidingBuffer
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This example is not really using a MFCC or a TensorFlow Lite node. It is just providing some wrappers to show how such a nodes could be included in a graph:
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The graph is:
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It is much more complex:
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- First we have a stereo source delayed by 10 samples ;
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- Then this stereo source is split into left/right samples using the default block Unzip
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- The samples are divided by 2 using a CMSIS-DSP function
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- The node HALF representing a constant is introduced (constant arrays are also supported)
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- The two streams are added using a CMSIS-DSP function
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- Then we have a sliding buffer
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- A block representing a MFCC (a fake MFCC)
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- Another sliding buffer
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- An a block representing TensorFlow Lite for Micro (a fake TFLite node)
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Note that those blocks (MFCC, TFLite) are doing nothing in this example. It is just to illustrate a more complex example typical of keyword spotting applications.
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Examples 5 and 6 are showing how to use the CMSIS-DSP MFCC.
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Let's look at the new features compared to example 1:
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## Delay
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```python
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g.connectWithDelay(src.o, toMono.i,10)
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```
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To add a delay on a link between 2 nodes, you just use the `connectWithDelay` function. Delays can be useful for some graphs which are not schedulable. They are implemented by starting the schedule with a FIFO which is not empty but contain some 0 samples.
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## CMSIS-DSP function
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Some CMSIS-DSP functions are automatically made available to the framework : mainly the functions with no state and which are pure stream based computation : Basic math functions etc ...
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To create a CMSIS-DSP node, just use:
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```python
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sa=Dsp("scale",floatType,blockSize)
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```
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The corresponding CMSIS-DSP function will be named: `arm_scale_f32`
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The code generated in `scheduler.cpp` will not require any C++ class, It will look like:
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```C++
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{
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float32_t* i0;
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float32_t* i1;
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float32_t* o2;
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i0=fifo3.getReadBuffer(160);
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i1=fifo4.getReadBuffer(160);
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o2=fifo5.getWriteBuffer(160);
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arm_add_f32(i0,i1,o2,160);
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cgStaticError = 0;
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}
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```
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## Constant node
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In the case of scaling, we need to connect the scaling factor to the node. So we need a constant node.
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A constant node is defined as:
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```python
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half=Constant("HALF")
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```
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In the C++ code, `HALF` is expected to be a value defined in `custom.h`
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Constant values are not involved in the scheduling (they are ignored) and they have no IO. So, to connect to a constant node we do:
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```python
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g.connect(half,sa.ib)
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```
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There is no "o", "oa" suffixes for the constant node `half`.
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Constant nodes are just here to make it easier to use CMSIS-DSP functions.
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## SlidingBuffer
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Sliding buffers and OverlapAndAdd are used a lot so they are provided in the `cg/nodes/cpp`folder of the `ComputeGraph` folder.
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In Python, it can be used with:
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```python
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audioWindow=SlidingBuffer("audioWin",floatType,640,320)
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```
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The first length (`640`) is the window size and the second length (`320`) is the overlap. So, in this case we have an overlap of 50%
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There is no C++ class to write for this since it is provided by default by the framework.
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It is named `SlidingBuffer` but not `SlidingWindow` because no multiplication with a window is done. It must be implemented with another block as will be demonstrated in the [example 3](example3.md)
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## Expected outputs
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```
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Schedule length = 302
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Memory usage 10720 bytes
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```
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And when executed:
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```
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Start
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Nb = 40
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```
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Execution is running for 40 iterations without errors.
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