Hi,
I'm starting to test/study the CMSIS lib in the Stellaris/Tiva. Using the paper "Using the CMSIS DSP Library in Code Composer Studio™ for TM4C MCUs", I got build the library and it works fine. The examples return the expected state.
Now I'm trying to use the cFFT in a signal. To the test I created a pure sine wave with amplitude of 1000 and 10 cycles in an array of 1024 samples, as in the image.
After the processing the indexer return the correct harmonic (10th), but I don't understand the result of the magnitude. The magnitude of the 10th hamonic is 727.98. How it relates to the amplitude of the harmonic? The RMS value, if yes, why the error?
This is the principal part of code:
#define N_points 1024 ... uint32_t ifftFlag = 1; uint32_t doBitReverse = 1; /* Process the data through the CFFT/CIFFT module */ arm_cfft_f32(&arm_cfft_sR_f32_len512, wave_float, ifftFlag, doBitReverse); /* Process the data through the Complex Magnitude Module for calculating the magnitude at each bin */ arm_cmplx_mag_f32(wave_float, wave_out, N_points/2); /* Calculates maxValue and returns corresponding BIN value */ arm_max_f32(wave_out, N_points/2, &maxValue, &testIndex);
I tried to scale the sine wave between 1 and -1, but the result was the same. Instead of 727 the magnitude of the harmonic was 0.727.
It's strange because for different numbers of cycles the result of respective "bin" change.
Maintaining the same buffer size (1024) I changed to just one cycle and the result was around 708, very close to the RMS value. See this little list:
1024 samples
01 cycle -> 708.8323
02 cycles -> 710.9644
04 cycles -> 715.2491
08 cycles -> 723.74
16 cycles -> 740.4352
I tried to change the buffer size to 64 samples to test. In this case the result of one cycle was around 740.
I'm reading about how to obtain the amplitude, but in this moment I'm lost!