Problems upsampling data using accelerate

Question

After I downsample a vector with a constant decimating factor, I want to upsample the vector back to the original sample rate (after performing some analyses). However, I am struggling with the upsampling.

For the downsampling I apply vDSP_desamp from the Accelerate framework, and for the upsampling I tried to apply vDSP_vlint:

// Create some test data for input vector
float inputData[10] = {0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9};

int inputLength = 10;
int decimationFactor = 2; // Downsample by factor 2
int downSampledLength = inputLength/decimationFactor;

// Allocate downsampled output vector
float* downSampledData = malloc(downSampledLength*sizeof(float));

// Create filter (average samples)
float* filter = malloc(decimationFactor*sizeof(float));

for (int i = 0; i < decimationFactor; ++i){
    filter[i] = 1.0/decimationFactor;
}

// Downsample and average
vDSP_desamp(inputData,
            (vDSP_Stride) decimationFactor,
            filter,
            downSampledData,
            (vDSP_Length) downSampledLength,  // Downsample to 5 samples
            (vDSP_Length) decimationFactor );


free(filter);

The output of downSampledData using this code is:

0.05, 0.25, 0.45, 0.65, 0.85

To upsample the (processed) data vector back to the original sample rate I use the following code:

// For this example downSampledData is just copied to processedData ... 
float* processedData =  malloc(downSampledLength*sizeof(float));
processedData = downSampledData; 


//  Create vector used by vDSP_vlint to indicate interpolation constants.
float* b = malloc(downSampledLength*sizeof(float));
for (int i = 0; i < downSampledLength; i++) {
    b[i] = i + 0.5;
}

// Allocate data vector for upsampled data
float* upSampledData = malloc(inputLength*sizeof(float));

// Upsample and interpolate
vDSP_vlint (processedData,
            b,
            1,
            upSampledData,
            1,
            (vDSP_Length) inputLength, // Resample back to 10 samples
            (vDSP_Length) downSampledLength);

However, the output of upSampledData is

0.15, 0.35, 0.55, 0.75, 0.43, 0.05, 0.05, 0.05, 0.08, 0.12

which is not correct, apparently. How should I apply vDSP_vlint? Or should I use other functions for upsampling the data?

Problems upsampling data using accelerate

0 Answers0