Programming Forums > Application Development > C++

BruceLeroy · Nov 7th, 2004, 7:17 AM

Suppose I have an array X of short ints with around a several thousand values.

What I want to do is generate another array Y of exactly 1000 elements whose content is representative of X. Y should also be an array of short ints.

But because X represents amplitudes of sound, I don't want to do uniform sampling because I may miss certain important peaks. I also don't want to use the naive approach of reading in a chunk of X, finding the average or max of that chunk and using that result as an element of Y, and repeating the process until I've got 1000 values. I call it naive because I recently bumped into an interesting concept called Root Mean Square, which, as google informs me, is perfect for waves (including soundwaves.

So! What I want to do is a read in a chunk of X, get the RMS square of that chunk and use that. Somehow what I have got so far ends up with unsatisfying result of getting back an array with all elements with the value 0. Here's my code:

(Toggle Plain Text)

short int* calculatePoints(short int speakerData[], long arraySize)
{

	//how many values to plot along x-axis

	int horizontalResolution = 1024; 

	//declare return value array

	short int* GraphPoints;

	GraphPoints = new short int[horizontalResolution];

	/* 0 this array out first to make sure we don't get any old data
	re-appearing again */

	for (int G=0; G<horizontalResolution; G++)
 GraphPoints[G]=0;

	//calculate how many samples each pixel is representative of

	int bufferSize = arraySize / horizontalResolution;

	if ( bufferSize % 2 ) bufferSize++;

	long i = 0; //index to current value in old array

	int x = 0; //index to pixel for the current iteration

	double RMS = 0; //variable for ROOT MEANS SQUARE calculation

	while ( x < 1024 )
	{	
        //read in a chunk, summing the squares of each value
 for (int j=i; j<bufferSize; j++)
 	RMS = RMS + (PCM_Data[i+j]*PCM_Data[i+j]);

        RMS = RMS / bufferSize; //get the mean

 RMS = sqrt(RMS); //get the square root

 GraphPoints[x]=(short int) RMS; //store the value
 
 i=i+bufferSize; //skip a few values

 x++; //increment the loop counter

	}

return GraphPoints;

}

Now, I'm guessing that the cast from double back to short int maybe causing a problem, but I'm not sure -- any ideas?

tempest · Nov 7th, 2004, 11:14 PM

Try adding tests to make sure the values are getting to the two functions...

(Toggle Plain Text)

 printf("Pre-sqrt: %f\n", RMS);
 RMS = sqrt(RMS); //get the square root

 printf("Pre-cast: %f\n\n", RMS);
 GraphPoints[x]=(short int) RMS; //store the value

BruceLeroy · Nov 8th, 2004, 1:57 PM

thanks it works now. stupid fence post errors

Nov 7th, 2004, 7:17 AM	#1
BruceLeroy Newbie Join Date: Nov 2004 Posts: 18 Rep Power: 0	Suppose I have an array X of short ints with around a several thousand values. What I want to do is generate another array Y of exactly 1000 elements whose content is representative of X. Y should also be an array of short ints. But because X represents amplitudes of sound, I don't want to do uniform sampling because I may miss certain important peaks. I also don't want to use the naive approach of reading in a chunk of X, finding the average or max of that chunk and using that result as an element of Y, and repeating the process until I've got 1000 values. I call it naive because I recently bumped into an interesting concept called Root Mean Square, which, as google informs me, is perfect for waves (including soundwaves. So! What I want to do is a read in a chunk of X, get the RMS square of that chunk and use that. Somehow what I have got so far ends up with unsatisfying result of getting back an array with all elements with the value 0. Here's my code: (Toggle Plain Text) short int* calculatePoints(short int speakerData[], long arraySize) { //how many values to plot along x-axis int horizontalResolution = 1024; //declare return value array short int* GraphPoints; GraphPoints = new short int[horizontalResolution]; /* 0 this array out first to make sure we don't get any old data re-appearing again / for (int G=0; G<horizontalResolution; G++) GraphPoints[G]=0; //calculate how many samples each pixel is representative of int bufferSize = arraySize / horizontalResolution; if ( bufferSize % 2 ) bufferSize++; long i = 0; //index to current value in old array int x = 0; //index to pixel for the current iteration double RMS = 0; //variable for ROOT MEANS SQUARE calculation while ( x < 1024 ) { //read in a chunk, summing the squares of each value for (int j=i; j<bufferSize; j++) RMS = RMS + (PCM_Data[i+j]PCM_Data[i+j]); RMS = RMS / bufferSize; //get the mean RMS = sqrt(RMS); //get the square root GraphPoints[x]=(short int) RMS; //store the value i=i+bufferSize; //skip a few values x++; //increment the loop counter } return GraphPoints; } short int* calculatePoints(short int speakerData[], long arraySize) { //how many values to plot along x-axis int horizontalResolution = 1024; //declare return value array short int* GraphPoints; GraphPoints = new short int[horizontalResolution]; /* 0 this array out first to make sure we don't get any old data re-appearing again / for (int G=0; G<horizontalResolution; G++) GraphPoints[G]=0; //calculate how many samples each pixel is representative of int bufferSize = arraySize / horizontalResolution; if ( bufferSize % 2 ) bufferSize++; long i = 0; //index to current value in old array int x = 0; //index to pixel for the current iteration double RMS = 0; //variable for ROOT MEANS SQUARE calculation while ( x < 1024 ) { //read in a chunk, summing the squares of each value for (int j=i; j<bufferSize; j++) RMS = RMS + (PCM_Data[i+j]PCM_Data[i+j]); RMS = RMS / bufferSize; //get the mean RMS = sqrt(RMS); //get the square root GraphPoints[x]=(short int) RMS; //store the value i=i+bufferSize; //skip a few values x++; //increment the loop counter } return GraphPoints; } Now, I'm guessing that the cast from double back to short int maybe causing a problem, but I'm not sure -- any ideas?

Nov 7th, 2004, 11:14 PM	#2
tempest Programming Guru Join Date: Oct 2004 Posts: 1,041 Rep Power: 6	Try adding tests to make sure the values are getting to the two functions... (Toggle Plain Text) printf("Pre-sqrt: %f\n", RMS); RMS = sqrt(RMS); //get the square root printf("Pre-cast: %f\n\n", RMS); GraphPoints[x]=(short int) RMS; //store the value printf("Pre-sqrt: %f\n", RMS); RMS = sqrt(RMS); //get the square root printf("Pre-cast: %f\n\n", RMS); GraphPoints[x]=(short int) RMS; //store the value __________________

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Nov 8th, 2004, 1:57 PM	#3
BruceLeroy Newbie Join Date: Nov 2004 Posts: 18 Rep Power: 0	thanks it works now. stupid fence post errors

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