Motorola DSP Developer's Kit

xcorr-c.asm

[C ]=mot###_xcorr( A, B, MAXLAG, 'flag')

Description

This function returns the length 2*M-1 cross-correlation sequence in a column vector where A and B are length N vectors.

Input/Output

Input: Vector A (includes the real part Ar, and the imaginary part Ai), vector B (includes the real part Br, and the imaginary part Bi), int MAXLAG, and int 'flag'

Output: Vector C (includes the real part Cr, and the imaginary part Ci)

Algorithm

N = LengthA;
for (i = Max(1, (N - MAXLAG)); i <= N - 1; i ++) {
    for (j = 0; j < = i - 1; j ++) {
        C[i]+= B[j+1] * A[j+ N - i + 1]+ BI[j+1] * AI[j+ N - i +1];
        CI[i]+= BI[j+1] * A[j+ N -i +1]-B[j+1] * AI[j + N -i +1];
        }
        CI[i] = -CI[i];// complex conjugate 
        if (flag == "biased") C[i] /= N;
        else if (flag == "unbiased") C[i] /= i ;
    }
    for (i = N; i <= Min((2 * N - 1), (N + MAXLAG)); i ++) {
        for (j = 0; j <= 2 * N - i - 1; j ++) {
            C[i] += A[j+1] * B[j+i- N+1]+ AI[j+1]* BI[j+i-N+1];
            CI[i]+= AI[j+1] * B[j+i- N+1]-A[j+1] * BI[j+i-N+1];
        }
        if (flag == "biased") C[i] /= N;
        else if (flag == "unbiased") C[i] /= (2 * N - i);
    }
    if (flag == "coeff") {
       for(i=Max(1,(N-MAXLAG));i<= Min((2*N-1),(N+MAXLAG));i++){
            C[i]=C[i]*C[N]+CI[i]*CI[N])/C[N]*C[N]+CI[N]*CI[N];
C[i] = (CI[i]*C[N] - C[i]*CI[N]) / (C[N]* C[N] + CI[N] *CI[N];
       }
    }

// Format vector C
    if (MAXLAG >= N) {
        LengthC = 2 * MAXLAG + 1;
        df = MAXLAG - N + 1;
        for (i = LengthC;  i >= 1 ; i --) {
            if ( i > df && i <= LengthC - df ) {
                C[i] = C[i - df];
                CI[i] = CI[i - df];
           }
            else {
C[i] = 0;
CI[i] = 0;
           }
        }
    } 
    else {
        C = C(N - MAXLAG : LengthC);
        CI = CI(N - MAXLAG : LengthC);
        }

Memory & Register

Memory allocation:

• X memory label X:INA stores the address of the input data of vector A
• Y memory label Y:INB stores the address of the input data of vector B
• X memory label X:INA+ 2*LENGTH(A) stores the output data of vector C

Register usage

• R2 stores the address of the length of vector A
• R3 = MAXLAG
• R7 = 'flag'
• R7 = 0 -- 'none'
• R7 = 1 -- 'biased'
• R7 = 2 -- 'unbiased'
• R7 = 4 -- 'coeff'
• N7 = 24 for dsp56300 or 16 for dsp56600
• R1 stores the address of the index of vector C
• R0 stores the address of the index of vector A
• R4 stores the address of the index of vector B
• R7 stores the temporary address of vector A or B
• R5 and R6 are used as loop control

Assumes M{0...7} = $ffffff

Status Register

The assembly function xcorr-c.asm does not explicitly set any status registers/bits during the function execution.

Data Size Limit

The total size of all vectors can't be larger than the continuous available data memory size.

Data Range Limit

The value of input vectors must be between -1.0 and +1.0.

Precision

In the case of DSP563, precision is 18 bits.

In the case of DSP566, precision is 10 bits.

Performance Limit

DSP563:

DSP566:

xcorr-r.asm

Motorola Toolbox Function Reference