Input and Output with the DSK - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 62

Build and run this project as sin1500MATL . Verify that the output is a 1500-Hz
sine-wave signal. Within CCS, be careful when you view the header file sin1500.h
so as not to truncate it.

Example 2.14: Amplitude Modulation ( AM )

This example illustrates an amplitude modulation (AM) scheme. Figure 2.23 shows
a listing of the program AM.c , which generates an AM signal. The buffer baseband
contains 20 points and represents a baseband cosine signal with a frequency of
f

400 Hz. The buffer carrier also contains 20 points and represents a
carrier signal with a frequency of f

= F s /20 =

= F s (number of cycles)/(number of points) =

F s /(number points per cycle) 2 kHz. The output equation shows the baseband

signal being modulated by the carrier signal. The variable amp is used to vary the
modulation. The polling-driven C source program AM.c implements this project.
Build and implement this project as AM . Verify that the output consists of the
2-kHz carrier signal and two sideband signals. The sideband signals are at the fre-
quency of the carrier signal

=

+ or - the frequency of the sideband signal, or at 1600

and 2400 Hz.
Load the GEL file AM.gel , increase the variable amp , and verify the baseband
signal being modulated (modulation index controlled by amp ,). Note that the

// AM.c AM using table for carrier and baseband signals

#include "DSK6713_AIC23.h" //codec-dsk support file

Uint32 fs=DSK6713_AIC23_FREQ_8KHZ; //set sampling rate

short amp = 1; //index for modulation

void main()
{
short baseband[20]={1000,951,809,587,309,0,-309,-587,-809,-951,
-1000,-951,-809,-587,-309,0,309,587,809,951};//400Hz baseband
short carrier[20] ={1000,0,-1000,0,1000,0,-1000,0,1000,0,
-1000,0,1000,0,-1000,0,1000,0,-1000,0}; //2kHz carrier
short output[20];
short k;

comm_poll(); //init DSK, codec, McBSP
while(1) //infinite loop
{
for (k=0; k<20; k++)
{
output[k]= carrier[k] + ((amp*baseband[k]*carrier[k]/10)>>12);
output_sample(20*output[k]); //scale output
}
}
}

FIGURE 2.23. Amplitude modulation program ( AM.c ).