Exercise 4: Developing signal generators in real time using the C5510 DSK

Transcription

1 Exercise 4: Developing signal generators in real time using the C5510 DSK The objective of this experiment is to use the C5510 DSK with its associated CCS, BSL, and AIC23 codec for generating sinusoidal and random signals. We will develop our programs based on the tone.c that is available in the folder c:\ti5510\examples\dsk5510\bsl\tone. We have run this sinewave generator on the C5510 DSK in Exp2 using the project tone.pjt located in the folder. In this experiment, we will modify that C program and build the project using CCS for execution on the C5510 DSK for real-time experiments. First, create a working folder, and copy the following files from the folder c:\ti5510\examples\dsk5510\bsl\tone into the new folder: tone.c, tone.cdb, and tonecfg.cmd. Follow the following steps to build a new project: 1. Select Project New to create a new project in the new folder. Add tone.c, tone.cdb and tonecfg.cmd into the project. In addition, add the DSPLIB 55xdspx.lib from the folder c:\ti5510\c5500\dsplib and dsk5510bslx.lib from the folder c:\ti5510\c5500\dsk5510\lib into the project. We will need DSPLIB functions to generate sine and random signals. 2. Select Project Build Options. Click on the Compiler tab. In the Category window, click on Advanced, and check the Large memory model (-ml) box, then click OK.

2 3. Select Option Customize, click on the Program Load Options tab, and check the Load Program After Build checkbox. 4. Build the project by selecting Project Rebuild All. 5. Connect a headphone (or a loudspeaker) to the HEADPHONE output of C5510 DSK. Run the program by selecting Debug Run. You should hear a tone playing through the headphone (or the loudspeaker). Select Debug Restart can rerun the program again. Now, we are ready to modify tone.c for additional exercises. Additional Exercises: 1. Read the C source code tone.c. In the C code, the array sinetable contains 48 samples (which cover exactly one period) of a pre-calculated sinewave and save it using signed 16-bit data. The sampling rate of codec is default at 48 khz, thus the codec outputs 48,000 samples per second. What is the frequency of the generated sinewave? Why the tone last for 5 second? (Answer: The time interval between two consecutive samples is T = 1/48,000 seconds. Each period of sinewave contains 48 samples, thus the period of sinewave is 48/48,000 = 1/1,000 seconds = 1 ms. Therefore, the frequency of the generated sinewave is 1,000 Hz. Since each period of sinewave is 1/1,000 second, the program generates 5000 periods, thus it lasts for 5 seconds). The array sinetable is shown below using hexadecimal format: /* Pre-generated sine wave data, 16-bit signed samples */ Int16 sinetable[sine_table_size] = {

3 }; 0x0000, 0x10b4, 0x2120, 0x30fb, 0x3fff, 0x4dea, 0x5a81, 0x658b, 0x6ed8, 0x763f, 0x7ba1, 0x7ee5, 0x7ffd, 0x7ee5, 0x7ba1, 0x76ef, 0x6ed8, 0x658b, 0x5a81, 0x4dea, 0x3fff, 0x30fb, 0x2120, 0x10b4, 0x0000, 0xef4c, 0xdee0, 0xcf06, 0xc002, 0xb216, 0xa57f, 0x9a75, 0x9128, 0x89c1, 0x845f, 0x811b, 0x8002, 0x811b, 0x845f, 0x89c1, 0x9128, 0x9a76, 0xa57f, 0xb216, 0xc002, 0xcf06, 0xdee0, 0xef4c We also can view the graphic of sinetable by selecting View Graph, and fill the Graph Property Dialog box with the values shown in the window: Pay special attention to the magnitude of sinewave. What is the value in the array sinetable corresponding to the maximum magnitude of sinewave? (Answer: 0x7ffd). Which one corresponding to the minimum magnitude? 2. The default setting of sampling rate for codec is 48 khz. As shown in Exp3, we can set different desired sampling frequency using the function DSK5510_AIC23_setFreq() available in the BSL dsk5510bslx.lib. For example, we can use the following command to set sampling rate to 8 khz:

4 DSK5510_AIC23_setFreq(hCodec, DSK5510_AIC23_FREQ_8KHZ); Now, modify the C program tone.c by inserting this line of code to set the sampling rate to 8 khz. Build a new project and perform a real-time testing. After the program is running, compare the sound effects for the sampling rates at 8 khz and 48 khz. What is the frequency of sinewave that you generated with 8 khz sampling rate? Why? Also, how many seconds the tone last? Why? 3. Modify the tone.c to generate 1 khz tone with 8 khz sampling rate. Hints: There are many ways and briefly introducing the following two methods: (a) You can re-calculate 1 period of sinewave with 8 samples (using MATLAB or hand calculation) to replace the original 48 samples sinetable. In this case, be sure to change SINE_TABLE_SIZE from 48 to? (b) You can use the same sinetable with 48 samples, but step through the table every 6 samples by modifying the outer loop as follows: for (sample = 0; sample < SINE_TABLE_SIZE; sample=sample+6) Try both methods and perform real-time testing. Are you sure that you have generate 1 khz tone with 8 khz sampling rate? 4. In Section (page 299), we have learned how to use a C55xx DSPLIB function rand16 to generate 128 random samples in the C program rangen.c that run under CCS simulator. In this experiment, we use the same DSPLIB function rand16 to generate 8 samples of random signal for 8 khz sampling rate (or 48 samples if the sampling rate is 48 khz) instead of using the pre-generated sinetable in tone.c. Partial changes of C code are: #define SINE_TABLE_SIZE 8 // no. of samples Int16 sinetable[sine_table_size]; // vector for random samples

5 for (msec = 0; msec < 5000; msec++) { rand16(sinetable, SINE_TABLE_SIZE); for (sample = 0; sample < SINE_TABLE_SIZE; sample++) { /* Send a sample to the left channel */ while (!DSK5510_AIC23_write16(hCodec, sinetable[sample])); } } /* Send a sample to the right channel */ while (!DSK5510_AIC23_write16(hCodec, sinetable[sample])); The modified C program (we still call it tone.c) is available for download in this webpage. Build a new project and perform a real-time testing. You should hear the sound of white noise, which is similar to the noise generated by modems during dial up. 5. You have already learned how to generate a single tone using the pre-generated table. (a) Try to generate multiple sinewaves at different frequencies using the same table such as the sinetable in tone.c. by stepping through the same table using different steps). (b) Try to use the DSPLIB function sine(x, r, Nx) to generate an array of sinewave. 6. You can combine both the sinewave and noise generator to generate a sinewave that is embedded in white noise for future experiments. Pay special attention to overflow problem when you add to Q.15 numbers. How can you prevent overflow? Build a new project and perform a real-time testing. Try different signal-to-noise ratio and compare the differences. 7. Finally, this is a challenging and practical problem: How to generate a tone (or multiple tones) at any frequency with any predetermined sampling rate using a table lookup technique? You may find you have to step through the table with a

6 non-integer step, thus you have to interpolate a value between two consecutive samples in the table. You also will find it is easier and better to design a new sinetable that have more samples (>48) to cover one period of sinewave. If you are ready, try to generate DTMF tones (which consist of two sinewaves) at 8 khz sampling rate. The frequencies of sinewaves are summarized in Figure 7.23, page 417.