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Altera_Forum's avatar
Altera_Forum
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10 years ago

I get a big noise when I echo the audio data from microphone

I designed a audio system in Qsys. I read the fifo data from the microphone input and write it to the audio write fifo. Although I can hear the microphone echo, it also generate a very big noise. I can't figure out why it will generate so much noise. Dose anyone have any idea why it is happened? I am not sure the setting of the audio codec is correct or not. Another problem is that how can I generate the sound with specific frequency?

I think the design in Qsys is correct. I generate the correct clock frequency to the audio and av_config core. I am using de2-115 board.

I use these functions :

alt_up_audio_read_fifo

alt_up_audio_write_fifo

Here is my c code to achieve the echo system. 

int main(){
    alt_up_audio_dev * audio_dev = alt_up_audio_open_dev ("/dev/audio");
    alt_up_av_config_dev* av_config_dev = alt_up_av_config_open_dev("/dev/audio_and_video_config");
    /* used for audio record/playback */
    unsigned int l_buf ;
    unsigned int r_buf ;
    // initialize the audio core
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x00, 0b000011000);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x01, 0b000011000);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x02, 0b000111100);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x03, 0b000111100);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x04, 0b000010100);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x05, 0b000000110);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x06, 0b000000000);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x07, 0b001001101); 
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x08, 0b000000000);
    alt_up_av_config_write_audio_cfg_register(av_config_dev, 0x09, 0b000000001);
    while(1)
    {
            // read audio buffer
            alt_up_audio_read_fifo (audio_dev, &(r_buf), 80, ALT_UP_AUDIO_RIGHT);
            alt_up_audio_read_fifo (audio_dev, &(l_buf), 80, ALT_UP_AUDIO_LEFT);
            // write audio buffer
            alt_up_audio_write_fifo (audio_dev, &(r_buf), 80, ALT_UP_AUDIO_RIGHT);
            alt_up_audio_write_fifo (audio_dev, &(l_buf), 80, ALT_UP_AUDIO_LEFT);
        }
    }
}

9 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    I would first suggest that you figure out if your problem is in audio input or output. Generate a synthetic audio signal and see if it comes out clean. Feed a test signal to the audio input and look at the data read.

  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    Thank you for the reply!

    How can I generate a synthetic audio signal? Since I don't have the line in cable...

    I try to do this, but it doesn't work... I think this two lines will generate the sound of "l_buf = 30000"?

    unsigned int l_buf = 30000;

    alt_up_audio_write_fifo (audio_dev, &(l_buf), 80, ALT_UP_AUDIO_LEFT);

    Many Thanks
  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    A simple counter that increments and is reset to zero results in a triangle wave. A counter that alternates between the min and max values gives you a square wave. Make sure a complete cycle runs in about 400-500 Hz for the most audible tone on cheap speakers.

  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    If I choose 16 bits as my audio data bit length, the max value becomes 65535 and min value becomes 0?

    I am not sure the C program is running under 50MHz or not.. If it is, i think my synthetic audio signal is: 

    
// for tri wave
int tri_signal = 0;
while (1){
	for(int n = 0; n<12500; n++){
		tri_signal = tri_signal + 5;
	}
	tri_signal = 0
}
// for sine wave (not smooth)
int sine_signal = 32767;
while (1){
	for(int n = 0; n<3125; n++){
		sine_signal = sine_signal + 10;
	}
	for(int n = 0; n < 6250; n++){
		sine_signal = sine_signal - 10;
	}
	for(int n = 0; n < 3125; n++){
		sine_signal = sine_signal + 10;
	}
}
  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    Check your data sheet. You must use a word length that matches the audio output chip. Most but not all are 16 bits. However, you cannot assume it is 0 to 64K. Min and max might be +/- 32K. Again check the data sheet.

  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    I can now generate a simple sine wave signal by a lookup table and send it to the line out now. I can hear very clear sound from the earphone. Thus I think the problem is not in the output. How should I send the sine signal to the input?

    Here is the code to test the output. 

    
for(n = 0; n<256; n++){
	alt_up_audio_write_fifo (audio_dev, &(sine), 16, ALT_UP_AUDIO_LEFT);
}
  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    Verilog is *not* a procedural language. For loops don't do things one by one, they infer hardware for each instance of the loop - so that will not do what you want.

    You need to create a counter (always @ ... counter <= counter+ 1), and use that to read from your fifo.

    EDIT:

    Never mind, that was C code, I misread. Ignore the above.
  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    I recommend using a triangle wave to test your input function. It is easier to verify that the received data is correct. You should use a function generator connected to the ADC input. Don't forget to get the impedance matched. Once you have a clean triangle wave going to your audio input you should be able to test your data input function.