====== Serial audio device with AVR microcontroller. ======

===== Summary =====

Progetto in fase embrionale, ma intanto ho realizzato un AVR-micro che campiona l'audio da un ingresso ADC e lo invia come stream su RS-232.
Lato pc è possibile ascoltare lo stream molto semplicemente con:
   play -1 -b8 -r 7940 -t raw -u --buffer 64 /dev/ttyS0

This project is still at its beginning. However I managed to have an AVR micro sampling audio from one of its ADC inputs. Then the stream is sent over RS-232.
On the host (pc) side, you can listen to the stream with:
   play -1 -b8 -r 7940 -t raw -u --buffer 64 /dev/ttyS0

//Note:// bitrate 7940 is just experimental. Once the timing will be optimezed bitrate would be standard 8000 (or greater with other kinds of serial i/f). However bitrate setting doesn't prevent play from working, just changes sound pitch and introduces small time-lapse gaps.

{{:audiodevice.jpg?direct&300|}}

In the photo you can see an avrnetio proto board. Network is not used in this project.
The led shows sampling activity. Audio comes from the red clip, through a capacitor, and goes in the avr's ADC6 pin. Two resistors polarize the signal at about half range. Serial port is going to the pc.

===== Firmware =====

Firmware is very simple and not optimized. No warranty about this (open source) code:
<code>
#include <avr/io.h>
#include <avr/interrupt.h>

volatile uint8_t i;

void main(void) {

    cli();
    DDRA |= _BV(PA4)|_BV(PA5);	// as output		Debug Leds
    TCCR0 = _BV(CS01) ;            // timer setup CLK/8
    OCR0=249;
    TIMSK|=_BV(OCIE0);

	ADMUX=_BV(ADLAR);                   //adc setup
	ADMUX|=0b00000110;
	ADCSRA=_BV(ADEN)|_BV(ADIE)|_BV(ADPS1);

	UCSRA=_BV(U2X);               //uart setup
	UCSRB=_BV(TXCIE)|_BV(TXEN);
	UCSRC=_BV(URSEL)|_BV(UCSZ1)|_BV(UCSZ0);
	UBRRH=0;
	UBRRL=16;

	sei();

    while(1) {
    }

}

ISR(TIMER0_COMP_vect){				//8000 Hz
	TCNT0=0;
	PORTA |= _BV(PA4);
	ADCSRA|=(1<<ADSC);	//START CONVERSION
}

ISR(ADC_vect){
	ADCSRA &= ~(1<<ADIF);
	PORTA&=~_BV(PA4);
	PORTA|=_BV(PA5);
	i=ADCH;
	UDR=i;
}

ISR(USART_TXC_vect){
	PORTA&=~_BV(PA5);
}
</code>

===== Application =====

Possible application is VOIP device. I want to create a cheap voip device using an Aria or other embedded module, but it needs an external audio capture device.

===== Future expansion =====

  * Stream to audio signal using PWM or external R2R ladder DAC.
  * Improving of performances.