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DS70297A-page 19

This chapter describes a simple program that demonstrates how to use the Audio 
PICtail

 

Plus Daughter Board for speech capture and loopback without the use of a 

codec. Topics covered include:
• Speech Loopback Demo
• Running the Demo

The CD that accompanies the Audio PICtail

 

Plus Daughter Board contains a Speech 

Loopback Demo application. As shown in Figure 3-1, this sample application uses the 
Audio PICtail

 

Plus Daughter Board to capture an input microphone signal and deliver 

the captured signal to the dsPIC33F device on the Explorer 16 board for audio 
processing. The application program running on the dsPIC33F or PIC24H then:
• Compresses the incoming digital signal from 16 bits to 8 bits using the G.711 

µ-law encoding algorithm

• Decompresses the 8-bit signal back to 16 bits using the G.711 µ-law decoding 

algorithm

• Outputs the decompressed signal to the OC PWM module, where it is converted 

to a Pulse-Width Modulated signal. 

The Audio PICtail

 

Plus Daughter Board then processes the PWM signal through the 

PWM filter to produce an analog audio signal. This signal is amplified and output 
through the speaker.
As a result, audio spoken into the microphone is digitally processed without the use of 
a codec device and played back through the speaker.

G.711

µ-LAW

ENCODE

12-BIT

ADC

OC

PWM

dsPIC33F

SPEECH INPUT

CAPTURE

SPEECH OUTPUT

PROCESSING

G.711

µ-LAW

DECODE

OR

PIC24H