User`s guide

MPLAB Starter Kit for dsPIC

DSCs User’s Guide

The amplifier has a variable gain from 3 db to 23 db, which can be adjusted to control

microphone sensitivity or boost a low line-input signal. The output of the amplifier is

biased at 1.65V.

4.1.2 Speech Playback

The mainstream speech playback interface processes the PWM digital signal from the

Output Compare module of the dsPIC33F device. A low-pass filter demodulates the

PWM signal as shown in Figure 4-2. The low-pass filter behaves like an integrator

whose output signal amplitude depends on the duty cycle of the input PWM waveform.

The PWM frequency should be an integral multiple of the audio sampling rate.

FIGURE 4-2: PWM DEMODULATION

The output of the low-pass filter feeds the headphone amplifier The headphone

amplifier drives an audio headphone. This amplifier can drive up to 75 mW into a 32

ohm headphone. The amplifier uses a digital volume control that is controlled by I/O

lines from the dsPIC33F device.

4.1.3 Codec

The audio codec can be used for a higher-end audio application. The input to the audio

codec is the output of the line/microphone pre-amplifier. The output feeds the

headphone amplifier. The codec must interact with the application program running on

the dsPIC33F device. Commands from the application program control the codec

operating parameters (such as communication protocol, sampling rate, volume control,

level control, filter settings, etc.). Command information is exchanged over the

Inter-Integrated Circuit™ (I

C™) module on the device.

The codec converts the incoming audio signal to a digital signal for the Digital

Converter Interface (DCI) module of the dsPIC33F device. Audio output from the

application program is sent to the codec via the DCI module. The codec converts this

digital signal to audio for the headphone amplifiers.

4.1.4 4 Mb Serial Flash Memory

The starter kit includes 4 Mb serial Flash memory that can be used for storing data. The

memory interfaces with the SPI bus on dsPIC33F device and might typically be used

by applications that require storage of speech samples for playback purposes.

PWM SIGNAL FROM OCPWM LOW-PASS FILTER DEMODULATED AUDIO SIGNAL