Datasheet

Audio PICtail
™
Plus Daughter Board User’s Guide
DS70297A-page 12 © 2007 Microchip Technology Inc.
2.1.2 Speech Playback
The mainstream speech playback interface processes the pulse-width modulated
digital signal from the Output Compare PWM module of the dsPIC33F or PIC24H
device. A low-pass filter demodulates the PWM signal as shown in Figure 2-2. 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 2-2: PWM DEMODULATION
The output of the low-pass filter feeds both the Line Output and Headphone amplifiers
(through Output Select Jumper J4). The Line Output amplifier drives external audio
recording equipment or an external audio power amplifier. The amplifier is an AC
inverting amplifier with adjustable gain. The gain can be adjusted to match the input
level requirements of the external audio equipment, and can be varied from -1 dB to
10 dB.
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 the clock switch and the control switch.
Volume is increased by pressing and releasing the clock switch while holding the
control switch down. Volume is decreased by pressing and releasing the clock switch
without operating the control switch.
2.1.3 Codec
The optional codec is supported by dsPIC33F devices only. The input is fixed to the
amplified incoming signal from the Line/Microphone amplifier. The output feeds both
the Line Output and Headphone amplifiers (through Output Select Jumper J4).
The codec must interact with the application program running on the dsPIC33F.
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
2
C)
module on the dsPIC33F.
The codec converts the incoming audio signal to a digital signal for the Digital
Converter Interface (DCI) module of the dsPIC33F. 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 Line Output and Headphone amplifiers.
2.1.4 DAC Differential Amplifier
The DAC Differential Amplifier allows theAudio PICtail Plus Daughter Board to interface
with the DAC module on dsPIC33F devices. Audio output from the DAC module is a
pair of differential analog signals. The DAC Differential amplifier converts this
complementary signal pair into a single-ended audio signal for the Microphone and
Line Output amplifiers. The DAC Differential amplifier provides 0 dB gain.
C
R
PWM SIGNAL FROM OCPWM LOW-PASS FILTER DEMODULATED AUDIO SIGNAL