Data Download Transmit Path; Stereo Audio Path - Motorola W208 Manual

W208 Level 3 Circuit Description
controlled by the Baseband serial port (BSP) or the MCU serial port (USP) via the serial
interfaces. The VUL path can be powered down by Program.
Figure 5: Voice Uplink Paths

2.2 Data Download Transmit Path

The External download cable is connected to the Earphone Jack J602 Pin 3, the headset
connector of the mobile phone. The download path is routed from J602 Pin 3 via U605
Pin 1 and U608 (Level shifter) Pin C2 to TX_Modem. The TX_Modem signal connects
to Locosto-Plus IC U101 Pin P3 to provide this capability. When software is set to
download mode, the signal HS_EN (U101 Pin T3) is applied low, the phone will entered to
download state till download cable pull out.

2.3 Stereo Audio Path

The stereo audio path receives Left and right signal samples at the rate of a programmable
frequency, from 8kHz to 48kHz, via the I2S serial interface and converts them to analog
signals to drive the external audio signal or speech transducers.
The digital audio signal is first fed to an audio digital filter that has two functions. The first
function is to interpolate the input signal and to increase the sampling rate to allow the
digital–to–analog conversion to be performed by an over-sampling digital modulator. The
second function is to band–limit the audio signal with a low–pass transfer functions. The
interpolated and band–limited signal is fed to a second order Σ-∆ digital modulator
sampled at fS1 frequency to generate a 4–bit (9 levels) over-sampled signal. This signal is
then passed through a dynamic element matching block and then to a 4–bit
digital–to–analog converter (DAC).
Due to the over-sampling conversion, the analog signal obtained at the output of the 4–bit
DAC is mixed with a high frequency noise. Because a 4–bit digital output is used, a
first–order RC filter (included in the output stage) is enough to filter this noise.
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