LG MG200 Service Manual page 29

3. H/W CIRCUIT DESCRIPTION
3.5.4 Audio Downlink processing
The VDL path receives speech samples at the rate of 8 kHz from the DSP via the VSP and
converts them to analog signals to drive the external speech transducer. The digital speech coming
from the DSP is first fed to a speech digital filter that has two functions. The first function is to
interpolate the input si gnal and to increase the sampling rate from 8 kHz up to 40 kHz to allow the
digital-to-analog conversion to be performed by an oversampling digital modulator. The second
function is to band-limit the speech signal with both low-pass and high-pass transfer functions. The
filter, the PGA gain, and the volumegain can be bypassed by programming bit 9 (VFBYP) in the
voiceband control register 1. The interpolated and band-limited signal is fed to a second order
digital modulator sampled at 1 MHz to generate a 4-bit (9 levels) oversampled signal. This
signal is then passed through a dynamic element matching block and then to a 4-bit digital-to-
analog converter (DAC). The volume control and the programmable gain are performed in the
voiceband digital filter. Volume control is performed in steps of 6 dB from 0 dB to -24 dB. In mute
state, attenuation is higher than 40 dB. A fine adjustment of gain is possible from -6 dB to +6 dB
in 1-dB steps to calibrate the system depending on the earphone characteristics. This configuration
is programmed with the voiceband downlink control register.
Auxiliary
AUXOP AUXOP
Amplifier
AUXON AUXON
-5 dB
Earphone Earphone
EARP EARP
Amplifier
Amplifier
EARN
1 dB
1 dB
Headset
Headset
HSO
Amplifier Amplifier
-5 dB
-5 dB
4-Bit Output 4-Bit Output
DAC and
LPF
Modulator Modulator
fs 1 = 1 MHz fs 1 = 1 MHz
Figure 14. Downlink Path
- 28 -
Side Tone
From Uplink
Receive PGA Downlink
(-6 dB to 6dB) Bandpass
1-dB Step Filter IIR
fs 2 = 40 KHz fs 3 = 8 KHz
From Voice
Volume
Serial
Control
Interface