LG CG225 Service Manual page 30

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 signal 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 volume gain 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.
AUXOP
Auxiliary
Amplifier
5 dB
AUXON
EARP
Earphone
Amplifier
EARN
1 dB
Headset
Amplifier
HSO
5 dB
4-Bit Output
DAC and
∑-∆
LPF
Modulator
fs1 = 1 MHz
Figure 12. Downlink Path
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3. H/W Circuit Description
Side Tone
From Uplink
Receive PGA
Downlink
( 6 dB
Bandpass
to 6 dB)
Filter IIR
1-dB Step
fs2 = 40 kHz
From Voice
Volume
Serial
Control
Interface
fs3 = 8 kHz