LG -T500 Service Manual page 54

3.12.2 Digital Part
The digital part of the X-GOLD™215 audio front-end comprises an interface to the TEAKLite® bus, interfaces to
the interrupt units of TEAKLite®, digital interpolation filters for oversampling digital-to-analog conversion,
digital decimation filters for analog-to-digital conversion and an interface to the analog part of the audio front-
end. For the digital microphone all the filtering is done in a dedicated hardware. The output sample stream is
then fed in a duplicated ring buffer structure like the data from the analog microphone path (after A/D
conversion and subsequent digital filtering).
Interpolation Filter
The interpolation path of the X-GOLD™215 audio front-end increases the sampling rate of the audio samples to
the rate of the digital-to-analog converter. Because the input sampling rates can vary between 8 kHz and
47.619 kHz the filter characteristic and oversampling ratio can be adjusted to the respective sampling rate. The
requirements for the interpolation filters depend on the sampling rate, because a sufficient out-of-band
discrimination in the audio frequency band (20 Hz,...,20 kHz) has to be ensured.
Decimation Filter
The digital decimation filter on X-GOLD™215 has two operating modes: 8 kHz output sampling rate and 16 kHz
output sampling rate (or 16 kHz output sample rate and 16kHz bandwidth in case of doubled ASMD clock).
3.12.3 Analog Part
The analog part of the X-GOLD™215 audio front-end in audio-out direction consists of a stereo digital to analog
converter (multi-bit oversampling converter) which transforms the output of the digital interpolation filter into
analog signals. It is followed by the gain control/amplifier section. The DAC outputs can be switched to several
output buffers. In audio-in section there is an input multiplexer which selects either one of two differential
microphone inputs to be connected to the low-noise amplifier and analog pre-filter. The signals from the
analog pre-filter are input to a second-order sigma-delta analog-to-digital converter. In addition there is a
connection for FM-radio playing.
Audio-out Part
The analog audio-out part consists of two multi-bit digital-to-analogue converters (DAC) and an output stage.
The signal sources are switched to the output drivers in the output stage. The output drivers consist of: a) one
mono, differential class-D Loudspeaker driver, b) one mono, differential Earpiece driver and c) one stereo,
single-ended (with uni- or bipolar signals), Headset driver.
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3. TECHNICAL BRIEF
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