W681307 Datasheet, PDF (117/160 Pages) Winbond – USB1.1 CODEC Microprocessor Control Unit with 32KB Mask ROM and 4KB RAM

14.9
The PCM CODEC
14.9.1
Block Diagram
Figure 14-6 shows the block diagram of the speech CODEC-filter.
W681307
Figure 14-6 The block diagram of the PCM CODEC-Filter
14.9.2
Analog Interface and Signal Path
The built in linear 14-bit PCM CODEC-filter uses Σ∆ technology. There are two paths in the block, a transmit path and a receive path.
14.9.2.1
Transmit Path in Σ∆ CODEC-Filter
An analog signal input, from a microphone interface, is passed to three terminal operational amplifiers (TI+, TI-, TG) driving a typical 2
KΩ load externally to amplify the input analog signal. The modulator block over samples the analog signal at 1.536 MHz with one bit
resolution. The next anti-aliasing decimation filter reduces the sampling frequency from 1.536 MHz (1 bit) to 32 KHz (15 bit). Digital bi-
quad filters perform the decimation from 32K to 8 KHz and CCITT low-pass filtering at 3400 Hz. The digital HPF block performs the
high-pass filtering at 300 Hz. In the final step, the 14 bit A/D conversed data is sent by the transmit path to the DSP engine for further
signal processing.
14.9.2.2
Receive Path in Σ∆ CODEC-Filter
A 14-bit linear digital signal from the DSP engine is first passed to the digital anti-aliasing interpolation filter block. The interpolation
block performs the reverse operation of the decimation filter (described above in the transmit path) and the sampling rate will be increased
from 8 KHz (14 bits) to 1.536 MHz (14 bits). The digital demodulator will then reduce the 14-bit samples (1.024 MHz) to 1 bit (1.536
MHz). The digital output signal will be passed to a 3400 Hz switched capacitor low-pass filter with sin(x)/x correction and an analog
smoothing filter to reduce the spectral components of the switched capacitor filter. Finally, the analog output signal is sent to the unit gain
power amplifier RO, which is capable of driving a 2 KΩ load connected to the VAG pin.
The last stage of the received path is a pair of power driver PO1- (PO2-) and PO1+ (PO2+) which is connected in a push-pull (differential)
configuration. The PO driver can accommodate large gain ranges by adjusting two external resistors for applications such as driving a
handset receiver (or a speaker). This differential circuit is capable of driving a 120 Ω (16Ω) load.
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Publication Release Date: May, 2007
Revision 1.3