SAF784X datasheet(11/93 Pages) NXP

11 / 93 page

SAF784X_2

Product data sheet

Rev. 02 — 9 May 2008

11 of 93

NXP Semiconductors

SAF784x

One chip CD audio device with integrated MP3/WMA decoder

6.1.2 HF acquisition

The HF data (EFM) signal is obtained by summing the signals from the three or four

central diodes of the OPU and ﬁltering and converting the signals to a digital

representation via a 6-bit HF ADC. Figure 4 shows a simpliﬁed block diagram of the HF

path.

The four diode signals D1, D2, D3 and D4 are summed in the ﬁrst RF ampliﬁer. The gain

of the ﬁrst ampliﬁer is controlled by register AGCGain[7:4].

A second gain stage has been added to lessen the gain bandwidth requirements of a

single gain stage operational ampliﬁer and also to act as dynamic Automatic Gain Control

(AGC). The gain of this ampliﬁer is set by register AGCGain[3:0] and can be changed

on-the-ﬂy from the ARM microprocessor. The gain range accommodates 12 dB of gain

needed to boost the signal when the laser tracks across a ﬁngerprint defect on the disc.

CD-R, CD-RW, and ﬁnger prints, not only reduce the AC signal amplitude compared to a

perfectly pressed disc but also reduces the DC pedestal voltage. The high-pass ﬁlter will

remove all DC present at the input, but offsets will be added by the second and third gain

stages. A 5-bit plus-sign DAC controlled by register OffsetComp[5:0] adds a current to

compensate for this offset. The current reduces in linear dB steps and follows the AC gain.

Fig 4.

HF acquisition block diagram

001aag308

20 k

Ω

RF AMP1

0 dB ... 24 dB

RFControl2[1]

HIGH-PASS

FILTER

20 k

Ω

20 k

Ω

20 k

Ω

20 k

Ω

20 k

Ω

20 k

Ω

80 k

Ω

80 k

Ω

RFControl2[2]

single-ended-to-

differential

× converter

OffsetComp[5:0]

decoded to [31:0]

HF_MON

RF AMP2

0 dB ... 12 dB

NOISE

FILTER

noise_filt_sel

RFControl2[3]

RFControl1[6:4]

RFControl1[3:0]

67.7376 MHz

from PLL

67.7376 MHz

sys_clk

RFControl2[4]

RFControl2[0]

RFControl2[1]

RFControl2[5]

rf_adc_out[5:0]

to channel decoder

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