SAA7824 Datasheet, PDF (29/89 Pages) NXP Semiconductors – CD audio decoder, digital servo and filterless DAC with integrated pre-amp and laser control (PhonIC)

Philips Semiconductors
CD audio decoder, digital servo and filterless
DAC with integrated pre-amp and laser control
Product specification
SAA7824
7.16 Servo part
7.16.1 DIODE SIGNAL PROCESSING
The photo detector in conventional two-stage three-beam
Compact Disc systems normally contains six discrete
diodes. Four of these diodes (three for single foucault
systems) carry the Central Aperture signal (CA) while the
other two diodes (satellite diodes) carry the radial tracking
information. The CA signals are summed into an HF signal
for the decoder function and are also differentiated (after
analog-to-digital conversion) to produce the low frequency
focus control signals.
The low frequency content of the six (five if single
Foucault) photo diode inputs are converted to digital Pulse
Density Modulated (PDM) bitstreams by six Sigma-delta
ADCs. These support a range of OPUs by interfacing to
Voltage mode mechanisms and by having 16 selectable
gain ranges in two sets, one set for D1-to-D4 and the other
for R1 and R2.
Table 13 Shadow register settings to control diode voltage ranges
SHADEN BITS
01
(bank 1)
SHADOW
REGISTER
A
signal magnitude
control for diodes
D1 to D4
(LF only)
ADDRESS
1010
DATA
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
VOLTAGE (mV)
20
25
30
40
60
75
100
120
150
200
270
350
450
600
720
960
INITIAL
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
reset
2003 Oct 01
29