Technics SL-P202A - Page 40

3-1-4 The photodiode current amplifiers
and I.f. a.g.c.
The low pass filters for the photodiode
inputs D1 - D4 are integrated in the
photodiode signal processor.
The outputs for RE1 (RE1 = D1 + D2) and
RE2 (RE2 = D3 + D4) are open collector
outputs. In case an external pull-up
has to be connected, it has to be
connected to Vdd.
Normally the pull-up function is done
by the inputs of the radial error
processor.
The a.g.c. for the photodiode
amplifiers keeps the d.c. sum current
constant for all dises. The control
loop has a LPF which is set by the
value of C2507.
3-1-5 The track loss detector
During playback of a disc, the radial
error signals keep the pick-up on
track. In case of searching for a
certain spot of music, the pick-up has
to jump over the tracks. To maintain
the control of the pick-up during a
search operation, the microprocessor
must be informed whether the pick-up is
on track or off track. This function is
performed by the track-loss detector.
Condition to be met for the track-loss:
e The gain controlled h.f. signal has
to be between 12% and 62% of the
nominal value (100% is set by the
h.f. a.g.c.).
® Average light intensity of the I.f.
Signal is more than 120% of the
nominal value (100% is set by the
I.f.a.g.c.).
All values’ are set during track
following.
Use of the TL signal:
1.Counting tracks when track jumping.
2.In case a TL is detected during
normal playback the control
microprocessor looks to the RE dig
Signal from the radial error
processor and compensates the
tracking error by means of the DAC
Signal.
2b ec
D1 D2 D3 D4
sass
Le) Lo
DISC TOO FAR AWAY
io °
DISC TOO CLOSE D4 Ss
nn
Such a situation can only occur due
to large knocks. This set up makes
the system extremely tolerant for
bumps.
(Formula TL=FS>120% and 63%>HFL>12%).
3-1-6 Focus processing
The CDM4 pick-up uses two focuault
knife focus pick-ups to keep the
reading spot focused on the data layer
of the disc.
The TDA8808 has two focus outputs: FE
and FE lag.
FE lag is the basic focus error signal.
FE is a normalized version of FE to
remove the effect of the unequal
illumination of photodiode pairs D1, D2
and D3, D4 which occurs in case of
tracking errors.
PRINCIPLE
FOCUSING ERROR
(D1+D4) — (D2+D3)
pe
DISC IN FOCUS D2
FE lag= (11 + I4)-(12 + 13).Ftransfer
ve -ll_- 12 (14 - 13
“Its ID +Ty 7B’ I control
Since in focus I1 + I2 = 13 + I4
and 11*12~+13 +14 with a.g.c. on
FE = FE lag on trace
Either one signal or both signals can
be used to control the focus actuator.
The FE lag signal gives the loop the
correct gain and phase margin. The FE
signal has an important role in the
start-up procedure.