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5) Automatic Gain Control (AGC)
The servo LS! UPD63700GF
contains a new function
which allows the microcomputer to automatically
adjust the gain of the focus and tracking servos every
time a new disc is inserted or the CD source is selected.
The block diagram of the AGC circuit is shown in figure
29. Basically,
a small disturbance signal is inserted into
the servo loop at a fixed frequency and the response of
the loop is measured via the filtered signals G1 and G2.
For a properly adjusted servo loop the amplitudes of G1
and G2 should be equal. The microcomputer reads in
these values, does a simply calculation and adjusts the
loop gain appropriately.
In order to achieve a high degree of accuracy this
adjustment is performed a number of times.
As
long
as
there
is
power
supplied
to
the
microcomputer it remembers the previous adjustment
point and uses this as a starting point. Thus, should the
system degrade with time (actuator sensitivity, dirt
_ build-up, circuit degradation etc.) the microcomputer
can follow this trend and keep the loop gain optimized.
If power to the microcomputer is removed, it forgets
the previous adjustment point and assumes a default
value.
:
&
i
ts
'
PRE AMP!
i
{C1001 |
iC
UPC2S7IGS |
4 KHz
MICROCOMPUTER
ADJUSTS GAIN
At shipping the CD player will be within 5dB of this
default and no problems should occur. For an older
player however this is not so and it is possible that
servo closure may not take place immediately.
In this
case, the microcomputer
adjusts the gain 'blind',
searching for a stable point.
In TEST MODE, the result of the AGC can be monitored.
Once tracking close (with AGC) has been performed
the set can be made to display the present value of the
gain block. The default value is displayed as ''20', which
is the value a typical PU unit, PCB & test disc would
result in. lf for some reason the loop gain had dropped
by, say, 6dB (1/2 the typical value) then the gain block
will be adjusted during AGC to twice it's default value ;
resulting in a gain of '40°. Similarly a set with a loop
gain twice the typical will display '10' as the present
gain.
Using this, it is possible to 'measure' the loop gain of
the servo without the need for any instrumentation. The
players shipped from the factory are checked with a test
disc so that the value of the gain block after AGC is
within the range 11 - 45.
PA3026
IkHz SIGNAL ON/OFF CONTROLLED
BY MICROCOMPUTER
ALMOST
PURE
©
AYIVVVVVVVV
VV
G tor G2 = AMPLITUDE
Fig.29 : AGC BLOCK DIAGRAM
1-314
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