SEARCH MODES

4 modes are available :

a) Automatic search

(digiatl AFT)

b) Automatic preset

c) Manual up/down (digital and linear AFT)

d) Manual up/down (linear AFT)

a) AUTOMATIC SEARCH. The search starts from

the actual tuning and band position. During the

search the tuning voltage is always changing from

lower to higher voltage levels. When the end of the

band is reached the search restarts from the begin-

ning of the next band after a 480 ms interruption

with the sequence of step-by-step band selection.

Disabled bands are automatically skipped.

The search is stopped when the first station is

found or if a channel selection command is given.

Stop of the automatic search is determined by the

STOP/AFT inputs controlled by the TDA4433 which

converts the AFC-S-curve into an up/down com-

mand.

At the end of the search the up/down command

controls the correct tuning acting on the counter of

the voltage synthesizer (Digital AFT).

It is important to call the attention to the Digital AFT

capture range which is larger than the normal linear

AFT as shown in fig. 14.

LINEAR AFT

CAPTURE RANGE

DIGITAL AFT

CAPTURE RANGE

Figure 14

Additionally the use of the Digital AFT allows stor-

age of the tuning information corresponding to the

zero point of the AFC-S-curve. This cannot be

guaranteed using the Linear AFT method only. The

latter is a cheaper system, because it does not

require the use of the TDA4433 but it cannot guar-

antee what described above.

As a result of the use of the Digital AFT, the require-

ments for stability of the tuner, of the reference

voltage source and of stability of the D/A converter

are less critical.

Tuning speed in automatic search, if no station is

found is :

VHF I

8 second

VHF III

8 second

UHF

32 second

CATV

32 second

The tuning and band information can be stored

using the store/memory addressing command.

The search can be stopped by a memory selection

command.

b) AUTOMATIC PRESET. The search starts from

the lowest memory address, tuning voltage and

VHF I band as described in automatic search

mode.

When an active station is encountered, the corre-

sponding tuning and band information is automat-

ically stored in the Non-Volatile Memory.

Afterwards the system starts to search for the next

station. The cycle is repeated until all bands have

been scanned or the tuning information has been

stored into all address locations. After completing

this cycle the system reads out the tuning informa-

tion of the lowest address.

c) MANUAL UP/DOWN WITH DIGITAL AND LIN-

commands pressed, the tuning voltage is in-

creased or decreased.

During this operation, the Digital AFT is automat-

ically defeated and can only be reconnected with

the ”AFT on” command or by an Automatic search

or preset command.

The search speed is kept at minimum (there is no

increment with the time)

Band

Sweep Timefor the

Complete Band

Number of Tuning

Steps/Second

VHF I

VHF III

UHF

CATV

seconds

128 seconds

128 seconds

512 seconds

512

16

64

64

16

In case of command received from remote control,

the counter is increased/decreased every two re-

ceived commands.

No band switching is provided at the upper or lower

tuning positions.

The volume is automatically muted 3 second after

the key pressure is immediately restored at the

release of the key.

d) MANUAL UP/DOWN WITH LINEAR AFT (pin 17

is disabled.

The Linear AFT output goes low after an up or down

command is issued and remains Low for 1 second

after the release of the key.

M491B

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