M494B1 datasheet(14/22 Pages) STMICROELECTRONICS

14 / 22 page

FUNCTIONAL DESCRIPTION (continued)

Manual Tuning

Manual tuning commands tune up or down, are

available from keyboard only and are provided to

allow both manual station search and tuning

adjustments. If a continuous tuning up or down

command is made from the keyboard the speed of

movement of the tuning counter is as shown in

fig. 10 for the UHF and CATV bands. Time to is the

start time for the key being pressed. When the

FB/sync. coincidence input is a logic HI the tuning

speed is reduced to 16 steps/sec. If, at time to the

FB/sync. coincidence input is at logic LO than the

tuning.sweep speed jumps immediately to 512

steps/sec.

For VHF III & I all these levels are shifted up by a

factor of 2 & 4 respectively giving slowest speeds

of 8 steps/sec. and 16 steps/sec. and highest

speeds of 1024 steps/sec and 2048 steps/sec. If

the continuity of command is broken by releasing

the keyboard for example then the tuning speed

returns to its slowest speed when the FB/sync.

coincidence input is at logic HI. If the upper or lower

limit of a band is reached during manual tuning then

tuning will continue in the same direction from the

opposite limit after a 480ms delay to allow for the

discharge of the external network.

The tuning counter is 13 bits in length giving a

range of 8192 steps. The UHF band has a band-

width of approx. 400MHz. Thus in the UHF band

the slowest speed of 4 steps/sec. gives a tuning

speed of about 200kHz/sec. The fastest speed of

512 step/sec. corresponds to a total band sweep

time of 16 seconds.

Program Memory Sequence

A continuous up/down program memory command

from keyboard produces a program change every

500ms. From remote control and data command

sources a continuous program memory sequence

command produces a program change approx.

every 500ms or every 5 received commands. A

memory sequence up or down command issued

from any source will bring the device out of standby

to the program selected before standby was com-

manded. The memory sequence up or down will

not then commence until the command is stopped

and reissued from any source (until an EOT has

been received or internally generated).

512

FB/SYNC.

COINCIDENCE

FB/SYNC.

COINCIDENCE

FB/SYNC.

COINCIDENCE

Time (sec)

123

TUNING SWEEP

SPEED (STEP/SEC)

Figure 10 : Tuning Sweep Speed (UHF & CATV BANDS)

M494

14/22

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