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large inductance. Parasitic layout and coil capacitance must
be considered for optimum performance. Suggested
component values are given in Table 3.
The sound AFT time constant is set by an external
capacitor that is connected from Pin 7 to ground. This
capacitor is driven by an internal 300 µA current source and
sink. The demodulated sound bandwidth is in excess of
100 kHz making this device well suited for MTS
(multi–channel television sound) stereo and SAP (second
audio program) TV applications. Sound de–emphasis is
controlled by the time constant of an internal 18 kΩ resistor
and an external capacitor that is connected from Pin 4 to
ground. The FM IF is active in PAL 1, PAL 2 and NTSC
modes, and provides 2.0 Vpp of audio at the Variable and
Constant outputs.
With the AM IF, intercarrier sound is amplified and
detected by a fully balanced exalted carrier demodulator. The
detector provides in excess of 2.0 Vpp recovered audio
output at Pin 24. An internal low pass filter is incorporated to
suppress any high frequency harmonics that may be present
at the demodulator output. The AM IF is active in both the
SECAM and NTSC modes.
Audio Input/ Audio–Video Switch
The Audio Input/Audio–Video Switch is a multifunction
input that selects the source for the audio that appears at
Pin 27, and the polarity of the video that appears at Pins 5
and 6. There are four possible modes for this input and they
are each selected by applying a specific dc voltage level to
Pin 3. Refer to Table 1 and to the circuit description for Pin 3
in Table 3. Audio 1 is intended for applications where
internally demodulated audio is present at the Variable and
Constant outputs. The Variable output can be used internal to
the TV chassis and the Constant output can be connected to
a jack for earphone or recorder use. Audio 1 is selected by
not having a dc path from Pin 3 to ground. Internally
demodulated audio (AM or FM) will appear at Pins 24 and 27,
negative video at Pin 5, and positive video at Pin 6. If there is
an ac coupled audio source present at Pin 3, it will be
internally disconnected. Audio 2 is intended for European
applications where internal and external audio sources must
be routed through the Peritel socket. Internally demodulated
audio present at the Constant output can be routed out the
Peritel socket while external audio can be routed in, ac
coupled to Pin 3, and level adjusted at Pin 1 for use within the
TV chassis. Audio 2 is selected by connecting a 22 kΩ
12
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resistor from Pin 3 to ground. Internally demodulated audio
(AM or FM) appears at Pin 24, negative video at Pin 5,
positive video at Pin 6, and the ac coupled external audio
source at Pin 3 appears at Pin 27 inverted. The audio level
into Pin 3 must be limited so that the selected mode of
operation is not changed during the peak excursions with
Audio 2 selected, and the valley excursion with Audio 1
selected. With the component values shown in Table 3, the
audio level should be limited to less than 1.1 Vrms. Video 1
and 2 modes provide a simple means to recover scrambled
video in systems that use some form of alternate line video
inversion. Descrambling is accomplished by switching
between the two video modes. Video 1 is selected by
connecting a 3.3 kΩ resistor from Pin 3 to ground. Internally
demodulated audio (AM or FM) will appear at Pins 24 and 27,
negative video at Pin 5, and positive video at Pin 6. Video 2 is
enabled when Pin 3 is grounded, usually by an IC or a
transistor that is gated on alternate or multiple lines. Internally
demodulated audio (AM or FM) appears at Pins 24 and 27,
positive video with white spot inversion at Pin 5, and negative
video at Pin 6. Note that Video 1 mode is identical to Audio 1.
Video 1 is provided so that when descrambling, Pin 3 does
not have to pass through the voltage range that selects
Audio 2. This prevents unwanted switching noise and buzz
from appearing at the audio outputs.
It should be noted that when combining the features of
Pin 3 with the Peritel socket, the TV chassis can provide the
audio and video source to drive an external monitor or video
recorder. Also an externally generated audio and video
source can be used to drive the TV chassis as a monitor.
DC Volume Control
The dc volume control consists of an electronically
controlled audio amplifier that has a range of 12 dB gain, to
60 dB attenuation. The audio output level is set by applying a
control voltage to Pin 1. This can be derived from an
electronic source such as a digital to analog converter, or a
manual source such as the wiper of a potentiometer that is
connected from V CC to ground. The potentiometer should be
20 kΩ or less. Because no audio signal is present on Pin 1,
any potential for hum and noise pickup can easily be
bypassed by connecting a capacitor from this pin to ground.
In most cases, an unshielded wire or printed circuit board
trace is all that is required to connect the variable voltage
source to the IF board.
MOTOROLA ANALOG IC DEVICE DATA
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