NTE1890 View Datasheet(PDF) - NTE Electronics

Part Name

Description

Manufacturer

NTE1890

Integrated Circuit NTSC Decoder for TV

NTE Electronics 

Functional Description (Cont’d):

Oscillator and ACC Detector

The 7.16MHZ reference oscillator operates at twice the subcarrier frequency. The reference signals

for the (R–Y) and (B–Y) demodulators, the burst phase detector, and ACC detector are obtained via

the divide–by–2 circuit, which provides a 90° phase shift. The oscillator is controlled by the burst

phase detector, which is gated with the narrow part of the sandcastle pulse (Pin8). As the burst phase

detector has an asymmetrical output the oscillator can be adjusted by changing the voltage of the out-

put (Pin21) via a high–ohmic resistor. The capacitor in series with the oscillator crystal must then have

a fixed value. When Pin6 (saturation control) is connected to the positive supply line the burst signal

is suppressed and the color killer is overruled. This position can therefore be used for adjustment of

the oscillator. The adjustment is visible on the screen.

The hue control is obtained by changing the phase of the input signal of the burst phase detector with

respect to the chrominance signal applied to the demodulators. This phase shift is obtained by gener-

ating a 90° shift sine–wave via a Miller integrator (biased via Pin19) which is mixed with the original

burst signal. A control circuit is required in the 90° phase shift circuit to make the chrominance voltage

independent of the hue setting. This control circuit is decoupled by a capacitor connected to Pin5.

As the shifted burst signal is synchronously demodulated in a separate ACC detector to generate the

ACC voltage, it is not affected by the hue control. The output pulses of this detector are peak detected

(Pin4) to control the gain of the chrominance amplifier, thus preventing blooming–up of the color dur-

ing weak signal reception. This ensures reliable operation of the color killer. During color killing the

color channel is blocked by switching–off saturation control and the demodulators.

Demodulators

The (R–Y) and (B–Y) demodulators are driven by the chrominance signal (Pin24) and the reference

signals from the 7.16MHZ divider circuit. The phase angle between the two reference carriers is 115°.

This is achieved by the (R–Y) demodulator receiving an additional phase shift by mixing the two sig-

nals from the divider circuit. The phase shift of 115° can be varied between 90° and 140° by changing

the bias voltage at Pin18. The demodulator output signals are fed to R and B matrix circuits and to

the (G–Y) matrix to provide the (G–Y) signal which is applied to the G matrix. The demodulator circuits

are killed and blanked by bypassing the input signals.

RGB Matrix and Amplifiers

The three matrix and amplifier circuits are identical and only one circuit will be described. The lumi-

nance and the color difference signals are added in the matrix circuit to obtain the color signal. Output

signals are 5V (p–p) (black–white) for the following nominal input signals and control settings.

D Luminance 450mV(p–p)

D Chrominance 550mV(p–p) (burst–to–chrominance ratio of the input 1: 2.2)

D Contrast–3dB max

D Saturation–6dB max

The maximum output voltage is approximately 7V(p–p). The black level of the blue channel is com-

pared with a variable external reference level (Pin12), which provides the brightness control. The

brightness control range is 1V to 3.2V. The control voltage is stored in a capacitor (connected to

Pin16) and controls the black level at the output (Pin15) between 2V and 4V, via a change of the level

of the luminance signal before matrixing.

Note

Black levels of up to approximately 6V are possible, but amplitude of the output signal is reduced to

3V(p–p).

If the output signal surpasses the level of 9V the peak–white limiter circuit becomes active and re-

duces the output signal via the contrast control.

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