TDA3858 datasheet(5/17 Pages) PHILIPS

номер детали	TDA3858

подробное описание детали	Quasi-split sound processor for all standards

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производитель	PHILIPS [NXP Semiconductors]

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5 / 17 page

June 1994

5

Philips Semiconductors

Product speciﬁcation

Quasi-split sound processor for all standards

TDA3858

FUNCTIONAL DESCRIPTION

The quasi-split sound processor is

suitable for all standards. Dependent

on the voltage at pin 6 either FM

mode (B/G) or AM mode (L) is

selected.

B/G standard (FM mode)

Pins 31 and 32 are active, AGC

detector uses peak sync level. Sound

carrier SC1 (5.5 MHz) provides AF1,

sound carrier SC2 (5.74 MHz)

provides AF2.

Muting

With no sound carrier SC2 at pin 25,

AF2 output is muted (in mid-position

of the standard select switch FM

mode without muting of AF2 is

selected).

The mute circuit prevents false signal

recognition in the stereo decoder at

high IF signal levels when no second

sound carrier exists (mono) and an

AF signal is present in the

identification signal frequency range.

With 1 mV at pin 25, under

measurement conditions, AF2 is

switched on (see limiting amplifier).

Weak input signals at pins 31 and 32

generate noise at pin 25, which is

present in the intercarrier signal and

passes through the 5.74 MHz filter.

Noise at pin 25 inhibits muting. No

misinterpretation due to white noise

occurs in the stereo decoder, when

non-correlated noise masks the

identification signal frequencies,

which may be present in sustained

tone signals. The stereo decoder

remains switched to mono.

L standard (AM mode)

Pins 1 and 2 are active, AGC detector

uses mean signal level. The audio

signal from the AM demodulator is

output on AF1, with AF2 output

muted.

Sound carrier notch ﬁlter for an

improved intercarrier buzz

The series capacitor Cs in the

38.9 MHz resonant circuit provides a

notch at the sound carrier frequency

in order to provide more attenuation

for the sound carrier in the vision

carrier reference channel. The ratio of

parallel/series capacitor depends on

the ratio of VC/SC frequency and has

to be adapted to other TV

transmission standards if necessary,

according to the formula

CS =CP (fVC/fSC)2 − CP.

The result is an improved intercarrier

buzz (up to 10 dB improvement in

sound channel 2 with 250 kHz video

modulation for B/G stereo) or

suppression of 350 kHz video

modulated beat frequency in the

digitally-modulated NICAM

subcarrier.

Intercarrier buzz ﬁne tuning with

250 kHz square wave video

modulation

The picture carrier for quadrature

demodulation in the intercarrier mixer

is not exactly 90 degrees due to the

shift variation in the integrated phase

shift network. The tuning of the LC

reference circuit to provide optimal

video suppression at the intercarrier

output is not the same as that to

provide optimal intercarrier buzz

suppression. In order to optimize the

AF signal performance, a fine tuning

for the optimal S/N at the sound

channel 2 (from 5.74 MHz) may be

performed with a 250 kHz square

wave video modulation.

Measurements at the

demodulators

For all signal-to-noise measurements

the generator must meet the following

specifications:

phase modulation errors

< 0.5° for

B/W-jumps

intercarrier signal-to-noise ratio as

measured with ‘TV-demodulator

AMF2’ (weighted S/N) must

be

> 60 dB at 6 kHz sine wave

modulation of the B/W-signal.

Signal-to-noise ratios are measured

with

∆f = ±50 kHz deviation and

fmod = 1 kHz; with a deviation of

±30 kHz the S/N ratio is deteriorated

by 4.5 dB.

.

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