TDA3858 Datasheet, PDF (5/17 Pages) NXP Semiconductors – Quasi-split sound processor for all standards

Philips Semiconductors
Quasi-split sound processor for all standards
Product specification
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 C_s 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
C_S = C_P (f_VC/f_SC)² − C_P.
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
f_mod = 1 kHz; with a deviation of
±30 kHz the S/N ratio is deteriorated
by 4.5 dB.
.
June 1994
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