TDA3857 Datasheet, PDF (5/14 Pages) NXP Semiconductors – Quasi-split sound processor with two FM demodulators

Philips Semiconductors
Quasi-split sound processor with two FM demodulators
Product specification
TDA3857
FUNCTIONAL DESCRIPTION
The quasi-split sound processor is suitable for all FM
standards (e. g. B/G).
B/G standard
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 17, AF2 output is muted.
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 17, under measurement conditions, AF2
is switched on (see limiting amplifier). Weak input signals
at pins 1 and 20 generate noise at pin 17, which is present
in the intercarrier signal and passes through the 5.74 MHz
filter. Noise at pin 17 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.
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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