TDA3856 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
TDA3856
FUNCTIONAL DESCRIPTION
The quasi-split sound processor is suitable for all
standards.
Dependent on the voltage at pin 5 either FM mode (B/G)
or AM mode (L) is selected.
B/G standard (FM mode)
Pins 23 and 24 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 19, 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 19, under measurement conditions, AF2
is switched on (see limiting amplifier). Weak input signals
at pins 23 and 24 generate noise at pin 19, which is
present in the intercarrier signal and passes through the
5.74 MHz filter. Noise at pin 19 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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