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TDA3856 데이터시트(PDF) 5 Page - NXP Semiconductors |
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TDA3856 데이터시트(HTML) 5 Page - NXP Semiconductors |
5 / 17 page June 1994 5 Philips Semiconductors Product specification Quasi-split sound processor for all standards 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 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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