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AD720JP датащи(PDF) 5 Page - Analog Devices |
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AD720JP датащи(HTML) 5 Page - Analog Devices |
5 / 8 page AD720/AD721 REV. 0 –5– (continued from page 1) All required low-pass filters are on chip. After the input signals pass through a precision RGB to YUV encoding matrix, two on- chip low-pass filters limit the bandwidth of the U and V color difference signals to 1.2 MHz prior to quadrature modulation of the color subcarrier; a third low-pass filter at 3.6 MHz (NTSC) or 4.4 MHz (PAL) follows the modulators to limit the harmonic content of the output. Delays in the U and V chroma filters are matched by an on-chip sampled data delay line in the Y signal path; to prevent aliasing, prefilter at 5 MHz is included ahead of the delay line and a post filter at 5 MHz is added after the delay line to suppress harmon- ics in the output. These low-pass filters are optimized for mini- mum pulse overshoot. The overall delay is about 170 ns, which precompensates for delays in the filters used to decode the NTSC or PAL signal in a television receiver. (This precompen- sation delay is already present in TV broadcasts.) The AD720 and AD721 are available in a 28-pin plastic leaded chip carrier for the 0 °C to +70°C commercial temperature range. THEORY OF OPERATION Referring to the AD720/AD721 block diagram (Figure 8), the RGB inputs (each 0 mV to 714 mV in NTSC or 0 mV to 700 mV in PAL) are first encoded into luminance and color difference signals. The luminance signal is called the “Y” signal and the color-difference signals are called U and V. The RGB inputs are encoded into the YUV format using the transformation Y = 0.299R + 0.587G + 0.114B U = 0.493 (B-Y) V = 0.877 (R-Y) For NTSC operation, the chroma amplitude is increased by the factor 1.06 prior to summation with the luminance output. The burst signal is inserted into the Y channel in the encoding matrix. The three outputs of the encoding matrix, now transformed into Y, U, and V components, take two paths. The Y (luminance) signal is passed through a delay line consisting of a prefilter, a sampled-data delay line, and a post filter. The pre- and post-filters prevent aliasing of harmonics back into the baseband video. The overall de- lay is a nominal –170 ns relative to the chrominance signal, in keeping with broadcast requirements to compensate for delays in- troduced by the filters in the decoding process. The U and V components pass through 4-pole modified Bessel low-pass filters with a 1.2 MHz –3 dB frequency to prevent aliasing in the balanced modulators, where they modulate a 3.579 545 000 MHz (NTSC) or 4.433 618 750 MHz (PAL) signal via a pair of balanced modulators driven in quadrature by the color subcarrier. The AD720/AD721 4FSC input drives a digital divide-by-4 cir- cuit (two flip-flops) to create the quadrature signal. The refer- ence phase 0 ° is used for the U signal. In the NTSC mode, the V signal is modulated at 90 °, but in the PAL mode, the V modulation input alternates between 90 ° and 270° at half the line rate as required by the PAL standard. The outputs of the balanced modulators are summed and low-pass filtered to re- move harmonics. SYNC DECODER BURST C-SYNC 4FSC ENCD RED GREEN BLUE QUADRATURE DECODER DELAYED C-SYNC SC 90 ° SC 0 ° CLOCK AT 8FSC DC RESTORE AND C-SYNC INSERTION 5MHz 2-POLE LP POST- FILTER COMPOSITE OUTPUT* –0.572V TO 2V NTSC –0.6V TO 2V PAL LUMINANCE OUTPUT* –0.572V TO 1.43V NTSC –0.6V TO 1.4V PAL CHROMINANCE OUTPUT* 572mVp-p NTSC 600mVp-p PAL NTSC/PAL C-SYNC DELAY ±180 ° (PAL ONLY) RGB-TO-YUV ENCODING MATRIX BURST Y U V SC 90 °/270° 5MHz 4-POLE LP PRE-FILTER 1.2MHz 4-POLE LPF 1.2MHz 4-POLE LPF SAMPLED- DATA DELAY LINE ∑ 3.6MHz (NTSC) 4.4MHz (PAL) 3-POLE LPF ∑ X2 X2 X2 POWER AND GROUNDS +5V AGND DGND LOGIC ANALOG ANALOG ONLY ANALOG LOGIC +5V –5V *NOTE: THE LUMINANCE, COMPOSITE, AND CHROMINANCE OUTPUTS ARE AT TWICE NORMAL LEVELS FOR DRIVING 75 Ω REVERSE-TERMINATED LINES. ASNC NTSC/PAL BALANCED MODULATORS NTSC/PAL X2 X2 X2 ROUT 1.5Vp-p GOUT 1.5Vp-p BOUT 1.5Vp-p AD721 (ONLY) NTSC/PAL Figure 8. AD720/AD721 Functional Block Diagram |
Аналогичный номер детали - AD720JP |
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Аналогичное описание - AD720JP |
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