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AD723 Arkusz danych(PDF) 9 Page - Analog Devices |
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AD723 Arkusz danych(HTML) 9 Page - Analog Devices |
9 / 20 page REV. 0 AD723 –9– After the luma prefilter, the bandlimited luma signal is sampled onto a set of capacitors at twice the master reference clock rate. After an appropriate delay, the data is read from the delay line, reconstructing the luma signal. The 8FSC oversampling of this delay line limits the amount of jitter in the reconstructed sync output. The clocks driving the delay line are reset once per video line during the burst flag. The output of the luma path will remain unchanged during this period and will not respond to changing RGB inputs. The reconstructed luma signal is then smoothed with a 4-pole low-pass filter. This filter has a –3 dB bandwidth of 7.5 MHz for NTSC (9 MHz for PAL), and is of a modified Bessel form with some high frequency boost introduced to compensate for Sinx/x roll-off in the sampled delay line. A final current mode buffer provides current drive for the LUMA output pin. The combined response of the luma input filter, delay line, and output filter has a bandwidth of 4.7 MHz for NTSC and 6.1 MHz for PAL. Chrominance Signal Path The chrominance path begins with the U and V color-difference matrices. The AD723 uses U and V modulation vectors for NTSC and PAL (+U being defined as 0 degrees phase), simplifying the design compared to I and Q designs. The U and V matrices com- bine the RGB inputs by the standard transformations: U = 0.493 × (B – Y) V = 0.877 × (R – Y) The Y signal in these transformations is provided by the lumi- nance matrix. Before modulation, the U and V signals are prefiltered to pre- vent aliasing. These 4-pole modified Bessel low-pass filters have a –3 dB bandwidth of 1.2 MHz for NTSC and 1.5 MHz for PAL. Between the prefilters and the modulators, the colorburst vec- tors are added to the U and V signals. The colorburst levels are defined according to the encoding standard. For NTSC, the colorburst is in the –U direction (with no V component) with a resultant amplitude of 286 mV (40 IRE) at 180 degrees phase. For PAL, the colorburst has equal parts of –U and ±V vectors (changing V phase every line) for a resultant amplitude of 300 mV alternating between 135 and 225 degrees phase. The burst gate timing is generated by waiting a certain num- ber of reference clock cycles following the falling sync edge. If the sync pulsewidth is measured to be outside the standard horizontal width, it is assumed that the device is in an h/2 period (vertical blanking interval) and the burst is suppressed. The U and V signals are used to modulate a pair of quadrature clocks (sine and cosine) at one-fourth the reference frequency input (3.579 545 MHz for NTSC, 4.433 618 MHz for PAL). For PAL operation, the phase of the cosine (V) clock is changed after each falling sync edge is detected. This will change the V-vector phase in PAL mode every horizontal line. By driving the AD723 with an odd number of sync edges per field, any indi- vidual line will flip phase each field as required by the standard. In order to suppress the carriers in the chrominance signal, the U and V modulators are balanced. Once per horizontal line the offsets in the modulators are cancelled in order to minimize residual subcarrier when the RGB inputs are equal. This offset cancellation also provides a dc restore for the U and V signal paths, so it is important that the RGB inputs be held at black level during this time. The offset cancellation occurs after each falling sync edge, approximately 8.4 µs after the falling sync edge, lasting for a period of 1.0 µs. If the inputs are unbalanced during this time (for example, if a sync-on-green RGB input were used), there will be an offset in this chrominance response of the inputs during the remainder of the horizontal line, includ- ing the colorburst. The U signal is sampled by the sine clock and the V signal is sampled by the cosine clock in the modulators, after which they are summed to form the chrominance (C) signal. The chrominance signal then passes through a final 4-pole modified Bessel low-pass filter to remove the harmonics of the switching modulation. This filter has a –3 dB frequency of 6 MHz for NTSC and 8 MHz for PAL. A final buffer provides current drive for the CRMA output pin. Composite Output To provide a composite video output, the separate (S-Video) luminance and chrominance signal paths are summed. Prior to summing, however, an optional filter tap for removing cross- color artifacts in the receiver is provided. The luminance path contains a resistor, output pin (YTRAP), and buffer prior to entering the composite summing amplifier. By connecting an inductor and capacitor on this pin, an R-L-C series-resonant circuit can be tuned to null out the luminance response at the chrominance subcarrier frequency (3.579 545 MHz for NTSC, 4.433 618 MHz for PAL). The center frequency (fC) of this filter will be determined by the external inductor and capacitor by the equation: f C = 1 2 π LC It can be seen from this equation that the center frequency of the trap is entirely dependent on external components. The ratio of center frequency to bandwidth of the notch (Q = fC/ BW) can be described by the equation: Q = 1 1000 L C When choosing the Q of the filter, it should be kept in mind that the sharper the notch, the more critical the tolerance of the components must be in order to target the subcarrier frequency. Additionally, higher Q notches will exhibit a transient response with more ringing after a luminance step. The magnitude of this ringing can be large enough to cause visible shadowing for Q values much greater than 1.5. |
Podobny numer części - AD723_15 |
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Podobny opis - AD723_15 |
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