SDA9280 Datasheet, PDF (17/43 Pages) Siemens Semiconductor Group – ICs for Consumer Electronics

SDA 9280 B22
An example of such non-harmonic distortions are periodic stripes with a frequency of
900 kHz appearing in a 4.8 MHz test pattern which is sampled with 13.5 MHz clock
frequency (2 _* 4.8 MHz – (13.5 – 4.8) MHz = 900 kHz).
The ideal postfiltering comprises an ideal lowpass filter with an edge frequency at the
maximum signal frequency and a stop band rejection of at least 30 or 40 dB. In practice
the postfilter can be greatly simplified when a large transition band is allowed. For this
purpose a digital interpolator is implemented with a steep transition at the half of the
sampling frequency and an out of band rejection of more than 30 dB before
D/A conversion. Combined with a two-fold oversampling the first image appears around
twice the sampling frequency, thus leaving considerably more space for the transition
band of an analog postfilter. There is another good reason for using a digital
interpolation. Since the output frequency may vary with different compression or
expansion factors an analog filter with varying edge frequencies is necessary. This
requirement can only be fulfilled in the digital domain because the edge frequency is
linearly controlled by the sample frequency.
The amplification factor of the implemented interpolation filter is 65/64. The maximal
output clock frequency is 8/3 times of the input sample frequency. The diagram
(figure 6) shows the frequency response.
Oversampling can be switched off (Ι²C signal: OVSAMP). Then the 4:4:4 format is
directly D/A converted. With activated oversampling it is possible to switch off
oversampling filtering (Ι²C signal: OVFILT). In this operation mode the input clock
frequency is doubled but each sample is simply repeated twice.
2.8 Insertion Facilities
Three different values are inserted into the video signal: black level, a colored
background area and an arbitrary colored pattern.
The blanking interval of the input signal is not processed by the compander. Therefore
the black level shifting in the luminance signal, caused by the peaking filter (coefficient
LCOF) and the amplification factor of the oversampling filter, has to be restored by
inserting the correct value (BLACK). BLACK is programmable and must be computed
according to the coding of the input data using the following formulas:
BLACK = 128 + 65/128 _* LCOF _* (BLACKIN – 128) for positive dual coding
BLACK = 128 + 65/128 _* LCOF _* BLACKIN
for 2’s complement
BLACKIN is the black level of the input signal, LCOF is the Lowpass coefficient of the
Luminance Peaking Filter: 0 ... [1/4] ... 1.5, 2
Black level insertion is controlled by the external signal HS. This signal also controls the
deflection circuit, consequently it has a stable phase referring to the horizontal blanking
interval. The value BLACK is inserted during 80 clock periods of the clock CLL. In the
Siemens MEGAVISION^® System HS is supplied by the MSC-circuit. To adjust the right
insertion phase a programmable delay of HS is available (Ι²C signal: HSDEL).
Semiconductor Group
17
1998-02-01