SDA6000 Datasheet, PDF (336/380 Pages) Infineon Technologies AG – Teletext Decoder with Embedded 16-bit Controller

SDA 6000
Slicer and Acquisition
12.2.1 Distortion Processing
For the full service slicer the digital bit stream is applied to a circuitry which corrects
transmission distortion. In order to apply the correct counter-measures, a signal
evaluation is done in parallel. This measurement device can detect the following
distortions.
Noise
The noise measurement unit incorporates two different algorithms. Both algorithm are
using the value between two equalizing pulses which corresponds to the black level. As
the black level is known to the system a window is placed between two equalizing pulses
of line four. The first algorithm compares successive samples inside a window placed in
line 4. The difference between this samples is measured and a flag is set as soon as this
difference over several TV lines is greater than a specified value. This algorithm is able
to detect higher frequency noise (e.g. with noise). The second algorithm measures the
difference between the black value and the actual sampled value inside this window. As
soon as this difference over several TV lines is greater than a specified value a second
flag is set. This algorithm is sensitive against low frequency noise as it is known from co-
channel distortion. Both flags can be used to optimize the correcting circuit characteristic
in order to achieve best reception performance.
Frequency Attenuation
During signal transmission the CVBS can be attenuated severely. This attenuation
normally is frequency depending. That means that the higher the frequency the stronger
the attenuation. As the clock-run-in (from now on CRI) for teletext represents the highest
possible frequency (3.5 MHz) it can be used to measure the attenuation. As only strong
negative attenuation causes problems during data slicing a flag is needed to notify highly
negative attenuation. If this flag is set a special peaking filter is switched on in the data-
path.
Group Delay
Quite often the data stream is corrupted because of group delay distortion introduced by
the transmission channel. The teletext framing code (E4_H) is used as a reference for
measurement. The delay of the edges inside this code can be used to measure the
group delay distortion. The measurement is done every teletext line and filtered over
several lines. It can be detected whether the signal has positive, negative or no group
delay distortions. Two flags are set accordingly. By means of these two flags an allpass
contained in the correcting circuit is configured to compensate the positive or negative
group delays. All of the above filters ca be individually disabled, forced or set to an
automatic mode via control registers.
Note: Slicer 2 does not have any compensation circuits.
Users Manual
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2000-06-15