3D Comb Filter; Overview; What Is A 3D Comb Filter; Circuit Description - Sony TVP-08 Training Manual

3D Comb Filter

Overview

The 3D Comb Filter is used to separate the Y and C signals in a compos-
ite video signal. In this section we will discuss what a 3D Comb Filter is
and why we need to use one. In addition we will discuss the comb filter
circuit in the RA-4 chassis.

What is a 3D Comb Filter?

History
In order to produce a picture, we need to separate the Y and C signals
from the composite video signal. This is necessary to extract the sepa-
rate R, G and B signals that are needed by the CRT. In the early days of
television through the late 70's, a trap filter was used to separate the Y
and C signals. This method is now referred to as a 1D filter. This system
functioned, but severely limited the resolution of the displayed signal. It
also produced unwanted picture artifacts such as dot crawl, which are
moving dots at the edge of a black and white transition.
A new filter was used in the late 70's. This filter was called a 2D-comb
filter because it used the signal in the horizontal and vertical dimensions.
It used delay lines to look at two consecutive horizontal lines and com-
pared them. For example, if line 140 could be delayed and then com-
pared to line 141, we could get a much better horizontal resolution and a
reduction in dot crawl. However, these filters have trouble with diagonal
lines and fine details. These problems result in a loss of vertical resolu-
tion.
Recently we have overcome these limitations through the use of the 3D-
comb filter. This filter not only uses the horizontal and vertical dimen-
sions, but adds a third dimension - time.
3D Comb Filter
As mentioned above, the third dimension in a 3D-comb filter is time. This
means that not only do we compare the line above or below a horizontal
line, but we also compare that line to the corresponding line in the frame
before and after it. This means that if we were processing line 140 in
45
frame 20, we would compare it not only to line 139 and 141 in frame 20,
but also with line 140 in frames 19 and 21. This type of processing re-
quires a great deal of memory since it must be capable of storing two full
frames of video for the constant comparisons that are occurring.
One problem with a full 3D-comb filter system is motion. Motion in the
picture between frames can cause some unwanted artifacts. Therefore,
the 3D comb filter senses motion in the frames. If no motion is detected,
then the line is processed in the 3D process described above. This is
referred to as interframe processing. When there is motion in the picture,
then the 3D filter reverts back to a 2D filter which uses interline process-
ing. Since many pictures contain both still and moving segments, the 3D-
comb filter has the ability to switch back and forth between interframe and
interline processing within a frame. The end result is a picture with higher
vertical and horizontal resolution, minimized dot crawl and less noise in
the video.

Circuit Description

When the set is first powered on, a pulse from Q1359/C resets IC1306 3D
Comb Filter. Q1359 inverts the pulse sent by Q1015, the main reset
transistor. This resets all the registers and clears memory so processing
can begin. Next, using the I²C data from IC1008 Main CPU, IC1306 3D
Comb Filter sets the levels of various adjustments. Examples of these
adjustments are YNRL and CNRL. They may need to be adjusted as per
Service Bulletin 378 located in the back of the book.
IC1309 8-Bit A/D Converter
Before the composite signal is input to IC1306 3D Comb Filter, IC1309
ADC first digitizes it. The composite video signal is input at IC1309/4
from the Y buffer circuit. In addition to the video signal input, IC1309 ADC
needs a clock and clamp pulse input. The clock input is a 4fsc (14.28
MHz) sine wave signal, which is sent from IC1309/75 ALTF to IC1309/24
CLK. The clamp pulse is sent from IC1306/61 to IC1309/6 PCL. This
signal is at the H rate. The RC network connected to IC1309/1, 7 and 10
is used to set the bias for the clamp circuit. If the DC voltages, other than
those that appear below, appear at these pins, the result may be a loss of
video. The digital output from IC1309 ADC is output from pins 13 – 17
and 20 – 22 to IC1306 3D Comb Filter.