mc44251 Freescale Semiconductor, Inc, mc44251 Datasheet - Page 8

• Manufacturer Part Number: mc44251
• Description: Triple 8-bit Video Three-state Outputs
• Manufacturer: Freescale Semiconductor, Inc

DITHERING
result of the digitizing process used to convert video infor-
mation between analog and digital forms. This method can
also be applied to non–video signals. When converting ana-
log signals to a digital format, errors can be introduced
because of the limited number of discrete levels that the digi-
tal system imposes. For example, an 8–bit digital word can
describe exactly 256 discrete analog levels. In some cases,
this may not be sufficient for the application involved.
the analog signal. The disadvantages of this approach are
the increase in cost of the converter and the loss in speed
associated with the increased number of bits. The addition of
analog filters at the point where the digital information is con-
verted back to analog is also possible, but this also has its
limitations.
an analog ramp is converted to a digital form and back to
analog, the limited number of vertical samples causes the
ramp to take the form of a “stair case” (see Figure 5). The
severity of this distortion depends on the number of digital
bits that generated the ramp. The “stair case” effect is less
pronounced when the number of bits producing the ramp is
increased.
a luminance ramp is displayed, granulation errors may be-
come noticeable. This condition can also occur when one of
the RGB signals is ramped and the other two are held
constant. When processing video information using 24 bits or
less, this “granulation” can be observed. Since the video is
processed as three 8–bit signals, the maximum number of
different luminance levels is reduced to 256. In cases where
only a 7–bit luminance signal is used, the maximum number
MC44251
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Dithering can be used to reduce the errors that are the
One solution is to increase the number of bits that describe
When a ramp voltage is generated from a digital source or
When processing video information using a YUV form and
EXPECTED
ACTUAL
Figure 5. Distortion Produced by Digitizing
Figure 4. Horizontal Timing
t BP
t H
of luminance levels is reduced to 128. The uniform lumi-
nance ramp is observed on the display as vertical bars rather
than a uniform luminance change.
the input signal. This offset (when it is equal to an integer
number of LSB) is then subtracted from the digital output
data. This offset is varied “line–to–line” by one half LSB
steps. It allows each point of the waveform to be interpolated
to higher precision by averaging the differential linearity
errors on the screen.
sample at a slightly different point on the input waveform (see
Figure 6). When translated to the display, the effect is to shift
this value slightly to the left. Overall accuracy of the display is
maintained by subtracting this value from the output when
the shift is equal to an integer number of LSB. For a uniform
and continuous video waveform, the result of incrementing
and decrementing the dither voltage is to effectively double
the luminance levels from 256 to 512. The effect is to broad-
en the luminance range. This results in a more pleasing dis-
play with less visible quantization.
counter, which is incremented for every line by input HZ, and
reset by every second vertical input VTN; thus, the dithering
pattern is synchronized to the deflection of the screen. The
effect of the dithering pattern is given in Figure 7. Subse-
quent subtraction of the introduced dithering signal at the
output of each ADC is performed by a binary adder. The
dither counter, and hence the dithering function, may be
stopped by holding VTN low. Depending on the state of the
dither counter, the output code range can be either from $00
to $FE or from $01 to $FF.
code range can be either from S00 to SFE or from S01 to
SFF.
The dithering technique consists of adding a small offset to
The addition of the small offset forces the comparators to
The dithering pattern is generated by means of a binary
Depending on the state of the dither counter, the output
VIDEO
HZ
MOTOROLA