Rainbow Electronics MAX7470 - Manual (Page 8)

MAX7469/MAX7470
HDTV Continuously Variable
Anti-Aliasing Filters
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Use the high level for bipolar signals, such as C (chro-
ma) or P
b /P r . See Table 1 for more details.
Component/Composite Selection
The MAX7469/MAX7470 accept component or com-
posite inputs. When configured for composite video
inputs, the color-burst filter is enabled; if configured for
component video inputs, the color-burst filter is dis-
abled. This filter is separate from the main filter and not
in the direct signal path so that it has no effect on the
overall frequency response. With normal video signals
and levels, the use of this color-burst filter has a negligi-
ble effect on the sync detection. It has a more signifi-
cant effect under conditions of low-signal amplitude
coupled with higher relative amplitude color burst.
External Sync Detection (EXTSYNC)
When filtering a video signal without embedded sync
information, such as computer formats (RGBHV) with
separate sync signals, use the external sync mode (see
Table 3) and apply the horizontal sync source to the
EXTSYNC pin. The sync detector determines when the
clamp circuit is turned on.
The MAX7469/MAX7470 are able to detect positive or
negative polarity external syncs with TTL logic levels.
Use the I
2 C interface to program the polarity of the
external sync signal.
Filter
The internal video filter delivers an optimized response
with a steep transition band to achieve a wide pass-
band along with excellent stopband rejection. In addi-
tion, the filter is optimized to provide an excellent time
domain response with low overshoot.
Setting the Filter Frequency
Use the I 2 C interface to vary the frequency response
(-3dB cutoff frequency) of the filter in the MAX7469/
MAX7470 from less than the SD passband to beyond the
HD passband in 256 linear steps. Write command byte
12h to access the frequency register, followed by an 8-bit
data word that corresponds to the desired frequency. See
the
Frequency Register section for more details.
The frequency set by the MAX7469/MAX7470 is the
-3dB point. Set the frequency according to the desired
flat passband response.
Optimizing the Frequency Response
Select the frequency response according to the resolution
of the video-signal format. High-definition signals require
higher bandwidth, while standard-definition signals
require less bandwidth. The actual bandwidth contained
in the video signal is a function of the visual resolution of
the signal. This bandwidth is typically less than what is
indicated by the format resolution (1080i, 720p, etc.). For
more information, see Maxim Application Note 750:
Bandwidth Versus Video Resolution , which is available on
www.maxim-ic.com.
The frequency response can be optimized to improve
the overall performance. It is important, at a minimum, to
meet the Nyquist criterion. Beyond this, the frequency
response can be further optimized. In oversampled sys-
tems, the sample rate is significantly more than the
desired passband response. The extra frequency span
between the passband and the sample rate contains
noise and other undesirable interferers that can be elimi-
nated by setting the corner frequency of the filter to just
pass the desired bandwidth. This results in a higher sig-
nal-to-noise ratio of the overall system.
Filter Bypass
The MAX7469/MAX7470 offer selectable filter bypass-
ing that allows the input video signals to bypass the
internal filters and reach the output buffers unfiltered.
Write the appropriate command byte to enable (0Eh) or
disable (0Fh) filter-bypass mode as shown Table 3.
Output Buffer
Each output buffer can drive a 2V P-P signal into a 150Ω
video load. The MAX7469/MAX7470 can drive a DC- or
AC-coupled load. Output AC-coupling capacitors can
be eliminated when driving a cable, thereby eliminating
the normal adverse effects caused by these large
capacitors, such as line, and field-time distortion, also
known as droop. The output DC level is controlled to
limit the DC voltage on the cable so that the blanking
level of the video signal is always less than 1V, meeting
digital TV specification. See the
Output Considerations
section for more information.
Gain Options
The MAX7469 features an overall gain of 0dB, while the
MAX7470 features an overall gain of +6dB. Use the
MAX7470 when driving a back-matched cable and the
Table 1. Clamp Levels
CLAMP LEVEL INPUT SIGNAL
FORMAT CHANNEL 1 CHANNEL 2 CHANNEL 3
Y P b P r Low High High
G S BR Low High High
CVBS Y C Low Low High
Y P b P r (sync on
all signals)
Low Low Low
R G B H V High High High