HMP8156 Intersil Corporation, HMP8156 Datasheet - Page 13

• Manufacturer Part Number: HMP8156
• Description: Ntsc/pal Encoder
• Manufacturer: Intersil Corporation

.
-10
-20
-30
-40
-50
-60
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
FIGURE 14. Cb AND Cr LOWPASS FILTER RESPONSE
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
0
0
0
0
0
0
FIGURE 13. Y LOWPASS FILTER RESPONSE
0.2
FIGURE 14A. FULL SPECTRUM
1
1
NTSC OR PAL
RECTANGULAR PIXEL
CLK2 = 27.00MHz
FIGURE 13B. PASS BAND
FIGURE 14B. PASS BAND
PAL SQUARE PIXEL
CLK2 = 29.50MHz
NTSC OR PAL
RECTANGULAR PIXEL
CLK2 = 27.00MHz
NTSC SQUARE PIXEL
CLK2 = 24.54MHz
0.4
2
NTSC SQUARE PIXEL
CLK2 = 24.54MHz
2
FREQUENCY (MHz)
FREQUENCY (MHz)
NTSC OR PAL
RECTANGULAR PIXEL
CLK2 = 27.00MHz
NTSC SQUARE PIXEL
CLK2 = 24.54MHz
FREQUENCY (MHz)
0.6
3
0.8
3
4
1.0
PAL SQUARE PIXEL
CLK2 = 29.50MHz
4
PAL SQUARE PIXEL
CLK2 = 29.50MHz
5
1.2
5
1.4
6
HMP8156
1.6
7
6
13
Chrominance Modulation
The HMP8156 uses a numerically controlled oscillator (NCO)
clocked by CLK2 and a sine look up ROM to generate the
color subcarrier. The subcarrier from the ROM is pre-scaled to
generate the proper levels for the various video standards.
Prescaling outside the CbCr data path minimizes color pro-
cessing artifacts. The HMP8156 modulates the filtered 8:8:8
chrominance data with the synthesized subcarrier.
Subcarrier Phase
The SCH phase is 0
cally over time due to residue in the NCO. In an ideal sys-
tem, zero SCH phase would be maintained forever. In reality,
this is impossible to achieve due to pixel clock frequency tol-
erances.
If enabled, the HMP8156 resets the NCO periodically to
avoid an accumulation of SCH phase error. The reset occurs
at the beginning of each field to burst phase sequence. The
sequence repeats every 4 fields for NTSC or 8 fields for PAL.
Resetting the SCH phase every four fields (NTSC) or eight
fields (PAL) avoids the accumulation of SCH phase error at
the expense of requiring any NTSC/PAL decoder after the
encoder be able to handle very minor “jumps” (up to 2
the SCH phase at the beginning of each four-field or eight-
field sequence. Most NTSC/PAL decoders are able to handle
this due to video editing requirements.
Composite Video Limiting
The HMP8156 adds the luminance and modulated chromi-
nance together with the sync, color burst, and optional blank-
ing pedestal to form the composite video data. If enabled in
the video processing register, the encoder limits the active
video so that it is always greater than one-eighth of full scale.
This corresponds to approximately one-half the sync height.
This allows the generation of “safe” video in the event non-
standard YCbCr values are input to the device.
Closed Captioning
If enabled in the auxiliary data control register, the HMP8156
generates closed captioning data on specified scan lines.
The captioning data stream includes clock run-in and start
bits followed by the captioning data. During closed caption-
ing encoding, the pixel and overlay inputs are ignored on the
scan lines containing captioning information.
The HMP8156 has two 16-bit registers containing the cap-
tioning information. Each 16-bit register is organized as two
cascaded 8-bit registers. One 16-bit register (caption 21) is
read out serially during line 18, 21 or 22; the other 16-bit reg-
ister (caption 284) is read out serially during line 281, 284 or
335. The data registers are shifted out LSB first.
The bytes may be written in any order but both must be writ-
ten within one frame time for proper operation. If the regis-
ters are not updated, the encoder resends the previously
loaded values.
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after reset but then changes monotoni-
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