HMP8156 Intersil Corporation, HMP8156 Datasheet - Page 14

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

The HMP8156 provides a write status bit for each captioning
line. The encoder clears the write status bit to ‘0’ when cap-
tioning is enabled and both bytes of the captioning data reg-
ister have been written. The encoder sets the write status bit
to ‘1’ after it outputs the data, indicating the registers are
ready to receive new data.
Captioning information may be enabled for either line, both
lines, or no lines. The captioning modes are summarized in
Table 8.
Analog Outputs
The HMP8156 converts the video data into analog signals
using four 10-bit DACs running at the CLK2 rate. The DACs
output a current proportional to the digital data. The full scale
output current is determined by the reference voltage VREF
and an external resistor RSET. The full scale output current
is given by
I
VREF must be chosen such that it is within the part’s operat-
ing range; RSET must be chosen such that the maximum
output current is not exceeded.
If the VREF pin is not connected, the HMP8156 provides an
internal reference voltage. Otherwise, the applied voltage
overdrives the internal reference. If an external reference is
used, it must decoupled from any power supply noise. An
example external reference circuit is shown in the Applica-
tions section.
The HMP8156 generates 1V
across 37.5 loads corresponding to doubly terminated 75
lines. The encoder may also drive larger loads. The full scale
output current and load must be chosen such that the maxi-
mum output voltage is not exceeded.
FULLSCALE
ENABLE BITS
CAPTIONING
CLOSED
00
01
10
11
(mA) = 3.6 * VREF (V)/RSET (k )
None
21 (NTSC)
18 (M PAL)
22 (Other PAL)
284 (NTSC)
281 (M PAL)
335 (Other PAL)
21, 284 (NTSC)
18, 281 (M PAL)
22, 335 (Other PAL)
OUTPUT LINE(S)
PP
nominal video signals
TABLE 8. CLOSED CAPTIONING MODES
Caption Data
Caption Data
(EQ 1.)
Ignored
Ignored
HMP8156
CAPTIONING REGISTER
284A
284B
14
Controlled Edges
The NTSC and PAL video standards specify edge rates and
rise and fall times for portions of the video waveform. The
HMP8156 automatically implements controlled edge rates
and rise and fall times on these edges:
1. Analog horizontal sync (rising and falling edges)
2. Analog vertical sync interval (rising and falling edges)
3. Color burst envelope
4. Blanking of analog active video
5. Overlay with internal mixing
6. Closed captioning information
Output DAC Filtering
Since the DACs run at 2x the pixel sample rate, the sin(x)/x
rolloff of the outputs is greatly reduced, and there are fewer
high frequency artifacts in the output spectrum. This allows
using simple analog output filters. The analog output filter
should be flat to F
Example filters are shown in the Applications section.
Composite + Y/C Output Mode
The HMP8156 provides three output modes: S-video, RGB,
and power down. When S-video outputs are selected, the
encoder outputs the luminance, modulated chrominance,
and two copies of the composite video signals. All four out-
puts are time aligned.
To reduce power dissipation, the second composite output
DAC may be turned off. The output may be disabled in the
host control register.
Composite + RGB Output Mode
When analog RGB video is selected, the HMP8156 trans-
forms the filtered 8:8:8 YCbCr data into 8:8:8 RGB data. The
transform matrix uses fixed coefficients to generate PAL
video levels for interfacing to a European SCART connector.
The encoder will not generate proper video levels if RGB
output is selected with NTSC format.
Caption Data
Caption Data
Ignored
Ignored
21A
21B
s
/4 and have good rejection at 3F
0 = Loaded
0 = Loaded
1 = Output
1 = Output
Always 1
Always 1
284
WRITE STATUS BIT
0 = Loaded
0 = Loaded
1 = Output
1 = Output
Always 1
Always 1
21
s
/4.