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HMP8156

NTSC/PAL Encoder

Intersil 

HMP8156

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.

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

CLOSED

CAPTIONING

ENABLE BITS

00

01

10

11

TABLE 8. CLOSED CAPTIONING MODES

CAPTIONING REGISTER

284A

21A

OUTPUT LINE(S)

284B

21B

None

Ignored

Ignored

21 (NTSC)

18 (M PAL)

22 (Other PAL)

Ignored

Caption Data

284 (NTSC)

281 (M PAL)

335 (Other PAL)

Caption Data

Ignored

21, 284 (NTSC)

18, 281 (M PAL)

22, 335 (Other PAL)

Caption Data

Caption Data

WRITE STATUS BIT

284

Always 1

Always 1

21

Always 1

0 = Loaded

1 = Output

0 = Loaded

1 = Output

Always 1

0 = Loaded

1 = Output

0 = Loaded

1 = Output

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

IFULLSCALE (mA) = 3.6 * VREF (V)/RSET (kΩ)

(EQ 1.)

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 1VPP nominal video signals

across 37.5Ω loads corresponding to doubly terminated 75Ω

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.

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 Fs/4 and have good rejection at 3Fs/4.

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.

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