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HMP8115CN

NTSC/PAL Video Decoder

Intersil 

HMP8115

square or rectangular pixels are output. The output format is

4:2:2 for all modes except the RGB modes which use a 4:4:4

output format.

CLK2 INPUT

Note that the color subcarrier is derived from CLK2. Any jitter

on CLK2 will be transferred to the color subcarrier, resulting

in color changes. Thus, CLK2 should be derived from a sta-

ble clock source, such as a crystal. The use of a PLL to gen-

erate CLK2 is not recommended. CLK2 must have a 50ppm

accuracy and at least a 60/40% duty cycle to ensure proper

operation.

The CLK2 clock rate must be one of the following frequencies:

24.54MHz

27.00MHz

29.50MHz

The frequency of CLK2 must be 2x the desired output sam-

ple rate. The values in Table 1 below indicate the CLK2 clock

rate based on the video standard and pixel mode. The out-

put sample rate for the given video standard and pixel mode

is half the CLK2 clock rate.

TABLE 1. VIDEO STANDARD CLOCK RATE SELECTION

SUMMARY

ALLOWABLE CLK2

FREQUENCIES (MHz)

VIDEO FORMAT

RECTANGULAR

PIXEL MODE

SQUARE

PIXEL MODE

(M) NTSC

27.00

24.54

(B, D, G, H, I, N) PAL

27.00

29.50

(M) PAL

27.00

24.54

(NC) PAL

27.00

29.50

Digital Processing of Video

Once the luma and chroma have been separated the

HMP8115 then performs programmable modifications (i.e.

contrast, coring, color space conversions, color AGC, etc.) to

the decoded video signal.

UV TO CbCr CONVERSION

The baseband U and V signals are scaled and offset to gen-

erate a nominal range of 16-240 for both the Cb and Cr data.

DIGITAL COLOR GAIN CONTROL

There are four types of color gain control modes available:

no gain control, automatic gain control, fixed gain control,

and freeze automatic gain control.

If “no gain control” is selected, the amplitude of the color dif-

ference signals (CbCr) is not modified, regardless of varia-

tions in the color burst amplitude. Thus, a gain of 1x is

always used for Cb and Cr.

If “automatic gain control” is selected, the amplitude of the

color difference signals (CbCr) is compensated for variations

in the color burst amplitude. The burst amplitude is averaged

with the two previous lines having a color burst to limit line-

to-line variations. A gain of 0.5x to 4x is used for Cb and Cr.

If “fixed gain control” is selected, the amplitude of the color

difference signals (CbCr) is multiplied by a constant, regard-

less of variations in the color burst amplitude. The constant

gain value is specified by the COLOR GAIN register 1CH. A

gain of 0.5x to 4x is used for Cb and Cr. Limiting the gain to

4x limits the amount of amplified noise.

If “freeze automatic gain control” is selected, the amplitude

of the color difference signals (CbCr) is multiplied by a con-

stant. This constant is the value the AGC circuitry generated

when the “freeze automatic gain” command was selected.

COLOR KILLER

If “enable color killer” is selected, the color output is turned

off when the running average of the color burst amplitude is

below approximately 25% of nominal for four consecutive

fields. When the running average of the color burst ampli-

tude is above approximately 25% of nominal for four consec-

utive fields, the color output is turned on. The color output is

also turned off when excessive phase error of the chroma

PLL is present.

If “force color off” is selected, color information is never

present on the outputs.

If “force color on” is selected, color information is present on

the outputs regardless of the color burst amplitude or

chroma PLL phase error.

Y PROCESSING

The black level is subtracted from the luminance data to

remove sync and any blanking pedestal information. Nega-

tive values of Y are supported at this point to allow proper

decoding of “below black” luminance levels.

Scaling is done to position black at 8-bit code 0 and white at

8-bit code 219.

A chroma trap filter may be used to remove any residual

color subcarrier from the luminance data. The center fre-

quency of the chroma trap is automatically determined from

the video standard being decoded. The chroma trap should

be disabled during S-video operation to maintain maximum

luminance bandwidth. Alternately, a 3MHz lowpass filter may

be used to remove high-frequency Y data. This may make a

noisy image more pleasing to the user, although softer.

Coring of the high-frequency Y data may be done to reduce

low-level high frequency noise.

Coring of the Y data may also be done to reduce low-level

noise around black. This forces Y data with the following val-

ues to a value of 0:

coring = 1: ± 1

coring = 2: ± 1, ± 2

coring = 3: ± 1, ± 2. ± 3

High-frequency components of the luminance signal may be

“peaked” to control the sharpness of the image. Maximum

gain may be selected to occur at either 2.6MHz or the color

7

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