VP 510

3

In the RGB to chrominance and luminance mode, when

pre interpolation does not occur, only 8 unsigned integer bits

are available from the look up table. Thus, within the 13 bit

total, the top 2 bits plus the bottom 3 bits will be made into

zero's.

Intermediate precision within the matrix multiplier grows to

15 signed integer bits plus 6 fractional bits. The least signifi-

cant 9 or 10 of the integer bits are selected at the output, and

the fractional bits are rounded to 3 bits. Ten integer bits are

used when the matrix is producing RGB from interpolated

chrominance and luminance. This allows for undershoot and

overshoot beyond the nominal 8 bit unsigned value.

Only 9 integer bits are necessary when the matrix is

producing chrominance, and the three fractional bits provide

additional precision into the decimating filter. In fact, if the

matrix is producing normalized chrominance, the coefficients

will have been chosen to produce an output in the range

± 127.

This range only requires 8 integer bits, and the ninth bit will be

a repeated sign bit. Note that

±127 is actually representing

±0.5 in this context. When the NORM bit in the Control

Register is reset, the chrominance outputs lie in the range

±1,

or

±256 in our internal representation. The full 9 integer bits are

then needed.

LUMINANCE FILTER

The luminance channel contains a 23 tap low pass filter

with internally defined 10 bit signed coefficients. When the

MODE bit in the Control Register is reset the filter will decimate

the sampling rate by two. When the MODE bit is set the filter

will interpolate the incoming data to produce outputs at twice

the incoming sampling rate. The filter coefficients remain the

same in both cases, but the gain is adjusted to preserve the

energy content..

When the filter is producing decimated luminance it ac-

cepts data from the matrix converter with 9 signed integer bits

plus 3 fractional bits ( 9.3 ). Since luminance is always positive,

however, the most significant bit will be zero. Words within the

filter calculation are allowed to grow to 15 integer bits plus 6

fractional bits. This is then rounded to 15 bits plus 3 fractional

bits, and finally the 10 least significant integer bits are chosen

to give a 10.3 result. The 10 bit integer component allows for

any undershoot or overshoot in the nominal 0 to 255 lumi-

nance range. The three fractional bits are used to round the

integer component to a 10 bit value. This is then clipped to a

value between 0 and 255. Negative values become zero, and

positive values greater then 255 will saturate at 255. Outputs

will not saturate under normal operating conditions, and the

circuit is only necessary to prevent overflow when the input

swings between the maximum and minimum values. Figure 2

illustrates the bit significance at various points in the data path.

When the filter is used to interpolate incoming luminance

data, the 8 bit input is padded to the 9.3 format used previ-

ously. The 13 bit output from the filter is applied to the matrix

converter without further rounding.

The response given by the filter is shown in Figure 3. Stop

band attenuation is approximately 45 dB, and the maximum

pass band ripple is 0.07 dB. These figures were obtained with

10 bit quantized coefficients and unquantized data. The

effects of the various quantization steps within the filter, plus

the reduction to 10 bits, is superimposed upon Figure 3. Also

shown is the CCIR601 specification for a luminance or RGB

0.1

0.2

0.3

0.4

0.5

0

10

20

30

40

50

60

70

80

CCIR601 Specification

Quantized Coefficients, FP Data

Quantized Coefficients and Data

0.0

-0.05

-0.1

0.05

0.1

0.05

0.1

0.15

0.2

PASSBAND RIPPLE

NORMALIZED FREQUENCY

Figure 3. Response of the Luminance Filter

Fig 2. Bit significance in the Y Filter

ROUND

MAC ARRAY

SELECT 10

INTEGER BITS

08.000

9.3

From

Pins

From

Matrix

1.9

Coefficents

15.6

15.3

10.3

To Matrix

Converter

in Interpolate

Mode

10.0

CLIP TO

0 - 255

Unsigned 8.0 to Pins

ROUND