STV0118 datasheet(16/42 Pages) STMICROELECTRONICS

16 / 42 page

IV - FUNCTIONAL DESCRIPTION (continued)

IV.8 - Burst Insertion

The color reference burst is inserted so as to

always start with a positive zero crossing of the

subcarrier sine wave. The first and last half-cycles

have a reduced amplitude so that the burst enve-

lope starts and ends smoothly.

The burst contains 9 or 10 sine cycles of

4.43361875MHzor 3.579545MHzaccordingto the

standard programmed in the Control Register

(Reg. 0, bits std[1:0]), as follows :

- NTSC-M

9 cycles of

3.579542MHz

- PAL-BDGHI

10 cycles of

4.43361875MHz

- PAL-M

9 cycles of

3.57561149MHz

- PAL-N

9 cycles of

3.5820558MHz

It is possibleto turn the burst off (no burst insertion)

by setting configurationbit ‘bursten’to 0 (register2).

Notes : - Two strategies exist for burst insertion: one is to merely

gate and shape the subcarrier for burst insertion, the other

is more elaborated and is to always start the burst with a

positive-going zero crossing. In the first case the phase

of the subcarrier when the burst starts is not controlled,

with the consequence that some of its first and last cycles

are more heavily dis torded. The second solution

guarantees smooth start and endof burst witha maximum

of undistorded burst cycles and can only be beneficial to

chroma decoders, it is the solution implemented in the

STV0118.

- While the first option gave constant burst start time but

uncontrolled initial burst phase, the second solution

guarantees start on a positive-going zero crossing with

the consequence that two burst start locations are visible

over successive lines, according to the line parity. This is

normal and explained below.

- In NTSC, the relation between subcarrier frequency and

line length creates a 180o subcarrier phase difference

(with respect to the horizontal sync) from one line to the

next according to the line parity. So if the burst always

starts with the same phase (positive-going zero crossing),

this means the burst will be inserted at time X or at time

X+TNTSC/2 after the horizontal sync tip according to the

line parity, where TNTSC is the duration of one cycle of the

NTSC burst.

- With PAL, a similar rationale holds, and again there will

be two possible burst start locations. The subcarrier

phase difference (with respect to the horizontal sync) from

one line to the next in that case is either 0 or 180o with

the following series: A-A-B-B-A-A-...-etc. where A

denotes ‘A-type’ bursts and B denotes ‘B-type’ bursts,

A-type and B-type being 180

° out of phase with respect

to the horizontal sync. So 2 locations are possible, one

for A-type, the other for B-type (see Figure 8).

- This assumes a periodic reset of the subcarrier is

automatically performed (see bits valrst[1:0] in Reg 2).

Otherwise, over several frames, the start of burst will drift

within an interval of one a subcarrier’s cycle. THIS IS

NORMAL and means the burst is correctly locked to the

colors encoded. The equivalent effect with a gated burst

approach would be the following : the start location would

be fixed but the phase with which the burst starts (with

respect to the horizontal sync) would be drifting.

IV.9 - Luminance Encoding

The demultiplexed Y samples are band-limited

and interpolated at CKREF clock rate. The result-

ing luminance signal is properly scaled before

insertion of any Closed-captions, CGMS or

Teletext data and synchronization pulses.

The interpolation filter compensates for the sin(x)/ x

attenuation inherent to D/A conversion and greatly

simplifies the output stage filter (refer to Figures 15

to 17 for characteristic curves).

9 10

811 12 13

Frequency (x106) (Hz)

-40

-35

-30

-25

-20

-15

-10

-5

Figure 15 : Luma Filtering Including DAC

Attenuation

234

567

9 10

111213

Frequency (x106) (Hz)

-40

-35

-30

-25

-20

-15

-10

-5

Figure 16 : Luma Filtering with 3.58MHz Trap,

Including DAC Attenuation

234

567

9 10

111213

Frequency (x106) (Hz)

-40

-35

-30

-25

-20

-15

-10

-5

Figure 17 : Luma Filtering with 4.43MHz Trap,

Including DAC Attenuation

STV0118

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