VP2611 Просмотр технического описания (PDF) - Mitel Networks

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VP2611

H.261 Encoder

Mitel Networks 

VP2611

OPERATION OF MAJOR BLOCKS

Motion Vector Estimator

The motion estimator calculates the mean absolute error

( MAE ) for each possible position of the combined luminance

block in a search window from the previous frame. The

combined luminance block consists of 16 x 16 pixels, and in

the search window this is displaced between -7 to +7 vertically,

and -8 to +7 horizontally. The two lsb's of each pixel are

discarded and the MAE value is contained within 14 bits.

The minimum MAE value, representing the best match

between the previous and current block, is passed to the

motion compensation decision block, together with the posi-

tion of this best fit in the search window. The zero displace-

ment MAE value is also passed to this block, which then

decides whether the best fit is sufficiently better than the zero

displacement fit. It uses the characteristic shown in Figure 3,

where the 14 bit MAE is a Hex value. In the area to the right

of the line all points defined by the two MAE values will cause

motion compensation to be applied. In this case the best fit

MAE value is used by the inter/intra decision processor,

otherwise the zero displacement value is used.

Inter/Intra Decision Processor

The MAE value passed by the motion compensation

decision block is compared to the simplified variance of the

current block. This simplified variance is calculated by sum-

ming the moduli of the differences between each luminance

pixel and the mean luminance value over the whole macrob-

lock. Eight bit pixels are used, and the variance value is

expressed in 14 bits by discarding the two lsb's from the actual

16 bit result. It can then be directly compared to the 14 bit MAE

value.

If the MAE value is below a user defined threshold inter

mode coding is always selected. The default threshold is 3, on

a scale from 0 to 255 using the 8 msb's from the 14 bit value.

Above this threshold inter mode is only selected if the variance

of the current block is greater than or equal to the MAE value

in use.

In order to avoid gradual picture degredation, every 61st

Macroblock input to the VP2611 is coded in intra mode

regardless of the above decision. As 61 is a prime number, this

will ensure that each macroblock will be transmitted in intra

mode at least once in every 61 transmissions. If FIX MAC-

ROBLOCK or SKIP PICTURE is invoked this `Force Intra'

180

140

MC Off

x = 1.125y

100

CO AB

80

5F

40

20

MC On

40 80 CO 100 140 180

Zero Movement Absolute Error in Hex

Fig 3 : MC Decision Slope

counter will be disabled.

The user may overide the internal Inter/Intra decision at

any time using the CBUS control port. A user generated

forced inter mode will overide an internally generated `Force

Intra'.

Low Pass Filter

The macroblock selected from the previous frame in

motion compensated inter mode coding, will be filtered before

it is subtracted from the current block. This decision can be

overidden externally by the system controller. The Filter uses

a simple [ 1 2 1 ] characteristic in both vertical and horizontal

dimensions as specified in H.261 on the macroblock boundaries

[010] is used.

SYMBOL

PARAMETER

MINIMUM

MAXIMUM

t RAC

t CAC

t RP

t CP

t RAS

t CAS

t REF

Access time from RAS

Access time from CAS

RAS precharge time

CAS precharge time

RAS pulse width

CAS pulse width

Time to refresh 256 rows

-

-

50ns or under

15ns or under

90ns or under

50ns or under

-

105ns or under

25ns or under

-

-

-

-

0.25ms or over

N.B. All times are quoted assuming 27MHz operation. For lower clock

frequencies increase the above values proportionately.

Table 1 : External DRAM timing requirements

3

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