VP2615 Просмотр технического описания (PDF) - Mitel Networks

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VP2615

H.261 Decoder

Mitel Networks 

VP2615

for use in the next frame and is also available on the output

pins.

Loop Filter

The best matched block from the search window in the

previous frame can be passed through a low pass filter to

reduce block boundary effects. The filter uses a simple [1 2 1]

characteristic in both horizontal and vertical dimensions as

laid down in the H261 Specification, on the macroblock

boundaries [010] is used. An instruction input at the DIN port

defines whether the filter should be used or not.

DCLK

DIN7:0

DMODE3:0

10ns

10ns

20ns

2ns

SCLK/2

20ns

2ns

N.B. All timings given are minimum values.

Reconstruction Adder

In Inter Mode, the IDCT data is added to the best fit block

from the previous frame store. In Intra Mode, the IDCT data is

added to zero. After the adder, the sign bit is removed from the

result to give 8 bit pixels. Clipping circuits ensure that any

pixels with values exceeding 255 are clipped to 255 and any

with negative values are clipped to zero (such values are

possible due to quantization effects).

OPERATION OF INTERFACES

DIN Input Port

The DIN port provides a glueless interface to the VP2614

Video Demultiplexer, from which it will accept run length

coded transform data and control information. The general

purpose nature of the interface will, however, allow other

sources of macroblock data to be used.

Data on the input bus is defined by means of the signals

DMODE3:0, and is strobed in with the DCLK signal which is

provided by the VP2614 and derived from SYSCLK. Set up

and hold times with respect to the rising edge of DCLK are

given in Figure 4. If DCLK is a continuous strobe, then the

WAIT state defined by DMODE 3:0 should be used to disable

any clocking actions. If preferred DCLK can alternatively be

DMODE3:0

FUNCTION

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

1010

1011

1100

1101

1110

1111

GOB Number

MB Number

Control Decisions

Quant Value

Horizontal MV

Vertical MV

Coded Blk Pattern

Sub-Block No

Zero Run Count

RLC Coefficient

Not used

Not used

Not used

Not used

Not used

Wait State

Table 2 . DIN Mode Functions

4

Fig 4 . DIN Port Timing

used as a strobe which is only present when data is valid and

action is needed. In this case WAIT states are not strictly

necessary.

The VP2615 always expects to receive a complete video

frame of data, even if error conditions have occurred in the de-

multiplexer. Skip Picture or Fixed Macroblocks should be

supplied if necessary once a frame has started. With the latter,

decoded data from the previously stored frame will be pro-

duced by the VP2615.

The asynchronous interface will allow the use of other

video de-multiplexers, as long as the protocol defined by

DMODE3:0 is observed. This protocol is defined below, and

summerized in Table 2.

Control Decisions : This byte must always be the first in the

sequence since it resets the internal control logic. It

defines which control decisions were taken when coding

the forthcoming MacroBlock. A high on DIN 0 indicates a

Fixed Macro Block (ie no change since the previous

frame), and a high on DIN1 indicates that Inter coding was

used. Similarly a high on DIN2 indicates that the

MacroBlock was filtered, a high on DIN3 indicates that

Motion Compensation was used. and a high on DIN6

indicates that SKIP PICTURE is in effect. In the latter case

the VP2615 will cease processing until SKIP PICTURE is

reversed by writing a new Control Decisions byte. Whilst

SKIP PICTURE is active, no further data will be output

from the YUV port. SKIP PICTURE effectively resets the

VP2615, and the next MacroBlock input should be the first

of a new frame. Since the frame store will not be updated

then the system should ensure that an Intra coded picture

is sent as soon as possible.

GOB Number: The correct GOB number is required for every

macro block in that group. (DIN3 is MSB).

MB Number: Each macroblock in a group requires an identi-

fication number. (DIN5 is MSB).

Coded Blk Pattern: This byte is defined in the H.261

Specification and is used to indicate which sub blocks

contain non zero coefficients. It is produced by the encoder

but is not used by the VP2615, and if provided will be

ignored. The sub block numbering sequence is actually

used to indicate blocks with zero coefficients.

Quant Value: This input represents the quantization value

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