VP2614 데이터 시트보기 (PDF) - Mitel Networks
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VP2614 Datasheet PDF : 12 Pages
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VP2614
VIDEO LOCK
Once the VP2614 has locked to the H261 frames it will
begin searching for the 20 bit unique Picture Start Code. Once
this has been identified the "Video Lock" status bit will be set,
and the bitstream will be translated on a code by code basis.
Video lock will be lost and translation process interrupted
under the following conditions:
1) A Picture Start Code or Group of Blocks ( GOB ) Start
Code is not present when expected.
2) The codeword is not valid for its context, causing no
match to be obtained. Each variable length code in the
bitstream is analysed by the VP2614, and invalid patterns will
force Video Lock to be lost.
3) Too many coefficients are transferred for the current
macroblock because the End of Block code was missing.
4) GOB number is not in the correct range for the operat-
ing mode.
5) A GOB number not in sequence will cause lock to be
lost and then regained.
Note that only the most frequently occurring coefficients
are variable length coded, the others being represented by an
escape sequence followed by a fixed length code. The Intra
DC coefficient is also a fixed length code. These fixed length
codes have bit patterns which are forbidden in the H.261
specification, but they could appear due to bit errors. These
invalid codes are trapped by the VP2614, but do not cause
Video Lock to be lost. Instead the run length coefficient is
replaced by a default value of magnitude 1. When video lock
has been lost the DMODE 3:0 outputs indicate a WAIT state.
When lock is regained any missing macroblocks are replaced
with Fixed Macroblocks.
A count is maintained of up 256 occurrences of faults 1 -
3, and a status bit is set when lock is lost ( the Video Lock
Achieved bit is also cleared ). An output signal is also provided
which can, if required, be used to interrupt the system control-
ler. This indicates any of the above errors which cause Video
Lock to be lost and also frame alignment errors; alternatively
it can be used to just indicate framing errors.
When Video Lock has been achieved, the detection of a
Picture or GOB start code when it is not expected will not
cause lock to be lost. Instead the VP2614 will resynchronize
to the new start code, and dummy macroblocks will be
generated for the missing GOB's. These dummy blocks will be
Fixed Macroblocks, and will cause the VP2615 Decoder to
use data from the previously decoded picture. Note that Video
LINE
STROBE I/P
(LCLK)
LINE I/P
DATA
0ns
2Ø +10ns
min
min
DATA VALID
DATA
ENABLE (LEN)
NOT
READY O/P
(LRED)
Asynchronous
Note. Ø is the system clock period
Fig 2 : Line Interface Timing
Lock is actually lost and re-gained under these conditions. The
status bit will momentarily be set and then reset, and the Video
Lock Lost Counter will be incremented.
Similarly any errors in the actual GOB number will not
cause lock to be lost and then gained again. Since sequential
GOB numbers are always produced by the encoder, then the
Decoder generates its own GOB numbers and ignores those
in the bitstream.
A Control Bit allows the system controller to take one of two
actions when Video Lock is lost. Either the VP2614 can be
forced to re-initialize to the next Picture Start Code, or it can
abandon the decoding operation until the next GOB Start
Code is detected. When lock has been lost, and a new start
code has been found, the VP2614 assumes its number to be
initially correct and starts its own sequence from that number.
If, however, the next number in the bitstream is not in se-
quence then this new number is used to start a new sequence.
This process continues until two sequential numbers are
obtained, and then no further checks on the GOB numbers are
made until Video Lock is again lost. The VP2614 will generate
"Fixed Macroblocks " for the missing GOB numbers since
Video Lock was lost, and will output these to the VP2615
decoder. This then uses data from the previous decoded
picture for those macroblocks.
A Video Hold bit is provided in one of the System Control
Registers which forces Video Lock to be lost immediately. No
further data is passed to the VP2615 whilst this bit is set, but
the Received Data Buffer is not allowed to fill unnecessarily.
Incoming data will be flushed out and lost. When the hold bit
is cleared a Picture Start Code must be detected to re-gain
Video Lock. The VP2615 will then be provided with any
missing GOB's as described above, before GOB's in the new
picture are processed.
A Freeze Frame Control Bit is also provided. This has a
similar action to the Video Hold Bit, except that it is only
actioned when PTYPE has been decoded in the picture layer,
and it also sets a Freeze Frame status bit. If Video Lock is lost
before the start of a new frame then Freeze Frame will become
active and a search will commence for a picture start code.
Even though Freeze Frame causes Video Lock to be lost, the
VP2614 will still search for picture start codes and will extract
PTYPE and Temporal Reference values.
If clear, a Release Mode Control Register Bit will allow the
freeze condition to be released when the Freeze Bit is cleared,
but is only actioned when the next Picture Header is decoded.
If the Release Mode Bit is set, then the freeze condition is only
released when the PTYPE bit in the H.261 stream specifies
that this is to occur. Even when automatic release has been
selected the system controller can still monitor the length of
time that the freeze has been in effect. It can then force a
release after a time out period by setting the Release Mode Bit
and clearing the Freeze Bit.
DE-MUX CORE
Once Video Lock has been achieved, the core of the
VP2615 will convert the H.261 bitstream into video param-
eters and run length coded coefficients. A state machine,
which is a hardware manifestation of the H.261 coding struc-
ture, maintains the current position in the bitstream. When
necessary variable length de-coding is performed, and side
information such as temporal reference and Picture Type
Information is stored in registers.
3
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