Note: Descriptions are shown in the official language in which they were submitted.
CA 02415292 2008-02-12
SINGLE CLOCK REP'ERENCE FOR COMPRESSED DOMAIN PROCESSING
SYSTEM WITH INTERSPERSED TRANSPORT PACKETS
The present invention relates to the generation of digital video signals. In
particular,
the invention is directed to improvemeitts to Asynchronous Serial Interface
(ASI) bitstream
formation in order to simplify the design requirements of a downstream
receiver, especially
by lowering the required processing speed.
The Moving Picture Experts Group (MPEG) created the ISO/IEC international
Standards 11172 and 13818 (generally referred to as MPEG-1 and MPEG-2 format
respectively) to establish a standard for coding/decoding strategies. AIthough
these MPEG
standards specify a general coding nZethodology and syntax for generating an
MPEG
compliant bitstreani, many variations and preconditions are permitted.
The locked clock approach facilitates the formation of various output format
standards. In the particular case of ASI output, a logic circuit uses the 27
Mhz to generate the
data clock for inputting data to an ASI formatting chip set, The DVB-ASI
format specifies a
data rate of 27 Mbytes/sec, and allows for insertion of specially defined idle
characters both
between transport packets and within transport packets.
It is possible to map the ATSC byte rate to this interface rate by only
inserting the
special idle characters between packets. Prior to the initiation of the packet
data output by the
Transport Stream Encoder (TSE) Software, special idle characters are
continuously output.
After ti=-e Transport packet data output is iiutiated, the Output Interface
fomiat is a flow of 188
packet bytes followed by a string of the special idle characters before the
next Traisport
packet bytes are issued. This forins an ASI Packet. The invention encompasses
alternate
systems and methods for implementing a locked clock approach.
Summary Of The Invention
The invention relates to a system and method for generating a transport packet
stream. The invention encompasses outputting an ASI group having a plurality
of short and
long ASI packets in a fixed sequence, wherein each short and long ASI packet
each
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has an associated transport packet and a fixed number of special idle
characters and
wherein the transport packets are dispersed among the special idle characters.
In one embodiment, the transport packets have a plurality of adjacent bytes
interleaved among special idle characters. In another embodiment, the
transport packets
have a plurality of separate bytes that are interleaved among the special idle
characters.
In yet another embodiment, 10 special idle characters are interleaved between
successive
transport packets bytes.
The invention encompasses systems and methods for generating a transport
packet stream wherein the short ASI packet has 2093 bytes formatted with a 188
byte
transport packet wherein 10 special idle characters are interleaved between
successive
transport packets bytes followed by 25 special idle characters.
The invention encompasses systems and methods for generating a transport
packet streain wherein the long ASI packet has 2094 bytes formatted with a 188
byte
transport packet wherein 10 special idle characters are interleaved between
successive
transport packets bytes followed by 26 special idle characters.
The invention encompasses outputting an ASI group having a plurality of short
and long ASI packets in a fixed sequence, wherein the short ASI packets each
have 2093
bytes including a 188 byte transport packet and 1905 special idle characters
and wherein
the long ASI packets each have 2094 bytes including a 188 byte transport
packet and
1906 special idle characters and wherein the transport packets are dispersed
among the
special idle characters. Again, the transport packet bytes can be a unified
group
interleaved among special idle characters or the transport packets bytes can
be separately
interleaved among the special idle characters.
Brief Description of the Drawings
Figure 1 shows a simplified block diagram of a transport stream encoder system
in accordance with the invention.
Figure 2A shows the basic format of an ASI packet.
Figure 2B shows the basic format of an ASI Group.
Figure 3 shows interspersed transport stream bytes in accordance with the
invention.
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Detailed Description of the Invention
Figure 1 shows a simplified block diagram of a transport stream encoder system
in accordance with the invention. The system preferably includes at least one
video
encoder 10 for receiving and encoding video data into an elementary video
bitstream.
The system also preferably includes at least one audio encoder 20 for
receiving and
encoding audio data into an elementary audio bitstream. In turn, these
bitstreams are sent
to packetizers 12 and 22 where the elementary bitstreams are converted into
packets. It is
understood that information for using the packets independently of the
transport stream
may be added when the packets are formed (e.g., non-audio/video data).
The packets are received and multiplexed by the transport stream encoder 40 to
produce a transport stream. It is understood that a variety of commercially
available
chipsets are suitable for implementing a transport stream encoder in
accordance with the
invention. In general, packets are constructed from elementary streams. One or
more
program groups are formed with associated "Packet Identifiers" (PIDs). Each
program
group may have associated video data and/or audio data. Data associated with a
single
PID generally shares a common time base. It is understood that the transport
stream may
contain one or more programs with one or more independent time bases.
It is also understood that the transport stream may be transmitted over a
transmission channel (not shown). Ultimately, the transport stream will be
demultiplexed and decoded by a transport stream demultiplexor (not shown). To
this
end, timing information is extracted by the transport stream demultiplexor for
synchronizing the video and audio decoders. Such synchronization is
accomplished
through the use of the "Program Clock Reference" (PCR) in the transport
stream.
The invention encompasses a locked clock approach to generate output formats
conforming to various standards. In the particular case of ASI output, a logic
circuit uses
the 27Mhz to generate the data clock for inputting data to an ASI formatting
chip set.
The DVB-ASI format specifies a data rate of 27 Mbytes/sec, and allows for
insertion of
specially defined idle characters both between transport packets and within
transport
packets.
In order to map the ATSC byte rate to this interface rate, it is possible to
only
insert the special idle characters between packets. Prior to the initiation of
the packet
data output by the Transport Stream Encoder (TSE) Software, special idle
characters are
continuously output. After the Transport packet data output is initiated, the
Output
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Interface format is a flow of 188 packet bytes followed by a string of the
special idle
characters before the next Transport packet bytes are issued. This forms an
ASI Packet.
Figure 2a shows the format of an ASI packet in having special idle characters
inserted only between transport packets. In order to insure that Program Clock
Reference (PCR) packets issued by the TSE are properly placed, two types of
ASI
packets, short and long, are created. In the short ASI packet, 1,905 special
idle character
bytes are inserted after the 188 byte Transport Packet. In the long ASI
packet, 1,906
special idle character bytes are inserted after the 188 Transport Packet byes.
Therefore,
the short ASI packet has a total of 2,093 bytes, and the long ASI packet has a
total of
2,094 bytes.
Both the Transport packet bytes and special idle character bytes are
serialized to
the 270 Mbps DVB-ASI output interface rate. This dictates that each byte of an
ASI
packet must be output at every cycle (tick) of the 27 MHz clock. To absolutely
minimize the jitter between successive PCR transport packets in the stream, an
ASI
Group, formed from a well defined assortment of short and long ASI packets is
identified. The ASI Group is defined as I short ASI packet, followed by 85
long ASI
packets, followed by 1 short ASI Packet, and finally followed by 84 long ASI
Packets, as
shown in Figure 2B.
The generation and output of an ASI group can also be described in terms of
ticks
of the 27 MHz clock. During the time interval of the ASI Group of 171 assorted
short
and long ASI packets there are exactly 358072 Program Clock ticks. Output byte
timing
is determined by allocating a distinct pattern of 27Mhz (PCR) clock cycles to
each ASI
group. Exactly 2093 PCR ticks are allocated to the first packet. This is
followed by
2094 PCR ticks for each of the next 85 packets, followed by 2093 PCR ticks for
the next
packet, followed by 2094 PCR ticks for each of 84 packets to generate a
smoothed flow
of output bytes as shown in Figure 1B.
Packet spacing is relatively uniform with only one clock cycle of 27Mhz
difference between a few of the packets. This approach locks the 27Mhz PCR
clock to
the output byte clock. Bytes are output at exactly the ATSC output rate
relative to the
PCR timing reference.
Extension of the ASI Formation Technique
The 188-byte Transport packet, shown grouped at one end of the ASI Packet of
Figure 2A, can be dispersed within the ASI Packet (i.e., placed anywhere
within the ASI
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Packet). It is also understood that the Transport packet need not be kept as a
unified
group (i.e., having all bytes adjacent), as described in the SAR 13662 filing.
In fact, the
individual bytes of the 188 byte Transport packet can be dispersed among the
special idle
character bytes, as shown in Figure 3.
The benefit of dispersing the Transport Stream Packet bytes is that this
lowers the
effective peak processing rate of the Transport Stream bytes at a receiver. In
Figure 1 A,
the Transport Stream information must be processed at a continuum of 270
Mbits/s
because the Transport Stream bytes are consecutive. By spreading the Transport
Stream
bytes among the special idle character, the sustained receiver processing rate
is lowered.
There are countably many ways to interleave the Transport Stream bytes among
the idle
characters. One convenient approach is to insert 10 idle characters between
each
Transport Stream byte, and eventually filling out the ASI packet with either
25 or 26 idle
characters depending upon whether it is a short (2095 byte) or long (2096
byte) ASI
packet. This approach tends to minimize the byte processing rate at the
receiver. The
formation of the ASI group, shown in Figure 1 B is unaffected.
While this invention has been described with an emphasis upon preferred
embodiments, it will be obvious to those of ordinary skill in the art that
variations in the
preferred devices and methods may be used and that it is intended that the
invention may
be practiced otherwise than as specifically described herein. Accordingly,
this invention
includes all modifications encompassed within the spirit and scope of the
invention as
defined by the claims that follow.