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Description  |
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BACKGROUND OF THE INVENTION
1. (a) Field of the Invention
The present invention relates generally to broadcast television and, more
particularly, to a method and apparatus for transmitting high definition
television (HDTV) programming using a digital satellite system transport.
2. (b) Description of Related Art
It is well known to transmit analog television signals over terrestrial
networks. Such networks typically broadcast signals over a relatively
small geographical area using the UHF or VHF frequency bands. Although
still widely used, these UHF, VHF broadcast systems have significant
shortcomings. For example, in order for consumers to receive a
sufficiently strong signal from a terrestrial network, the consumer must
typically be located near a major city (e.g., Los Angeles, Chicago or
Denver). Additionally, terrestrial networks broadcast a relatively small
amount of information for the bandwidth that they occupy. For the
broadcast of a given program, analog terrestrial networks devote one
frequency entirely to that program. Because there must be separation
between transmission frequencies in a traditional broadcast system, there
are relatively few programming channels available using analog terrestrial
broadcast methods.
Cable television networks transmit programming channels over coaxial cable.
While cable technology has greatly increased the number of channels
available to a television user, the cost, materials and manpower
associated with the installation and maintenance of the cable system
infrastructure is quite high. These costs are typically passed on to the
cable system's subscribers in their subscription fees. Additionally, cable
television is not available in areas of the country where demand is
insufficient.
Digital direct-to-home (DTH) satellite systems broadcast hundreds of
programming channels to a very wide geographical area (e.g., the
continental United States). One example of such a broadcast satellite
television system is the DIRECTV.RTM. system. Broadcast satellite systems
can provide many channels due to their efficient use of bandwidth. MPEG-2
video compression is one particular method of efficient bandwidth usage
employed by broadcast satellite systems. The information for broadcast is
converted into digital signals that are divided into packets. Each packet
is assigned a header that is used to identify the information for a
particular television service. The identifying information in the header
is referred to as a service channel identification (SCID) or a program
identification (PID). After the data have been put into packets, the
packets are transmitted to a satellite, which rebroadcasts the packets
over the satellite's coverage area. Each subscriber within the coverage
area can receive the broadcast programming by tuning their receiver to the
proper frequency and obtaining the appropriate packets based on the SCID
in the header of each packet. Broadcast satellite systems eliminate the
need for the massive infrastructure that cable systems require, thereby
making it easy to add subscribers to the system.
The progression from terrestrial television broadcast to cable television,
to DTH satellite television has allowed consumers to obtain more and more
programming information while limiting the costs that are passed to the
consumers. However, all of the DTH programming has been standard
definition television (SDTV), which is also called conventional definition
television (CDTV).
High definition television (HDTV) has a resolution of approximately twice
that of SDTV in both the vertical and the horizontal dimensions. HDTV
provides motion picture video resolution and CD-quality sound to a viewer
at home. Additionally, the aspect ratio selected for HDTV is 16:9, which
is similar to the 1.76:1 ratio used in the motion picture industry. The
HDTV standard, as set forth in advanced television standards committee
(ATSC) documents A/53 and A/54, specifies the use of MPEG-2 video
processing in accordance with ISO/IEC standard 13818, and digital audio
processing in accordance with ATSC document A/52. Plans are in place in
the United States and many other countries of the world to transition from
SDTV to HDTV in the near future. However, there are no known methods that
enable DTH satellite systems, or any other systems (e.g., cable systems)
using defined data transfer protocols, to broadcast data that is generated
in accordance with HDTV standards.
SUMMARY OF THE INVENTION
In one embodiment, the present invention is embodied in a transmission
station for transmitting programming in a first format and a second
format. The transmission station includes a video encoder for generating
digitally encoded video signals in the first format, a packetizer coupled
to the video encoder for packetizing the digitally encoded video signals
in the first format into a first packet format, and a repacketizer
connected to the packetizer for repacketizing the digitally encoded video
signals in the first format into a second packet format. The present
invention may also include an audio encoder for generating digitally
encoded audio signals in the first format, a second packetizer coupled to
the audio encoder for packetizing the digitally encoded audio signals in
the first format into the first packet format, the second packetizer
further coupled to the repacketizer. Wherein, the first format is a high
definition television (HDTV) format and the second format is a standard
definition television (SDTV) format.
In another embodiment, the present invention may be a receiver station for
receiving transmissions in a first format and a second format. The
receiver station includes, a transport demultiplexer for demultiplexing a
received signal into an audio component and a video component, a video
decoder coupled to the transport demultiplexer for decoding the video
component, and an audio decoder coupled to the transport demultiplexer for
decoding the audio component. Wherein, the first format is a high
definition television (HDTV) format and the second format is a standard
definition television (SDTV) format.
Alternatively, the present invention may be embodied in a method of
transmitting programming in a first format and a second format. The method
includes the steps of digitally encoding video signals into the first
format, packetizing the digitally encoded video signals in the first
format into a first packet format, and repacketizing the packetized
digitally encoded video signals in the first format into a second packet
format. The method may further include the steps of digitally encoding
audio signals into the first format, packetizing the digitally encoded
audio signals in the first format into the first packet format; and
repacketizing the packetized digitally encoded audio signals in the first
format into a second packet format. Wherein, the first format comprises a
high definition television (HDTV) format and the second format comprises a
standard definition television (SDTV) format.
The invention itself, together with further objects and attendant
advantages, will best be understood by reference to the following detailed
description, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a satellite broadcast system capable of implementing
the present invention;
FIG. 2 is a detailed diagram of the transmission station shown in FIG. 1;
FIG. 3 is a diagram of a conventional PES packet showing details of the PES
header fields;
FIG. 4 is a detailed diagram of the receiver station shown in FIG. 1;
FIG. 5 is a detailed diagram of an alternate embodiment of the receiver
station shown in FIG. 1; and
FIG. 6 is a diagram of the transmit and receive protocol stacks used in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is embodied in a method and apparatus for
broadcasting high definition television (HDTV) signals and programming. As
used herein, the term programming refers to audio, video, data or any
other information that may be broadcast. The HDTV signals may be broadcast
via a variety of different media (e.g., via satellite in a DTH system). In
particular, the present invention allows for the broadcast of HDTV signals
and standard definition television (SDTV) signals in the same system
without changing existing transport protocols. For example, the present
invention uses the current SDTV satellite broadcast systems and protocols,
while supporting the broadcast of HDTV signals using standard HDTV
equipment and protocols. According to the present invention, a number of
audio and/or video signals are packetized, according to standard HDTV
protocol into packetized elementary streams (PES) and combined (e.g.,
using statistical multiplexing) with additional data to form a master data
stream. The master data stream (which is characterized primarily by data
that is in the HDTV format) is packetized into transmission packets that
are compatible with the transmission system (e.g., a DTH system, which is
typically characterized primarily by data/signals that are in the SDTV
format). The transmission packets are transmitted (e.g., via satellite
retransmission) to receiver stations. A method is provided at the receiver
stations for resolving the different timing designations and clock speeds
that may be needed in order to decode the SDTV and HDTV data/signals
Referring now to FIG. 1, a diagram of one preferred embodiment of a
satellite broadcast transmission system 40 capable of utilizing the
present invention is shown. The system 40 includes a transmission station
50, a satellite 55, and a plurality of receiver stations 60. The
transmission station 50 processes SDTV and HDTV signals (in a manner
described in more detail later in this disclosure) and
transmits/broadcasts them to the satellite 55. The HDTV signals may
include video, audio or data. The satellite 55 receives the signals and
appropriately processes them for rebroadcast. Processing may include, but
is not limited to, frequency conversion and power amplification. The
processed signals are then rebroadcast by the satellite 55 to the receiver
stations 60, which may be located in geographically remote locations.
FIG. 2 is a diagram illustrating how data flows through the transmission
station 50 shown in FIG. 1. In general, the transmission station 50
provides a variety of video, audio, and data processing functions. The
video processing function includes an MPEG-2 encoder 65 and a PES
packetizer 70 for placing the HDTV signals in an appropriate HDTV format
In operation, a video signal for transmission is coupled to the MPEG-2
encoder 65, which processes the video signal. Video is processed on a
frame-by-frame basis in accordance with the MPEG-2 standard to create
access units for each frame.
As is well known, MPEG-2 encoding is based on the principle that successive
frames of a video image are largely redundant. For example, the background
of an image may stay constant over numerous video frames. Generally, video
compression in the MPEG-2 system is accomplished by predicting motion that
occurs from one frame of video to another and transmitting motion vectors
with background information, which enables a receiver to create the next
video frame from the current video frame. Accordingly, only the motion and
background difference between two frames of video need be broadcast. More
specifically, an MPEG-2 cycle generates intra-coded pictures (I-frames),
predictive coded pictures (P-frames), and bi-directionally predictive
coded pictures (B-frames) from a sequence of video frames. I-frames
exploit the spatial redundancy within a single picture (frame or field).
I-frames do not take advantage of temporal characteristics of the video
and do not use any interframe coding. More data is associated with
I-frames than with P or B-frames.
P-frames are frames that utilize temporal prediction in the forward
direction (i.e., predictions for the P-frame are formed only from pixels
in the most recently decoded I-frame or P-frame). P-frames exploit
interframe coding techniques to improve compression efficiency and picture
quality.
B-frames are frames that include prediction from a future frame as well as
from a previous frame. The referenced future or previous frames are either
I-frames or P-frames.
Accordingly, the size of an access unit created from a video frame varies
based on the video content of the frame and the MPEG-2 cycle. The MPEG-2
cycle may create an I-frame that has a large access unit, or P or B-frames
that have relatively small access units.
The access units from the MPEG-2 encoder 65 are passed to the program
encapsulated stream (PES) packetizer 70, which creates a video PES stream
75, in a known manner. The video PES stream 75 consists of a number of PES
packets, which include PES headers 80 and PES payloads 85, 90. PES packets
are variable length packets and may have a maximum size of 64 kilobytes
(KB). Alternatively, for certain applications such as video processing,
PES packets may be unconstrained in size. The diagram in FIG. 2 shows two
video PES packets having payloads P.sub.1 V.sub.1 85 and P.sub.2 V.sub.1
90 representing first and second PES payloads created from the video-1
source.
Audio is processed in a similar manner to video. Specifically, audio for
broadcast is coupled to a digital encoder 100 (e.g., an AC-3 Dolby Digital
Encoder). The digital encoder 100 creates one data packet for each 32 ms
of audio. The encoded audio data packets are passed to a PES packetizer
105, which processes the encoded audio data to create an audio PES stream
110, in a known manner. The audio PES stream 110 consists of a number of
PES packets that include PES headers 115 and PES payloads, which may be
variably sized. FIG. 2 shows two audio PES packets having payloads P.sub.1
A.sub.1 120 and P.sub.2 A.sub.1 125 representing first and second PES
payloads created from the processing of the source audio-1.
Data for transmission is in the form of a data stream 135. Data for
transmission may include, but is not limited to, electronic program guide
data or conditional access data. Since the data stream 135 does not
contain time-sensitive data, it is not PES packetized but, rather, is
provided in pre-packetized form to the transmission station 50. The data
stream 135 shown in FIG. 2 includes a number of packets, each having a
header 140 and a payload 145, 150. FIG. 2 represents two payloads P.sub.1
D.sub.1 145 and P.sub.2 D.sub.1 150 representing first and second payloads
provided by the data source data-1.
In accordance with the present invention, the video PES stream 75, the
audio PES stream 110 and the data stream 135 are coupled to a transport
multiplexer and repacketizer 155, which selectively combines the streams
75, 110, 135 into a master stream 160, composed of a number of transport
packets. The master stream 160 includes data from the video PES stream 75,
the audio PES stream 110, and the data stream 135, as represented by a
stream of payloads having reference numerals 170, 175, 180, 185, 190, 195,
200, 205, and 210. In keeping with the present invention, the transport
multiplexer and repacketizer 155 adds a transport header 165 to each
portion of information pulled from the streams 75, 110, 135. The
information pulled from the streams 75, 110, 135 is referred to as
transport payload and, in one preferred embodiment of the present
invention, is preferably 127 bytes long. Preferably, the transport header
165 is three bytes in length and contains a service channel identification
(SCID) that can be used to filter each transport data packet at the
receiver stations 60. In the disclosed embodiment, there is no requirement
that the PES packets and transport packets be aligned. For example, the
beginning of a PES packet need not come at the beginning of a transport
packet, nor is it required that the end of a PES packet correspond to the
end of a transport packet. This is most clearly seen with reference to
payload 200 within the master stream 160. In payload 200, data associated
with P.sub.1 V.sub.1 ends and data associated with P.sub.2 V .sub.1 begins
in the middle of the transport packet payload.
Once assembled, the master stream 160 is passed to an RF conversion
function 215, which appropriately modulates the transport packets onto an
RF carrier signal. The transport packets include headers 165 and payloads
170, 175, 180, 185, 190, 195, 200, 205, and 210, The RF carrier signal
containing the modulated information is coupled to a transmit antenna 220,
which broadcasts the information to the satellite 55.
FIG. 3 is a detailed diagram of a conventional PES packet shown in FIG. 2.
The following description of the conventional PES packet focuses on
information that is key to the understanding of the present invention.
More thorough information regarding PES packets may be found in the "ATSC
Digital Television Standard" and "Guide to the Use of the ATSC Digital
Television Standard," which are published by the Advanced Television
Systems Committee and are referred to as documents A/53 and A/54,
respectively, and are hereby incorporated by reference.
The conventional PES packet shown in FIG. 3 includes a PES header (e.g.,
80) and a PES payload (e.g., 85), which may have a variable length. As is
known in the art, the PES header 80 includes a number of fields including
a packet start code prefix 235, which is a predetermined code that is used
to identify the start of a new PES packet. A one byte stream ID 240 is a
number that is unique to each PES stream (e.g., video, audio or data) and
is used to identify each PES stream during filtering at the receiver
station 60. A two byte PES packet length field 245 is used to represent
the length field of the PES packet on which the header is placed. A two
bit field 250 containing 1 0 is located after PES packet length 245. A
fourteen bit PES header flag field 255 is used to indicate various flags
that are placed in the PES header 80. A one byte PES header data length
field 260 is used to indicate the length of optional fields and stuffing
bytes . Additional PES header fields 265 include a number headers that are
used in processing the PES packet at the receiver station 60.
As is known, a PTS/DTS field 275 contained in the additional PES header
fields 265 contains information representative of a presentation time
stamp (PTS) and a decoding time stamp (DTS) The PTS is used to inform the
receiver station 60 of the intended time of presentation of the
presentation unit following the PES header 80. The PTS refers to the
presentation time of the first access unit occurring in the PES payloads.
The DTS specifies the time at which an access unit should be decoded. If a
PES packet does not contain an access unit, the PTS/DTS field 275 of the
header will not contain a PTS/DTS.
Conventionally, in a satellite broadcast system a reference time clock
(RTC) for a transport encoder is a 32 bit binary counter that is clocked
at 27 MHz. This counter "wraps" or "loops" approximately every 2.5
minutes. The wrapping time was selected as sufficiently long enough to
prevent transmitted elements from being confused with one another.
However, as is known, the MPEG-2 transport utilized by the HDTV system
uses a 33-bit counter, clocked at 90 KHz. This rate is 1/300+L th the rate
of the satellite broadcast RTC. The MPEG-2 transport clock wraps every
26.5 hours.
In accordance with one preferred embodiment of the present invention, one
satellite may carry both SDTV and HDTV information. Typically, auxiliary
data packets, which contain the value of the RTC, reference time stamps
(RTS) or encryption control word packets, are sent to receiver stations.
The RTS represents the time at which the last bit of the packet left the
encoder. The RTS is used by the receiver station 60 to synchronize its 27
MHz clock with the transmission station 50 clock. In traditional satellite
broadcast systems the RTS is carried on the same SCID as the program with
which it is associated, but the RTS is contained in auxiliary data
packets.
Conventionally, transport demultiplexers in the receiver stations 60 use
the RTC in the auxiliary data packets to synchronize the local 27 MHz
clock reference with the clock at the transmission station. Audio and
video are then displayed according to the RTC, which is synchronized with
the receiver's local clock. Since the MPEG-2 clock and the RTC, which is
used to synchronize the receiver station 60 27 MHz clock, operate at
different speeds, a comparison between the PTS/DTS field 275 and the
receiver station 60 clock will not appropriately indicate when a video or
audio packet should be decoded and presented. Accordingly, the contents of
the PTS/DTS field 275 must be converted before a comparison is made to the
receiver station 60 clock. In accordance with one embodiment of the
present invention, the PTS/DTS field 275 is defined as the RTC value
divided by 300. The maximum value of the PTS is (2.sup.32 -1)/300, which
is the wrap value of an unsigned 32 bit counter. Since both audio and
video PES packets wrap at the same rate, the audio and the video will be
synchronized and will be appropriately displayed. Accordingly, in keeping
with the present invention, to resolve timing between PTS/DTS field 275
and receiver station 60 clock, the receiver stations 60 multiply the
contents of PTS/DTS field 275 by 300 before comparing it to the receiver
station 60 clock. This scheme provides a communications transport capable
of transferring HDTV data along with conventional SDTV data to a number of
receiver stations 60 using existing hardware and minor software
modifications to the receiver stations 60. Specifically, in keeping with
the present invention, the receiver stations 60 must be programmed to
recognize HDTV PES headers and to multiply the content of the PTS/DTS
field 275 of a HDTV PES header by 300 before comparing its value to the
clock located in the receiver station 60.
FIGS. 4 and 5 are detailed block diagrams of the receiver station 60 shown
in FIG. 1. In one embodiment the receiver station 60 includes an antenna
310, an RF conversion function 315, and a transport demultiplexer 320. The
receiver station 60 further includes an MPEG-2 decoder 325, an audio
decoder 330, a data processing function 335, and a display 340. Signals
from the satellite 55 are received by the antenna 310 and are passed to
the RF conversion function 315, which appropriately processes the signals.
Processing may include downconversion, filtering, amplification or other
processing.
After the received signals are appropriately processed, they are passed to
the transport demultiplexer 320, which demultiplexes the data packets in
the received signals into video, audio and data streams. The video, audio,
and data streams are coupled to the MPEG-2 decoder 325, the audio decoder
330, and the data processing function 335, respectively. The MPEG-2
decoder 325 appropriately processes the video signal and passes the signal
to the display 340. Similarly, the audio decoder 330 processes the audio
PES stream to produce signals that are passed to the display 340. In
accordance with one embodiment of the present invention, processing of the
audio and video PES streams includes determining if the PES streams are
HDTV PES streams. If the PES streams are HDTV stream, the contents of the
PTS/DTS field 275 are multiplied by 300 before it is compared to the
receiver station 60 clock. Accordingly, this processing allows SDTV
signals to be handled by the receiver in a conventional manner and allows
special processing on HDTV signals. The need to resolve the differences
between the PTS/DTS filed 275 and the receiver station 60 clock is unique
to this particular embodiment of the present invention. Because the DTH
satellite hardware is in place, there is a need to resolve timing
reference differences. Accordingly, not all, or even any other,
embodiments of the present invention will need to resolve timing reference
differences.
The data processing function 335 processes any data that was sent as a data
stream. The data processing function 335 may perform the function of
creating a digital program guide, providing conditional access or other
data related tasks within the receiver station 60.
In an alternate embodiment shown in FIG. 5, data from the transport
demultiplexer 320 is passed to an IEEE 1394 serial data interface 345,
which converts the signals to standard serial digital interface signals.
The IEEE interface 345, in turn, passes the serial data to a number of
IEEE 1394 compliant devices 350. These devices may include video displays,
video tape recorders, and read/write digital video disk units.
FIG. 6 represents a protocol stack representation of the data transfer from
the transmission station 50 to the receiver station 60. Conventional
MPEG-2 PES streams 360, such as the streams generated by the PES
packetizers 70, 105 (shown in FIG. 2) are transferred to a transport layer
365 along with any other data 367 that is to be broadcast. In accordance
with the present invention, the transport layer 365 processes the MPEG-2
PES streams and the data for transmission into, for example, 127 byte
payloads with 3 byte headers. The processing may be carried out by a
multiplexer (e.g., a statistical multiplexer), such as the transport
multiplexer and repacketizer 155. The functionality above the dotted line
in the transmit protocol is conventional and known. However, the
processing performed on the MPEG-2 PES streams 360 by the transport 365,
is novel. The payloads and headers are passed from the transport layer 365
to a modulator 370 having forward error correction (FEC) capabilities 370,
which modulates a carrier signal to encode the information from the
payloads and the headers. Many modulation schemes such as QPSK, DQPSK, FSK
or QQPSK may be used in accordance with the present invention.
The receive protocol receives the modulated carrier signal at a demodulator
375 having forward error correction (FEC) capabilities. The demodulator
375 demodulates the carrier signal to obtain the information contained in
the packets and the headers. The demodulated information is passed to a
transport layer 380, which, in accordance with the present invention,
strips off the transport header added by the transport layer 365, and
re-assembles the MPEG-2 PES streams and the data stream. Re-assembly may
be accomplished through examination of PES headers and concatenating
individual PES packets together to create a PES stream. In keeping with
the present invention, the assembled MPEG-2 PES streams are passed to an
MPEG-2 PES layer 385, which appropriately processes the MPEG-2 PES stream
in to an audio PES stream and a video PES stream. In accordance with the
present function, processing includes determining if the PES streams are
HDTV PES streams, and if the PES streams are HDTV streams, the PTS/DTS
field 275 from the PES headers are multiplied by 300 before they are
compared to the receiver station 60 clock. This comparison allows the
receiver station to appropriately determine the time for presentation of
the data in the PES stream. The transport 380 also passes data that is not
part of a PES stream to a data function 390. Accordingly, except for the
novel feature of multiplying the contents of the PTS | | |
