Television – Bandwidth reduction system
Reexamination Certificate
1998-03-03
2004-10-19
Kostak, Victor R. (Department: 2614)
Television
Bandwidth reduction system
C348S515000, C348S722000, C375S240280
Reexamination Certificate
active
06806909
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to the field of digital multimedia transmissions and especially to MPEG-2 bit streams.
BACKGROUND
One of the most common operations in TV is switching from one program to another. At the studio, cameras and microphones are switched and mixed to form a program. At the broadcaster (whether broadcast by cable or airwaves), programs are regularly switched to commercials and to other programs. Finally, the viewer is given a choice of several program channels and often switches between the channels, especially between programs.
Currently the switching of analog signals at the studio and at the broadcaster, occurs during vertical intervals. In order to form a picture on a TV screen, first the odd lines of the picture are drawn by an electron gun, from the upper left, across each line, to the lower right side. Then during a vertical interval, the aim of the electron gun is moved from the lower right back to the upper left corner. Then, in a similar manor the electron gun draws the even lines of the picture interlaced with the odd lines. An independent unit of video such as all the even lines (or all the odd lines) is usually referred to as a “frame”.
Currently, play-to-air (PTA) switchers are used to switch analog TV signals. Such switchers include synchronizing circuits, so that when a switch command is received, the PTA switcher waits until the next vertical interval between and then switches. When the program is switched during the vertical interval, there are no resulting flickers or flashes or other anomalies in the picture display during switching. This is known as seamless switching.
In a typical implementation of a PTA switcher, there are two output channels: a program channel and a preview channel. The program channel carries the material that is being broadcast (“aired”), whereas the preview channel is used for viewing only within the studio and it usually carries the program to be switched to next (i.e., the next program to be aired and transmitted over the program channel). The focus herein is on the output stream carried over the program channel since this is the stream that is received by the viewers and has to be displayed seamlessly. Therefore, and unless specified differently, output stream refers to the stream output over the program channel.
Many broadcasters are considering adding digital channels to their broadcasts. In the world, colors, brightness, sounds have practically infinite variations. (i.e. they are analog) For digital broadcasting, analog scenes and sounds, usually must be converted into digital representations in a process known as digitalizing or analog-to-digital (A/D) conversion. Due to the high bandwidth required for uncompressed digital video signals, it is expected that the video signals will require compression even in the production studio. For example, a single channel of uncompressed standard definition, digital video requires transmission of about 250 Mbs (million bits per second) of information (high definition video requires 1.5 Gbs). In digital video, pictures may not be interlaced and the term “video frame” is used to refer to a complete picture.
The digital compression/decompression system can be conceived as: multiple encoders, each of which convert an uncompressed digital signal stream to a compressed stream; a switcher or splicer which switches between the input stream from each encoder to an output stream; and a decoder which decompresses the output stream from the splicer.
The standard for handling digital multimedia data is known as MPEG-2. In MPEG-2, the digital representations of elements (e.g. video, 2-4 audio channels, captions) of a program are compressed (encoded) in a lossy manner (i.e. some information is lost) and the encoded information is transmitted as a continuous stream of bits. At the end of transport, the encoded information is decompressed (decoded) to approximately reproduce the original digitalization of the elements, and the decoded elements are displayed to the viewer.
MPEG-2 streams are organized hierarchically. First, the digital representations for each element are encoded (compressed) into a bitstream known as an elementary stream (ES). Then headers are inserted into each ES to form a packetized elementary stream (PES). The header of each PES contain a decode timestamp (DTS) which specifies when the decoding of the following ES is to be completed, and a presentation timestamp (PTS) which specifies when the decoded information for the following ES is to be presented. For example, a PES header will be inserted before each picture of a video elementary stream and before each frame of an audio elementary stream. Each PES stream is encapsulated (packaged) into a series of transport packets each of which are 188 bytes long and include a header and payload such as the bits of a PES stream. A typical PES stream such as a picture, requires a large number of packets. The header of each packet includes flags, a countdown field, and a 13 bit packet identifier (PID) field which identifies the portion of the PES that the packet is for. For example, all the packets for an MPEG group of pictures may have the same PID. All the packets with the same PID are called a PID stream.
There are several auxiliary PID streams for each program, one of the streams is the program clock reference (PCR) which contains samples of a 27 MHz clock used by the video and audio encoders and decoders. The PID that carries the PCR is called the PCR_PID. Another auxiliary PID stream for each program, contains a program map table (PMT) which lists all the PID's which belong to the program and defines which PID streams contain which elements (video, audio channels, captions, PCR_PID). All the PID streams for a program are multiplexed together (the packets are intermixed, but bits of different packets are not intermixed) so that, for example, the packets for pictures and the packets for audio frames are mixed together.
An MPEG-2 bit stream may include multiple programs. For example, the stream in a cable TV system may include hundreds of programs. The packets for different programs are also multiplexed together so that the decoder has to select the packets of a program in order to decode a particular program. Thus, another auxiliary PID stream is provided containing a program association table (PAT) which lists the PID streams containing the PMT's for each of the programs. The packets of the PAT stream are all identified by a PID=0.
The packets for each program in a multi-program stream may be referred to as a stream or sub-stream. Similarly, the packets for each element or component of a program may be referred to as a stream or substream. Those skilled in the art are accustomed to this terminology.
FIG. 1
schematically illustrates a stream of packets with a packet identifier in the header and video, audio, PCR or PMT data in the payloads. Each packet is actually a continuous stream of bits representing one formatted block as shown. The packets containing data for a first video picture V
1
are mixed with packets containing data for a first audio frame A
1
and packets containing data for a second audio frame A
2
as well as with packets containing PCR times and packets containing PMT information. Note that packets for different video frames in the same program are not mixed and packets for different audio frames in the same program are not mixed. However, for multi-program streams, the packets for a picture of one program would be mixed with packets for pictures of another program. Also, note that the bits of different packets are not mixed, that is, the stream transmits all the bits for one packet sequentially together then all the bits for the next packet sequentially together.
FIG. 2
schematically illustrates the same streams as
FIG. 1
in a different way, by showing a separate bar for each component (element) of the program with vertical lines between PES streams for each picture or audio frame. The separate packets are not shown. In
FIG. 2
, the intermixing of packets for au
Balakrishnan Mahesh
Parthasarathy Kavitha
Radha Hayder
Belk Michael E.
Koninklijke Philips Electronics , N.V.
Kostak Victor R.
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