Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-07-11
2003-11-18
Le, Vu (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06650710
ABSTRACT:
The present invention relates to a method and apparatus for filtering a digital packet data stream as used in a digital audiovisual transmission system, in particular a packet data stream conforming to an MPEG format for a digital television system.
Conventional digital television broadcast systems transmit data in the form of discrete transport stream packets or transport packets, each packet being of a predetermined length and containing a header and a payload. The MPEG-2 standard is the currently favoured standard in this domain and sets out a predetermined format for transport packets.
The packet header comprises general descriptive data regarding the packet, whilst the payload comprises the data to be processed at the receiver. The packet header includes a packet ID or PID identifying the packet. The payload of the packet may contain audio, video or other data such as application data processed by the receiver/decoder to provide extra functions, for example to generate a program guide or other visual data that may be superimposed on the screen over the normally displayed program.
Conventionally, the incoming data stream is filtered by a receiver/decoder according to the PID of each packet. Demultiplexed visual and audio data is then passed directly to dedicated processor elements in the receiver/decoder adapted to generate real time visual and audio images respectively. Teletext and subtitle data are similarly passed directly to the general microprocessor of the receiver/decoder to generate real time teletext and subtitle displays.
Data requiring immediate processing such as visual, audio, teletext and subtitle data is communicated to an appropriate processor in the form of what is conventionally known as a packetised elementary stream or PES. This continuous flux of data, which is formed by assembling the payloads of the transport packets, itself comprises a sequence of packets, each PES packet comprising a packet header and payload. Unlike the transport packet data, the length of a PES packet is variable. The filtered PES streams are processed immediately in order to provide a real time operation.
Whilst visual, audio, teletext or subtitle data are the most common types of data defined by a PES stream, other flux type data may equally be organised as a PES stream.
In addition to PES stream data, other data not requiring immediate processing may also be contained within the payloads of the transport packets. Unlike PES data, which is treated immediately by a processor to generate a real time output, this data is typically processed in an asynchronous manner by the processor. The data is often cyclical, i.e. the functions defined by the data are called up at several intervals by a processor. An example of data of this kind is the so-called Programme Specific Information or PSI data.
In this case, the data contained in the payload of the transport packets is divided into a series of sections or tables, each including a header and a payload, the header including a table ID or TID identifying the table. After the initial PID filtering, a second stage of filtering based on the section header is used to divide out modules with the same TID, and the payloads of these modules are stored and gradually assembled in a memory element in the receiver/decoder to build up the downloaded application. Unlike PES data, the order in which the sections arrive is not important, since the application is assembled over a period of time.
An example of a conventional decoder architecture for filtering PES and PSI type data is shown in the European Patent Application EP 0735776 (Hitachi). Transport packet data is initially stored in a packet landing buffer, after which an initial stage of PID filtering is carried out. Audio and video PES data is immediately passed to audio and video decoders for real-time processing, whilst a second stage of filtering at the PSI section header or TID level is carried out for PSI data, such data being thereafter stored in a system decoder buffer.
The European Patent Application EP 0714213 (LG Electronics) describes a similar decoder architecture and, in addition, discusses the analysis of adaptation field or ADF data contained in the header of transport packets used to convey PES data.
Whilst the use of a set of assembled sections enables complex applications to be downloaded and executed by the microprocessor, the steps required in this process lead to a relatively long processing time since all tables having the same TID must be identified, filtered out and stored in the memory before the application is complete.
It is an object of the present invention to provide a method and apparatus for filtering transmitted data packets which overcomes the disadvantages of the known packet filtering systems.
According to one aspect of the present invention, there is provided a method of filtering a packet data stream as transmitted in an digital audiovisual transmission system and received by a receiver/decoder characterised in that transport packet data is filtered at a first stage by a first digital filter according to the characteristics of the transport packet header, selected filtered data then being passed directly to a memory element within the receiver/decoder.
This method of treating the packet data represents a radical departure to the conventional method, in which data filtered according to the PID packet header value is either sent directly (in the form of a PES stream) to a dedicated processor or is filtered in a second stage according to a TID or PSI header value before being assembled in the memory of the decoder.
By storing transport packet data directly after a first stage filtering, the present invention permits the direct and rapid processing of data that would otherwise have to be compiled over a period of time by assembling a series of tables etc. For example, raw or native data may be carried in the payload of the transport packet for storage in the memory and subsequent use by the main processor of the receiver/decoder.
The transport packet data may be filtered at least according to the value of a packet ID or PID within the transport packet header. However, in a preferred embodiment the data may be filtered according to other bit values within the transport packet header in addition to the value of the packet ID.
The data passed to and stored in the memory element may correspond to only a part of a transport packet, for example, the payload of a packet. Preferably, however, a complete transport packet including the packet header and payload is passed to and stored in the memory element. This facilitates later treatment of the packet data. For example, it may be desired to carry out an analysis of the transport packet header characteristics.
In the case where an MPEG-type format is used, the transport packet data is transmitted in packets of fixed size. In one advantageous embodiment, a filtered packet is passed to a memory element having the same size as the fixed packet size.
Data contained within the payload of the transport packet may itself be organised in the form of secondary packets of data each with a header and a payload. Unlike the transport packets, these secondary data packets may have a variable packet length.
In particular, in the case where the transmitted data is organised in an MPEG format, the data contained in the payload of an MPEG transport packet may be organised in the form of a packetised elementary stream or PES. This packetised elementary stream may correspond to a video, audio, teletext, subtitle or other packetised elementary stream which may be processed directly by a corresponding processor of the receiver/decoder.
In addition to passing selected packets to a memory element, the digital filter may also be used to pass selected data to a processor, e.g. to pass selected data to a visual processor to generate an image, without first being stored in a memory element.
Data may be passed to a processor for execution either directly or via a second digital filter stage. In the case where data contained in the payload of trans
Declerck Christophe
Hamery Dominique
Tranchard Lionel
Le Vu
Rosenthal & Osha L.L.P.
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