System time clock capture for computer satellite receiver

[0001] This application claims the priority of U.S. Prov. Pat. Appl. No. 60/152,192, filed Sep. 2, 1999, which is herein incorporated by reference for all purposes.

[0002] The Digital Video Broadcast (DVB) standards and the Digital Satellite System (DSS) standards are designed for the delivery of digital video and digital audio “programs” to in-home set-top box appliances. More recently, interest has risen in using these standards for high bandwidth data delivery, for example, to personal computers for applications such as Internet access. The data broadcast may include, for example, internet protocol (IP) packets carrying electronic mail, world wide web content, and other data.

[0003] In data broadcast systems, various elementary streams of digital video, digital audio, and digital data typically are time-division multiplexed onto a single transport stream which is broadcast by way of a satellite transponder to multiple receivers. The elementary streams of information or data are generally packetized, as is the transport stream.

[0004] A receiver, for example, may be coupled to a personal computer. Such a receiver would receive the transport stream comprising the information, demultiplex elementary streams of digital video, digital audio, and/or data from the transport stream, and filter (select) those elementary streams destined for that receiver. The receiver further should be able to deliver the elementary streams to host memory of the personal computer for processing or display.

[0005] However, designing a receiver capable of demultiplexing, filtering, and delivering the elementary streams of information poses difficult problems and challenges. For example, critical timing constraints govern the delivery of elementary streams containing digital video and digital audio programs. These critical timing constraints derive, for example, from the specific timing required for proper decoding and presentation of digital video frames on the host computer. Hence, the particular elementary streams must be demultiplexed, filtered, and delivered in real-time in such a way that these critical timing constraints are met. Moreover, accurate synchronization is required in order to meet these critical timing constraints.

[0006] In accordance with embodiments of the present invention, at the start of every transport packet reception, a first latch in the receiver captures a first system time clock (STC) timestamp. The first STC timestamp may be used to keep the receiver and the transmitter synchronized. In particular, if the transport packet contains a program clock reference (PCR) timestamp from the transmitter, then the PCR timestamp is compared with the first STC timestamp. Based on the comparison, control adjustment may be made to the STC frequency in order to maintain synchronization. In a preferred embodiment, this synchronization is accomplished using firmware in a transport controller within the receiver.

[0007] Further in accordance with the present invention, the contents of the transport packet are to be utilized by a decoder that is on a separate node of a host system from the receiver's node. The decoder may comprise hardware and/or software and may, for example, be part of an audio-visual or computer-network interface. To overcome the fact that the decoder and receiver are on separate nodes of the bus, a system and method is provided for the decoder to access the STC counter. The decoder captures a second STC timestamp by writing across the bus to a first register in a bus interface on the receiver. Writing to the first register causes the second STC timestamp to be latched in a second register in the bus interface. The decoder may then complete capture of the second STC timestamp by retrieving it over the bus. The second STC timestamp may be compared to the last PCR timestamp received to provide an accurate indication of elapsed time since receipt of the last PCR timestamp.

[0008] Synchronization between the decoder and the receiver is maintained with a system timestamp captured by the decoder. This system timestamp is adjusted with a scaled offset based on a message delay time between the decoder and the receiver.

[0009] A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.

[0011] FIG. 1 is a schematic diagram including a system in accordance with a preferred embodiment of the present invention. A satellite transponder 102 transmits a signal carrying a transport stream to multiple receiver dishes 104, one of which is shown in FIG. 1. Receiver dish 104 in FIG. 1 is coupled to a host computer system 106 by way of a link interface 111 on a receiver circuit 108. Receiver circuit 108 may comprise an integrated circuit, a circuit board having the different receiver components attached thereto, or any other suitable circuit design or configuration.

[0012] Receiver circuit 108 includes a local bus 110 to which is coupled a transport controller 112, a local CPU 114, and local memory 116. Transport controller 112 receives the transport stream from link interface 111 and handles low-level transport stream parsing. Local CPU 114 comprises a microcontroller that controls operations on receiver circuit 108. Local memory 116 comprises memory such as static RAM, dynamic RAM, or ROM located on receiver circuit 108.

[0013] A bus interface 118 couples local bus 110 on receiver circuit 108 to a host system bus 120 of host computer 106. In a preferred embodiment, host system bus 120 comprises a PCI bus, and bus interface 118 comprises a PCI interface. Also coupled to host system bus 120 are a host CPU 122, host memory 124, an audio-visual interface 126, and a network interface 130, among other components. Host CPU 122 comprises the microprocessor of host computer 106, and host memory 124 comprises memory of host computer 106. Audio-visual interface 126 comprises a graphics interface that is coupled to an audio-visual system 128. Audio-visual system 128 may include, for example, speakers and a CRT monitor or a flat panel display. Network interface 130 comprises a network interface card which is coupled to a local area network (LAN) 131 which is in turn coupled to one or more networked computer system(s) 132. For example, network interface 130 may comprise an ethernet card, LAN 131 may comprise an ethernet network, and networked computer system(s) 132 may comprise personal computers.

[0014] The present invention relates to timing information based on a transmitter System Time Clock (STC) counter that is periodically inserted by the transmitter into a transport stream. There are various circumstances in which it is highly desirable to account for the delay between reception of a packet from the transmitter and actual applications using that packet. This is the case, for example, in applications where a direct memory access engine is used to transfer elementary streams from the receiver, such as described in the concurrently filed and commonly assigned U.S. patent application Ser. No. 09/651,539, entitled “MULTITHREADED DIRECT MEMORY ACCESS ENGINE FOR BROADCAST DATA DEMULTIPLEX OPERATIONS,” having Thomas Gene Adams and Gene Maine as coinventors, which is herein incorporated by reference for all purposes. Such applications may also include flexibile media access control as described in the concurrently filed and commonly assigned U.S. patent application Ser. No. 09/649,792, entitled “FLEXIBLE MEDIA ACCESS CONTROL AND SECTION FILTER HARDWARE ENGINE FOR SATELLITE DATA RECEIVER,” having Thomas G. Adams and Randy R. Fuller as coinventors, which is also herein incorporated by reference for all purposes. Since there is considerable variability in the delay through the steps of demultiplexing the elementary streams and performing the direct memory access through the firmware and software layers, it is desirable to use the calculation of the delay to synchronize the operations.

[0015] Receiver circuit 108 receives the transport stream and utilizes the injected timing information for purposes of synchronizing a receiver System Time Clock (STC) counter with the transmitter STC counter. In accordance with a preferred embodiment, the timing information based on the transmitter STC clock is inserted into a special segment of a MPEG-2 transport packet that is called an “adaptation field.” In particular, the adaptation field includes a Program Clock Reference (PCR) timestamp, which is a sampled value of the transmitter STC counter. When a packet containing a PCR value is initially received, it is timestamped with its local timer value. As the packets are passed to various drivers for processing, uncertainties in the hardware and software timing accumulate. It is therefore not possible simply to receive a PCR value from the received data stream, pass it between host drivers, and use it for decoder and presentation timing. According to embodiments of the invention, the PCR timestamp is instead referenced to a time base accessible to all host software elements. This system time value is referred to herein as “system time,” with a latched value of system time being referred to as a “system timestamp.” Various methods may be used to generate system time, including the use of performance timers or other computer timing references, provided that all host drivers have access to the same system time reference so that system timestamps may be compared.

[0016] Thus, in accordance with one embodiment of the present invention, at the start of every transport packet reception, a first latch in receiver circuit 108 captures a first STC timestamp. The first STC timestamp may be used to keep the receiver and the transmitter synchronized. In particular, if the transport packet contains a PCR timestamp from the transmitter, then the PCR timestamp is compared with the first STC timestamp. Based on the comparison, control adjustment may be made to the STC frequency in order to maintain synchronization. In a preferred embodiment, this synchronization is accomplished using firmware in a transport controller within receiver circuit 108.

[0017] Further in accordance with the present invention, the contents of the transport packet are to be utilized by a decoder that is on a separate node of a host system bus 120 from the receiver's node. The decoder may comprise hardware and/or software and may, for example, be part of an audio-visual interface 126. In an alternative embodiment, the decoder is part of a computer network interface 130. To overcome the fact that the decoder and receiver are on separate nodes of bus 120, a system and method is provided for the decoder to access the STC counter. The decoder captures a second STC timestamp by writing across bus 120 to a first register in a bus interface 118 on receiver circuit 108. Writing to the first register causes the second STC timestamp to be latched in a second register in bus interface 118. The decoder may then complete capture of the second STC timestamp by retrieving it over bus 120. The second STC timestamp may be compared to the last PCR timestamp received to provide an accurate indication of elapsed time since receipt of the last PCR timestamp.

[0018] The time between PCR reception and the decoder capturing the system timestamp may be quite variable. This delay time can be determined accurately by using the receiver and decoder timestamps. The system timestamp is then adjusted to account for this delay:
T_system′=T_system−x(T_decoder−T_receiver).

[0019] The adjusted system timestamp is calculated by offsetting the system timestamp by a scaled message delay. The message delay is the difference between the decoder timestamp T_decoder and the receiver timestamp T_receiver. This message delay is scaled by the scale factor x to take into account that system time may be referenced to a different clock reference than the decoder and receiver timestamps. If they are referenced to the same clock reference, the scale factor may take the value x=1. However, in circumstances, for example, where system time is extracted from a personal computer clock and the decoder and receiver times are extracted from the receiver reference clock, the scale factor may be nonzero.

[0020] Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative embodiments, and equivalents may be used without departing from the spirit of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.