Field of the Invention
The present invention relates to signal processors and to a signal format. In particular the invention relates to a signal format, a signal encoder for encoding a signal according to the format, a corresponding decoder, and a signal transmission system including the encoder and decoder.
It is desired to transmit packetised signals such as MPEG 2 TS (Transport Stream) packets from one location or piece of equipment to another. It is known to transmit MPEG 2 TS packets via a DVB Asynchronous Serial Interface (ASI). DVB ASI is effective for the transmission of one transport stream from point to point such as between specific items of equipment but is otherwise relatively inflexible.
According to one aspect of the invention, there is provided a transmission system in which MPEG 2 TS packets are transmitted via an SDTI system.
Such a system provides greater flexibility than using DVB ASI. The SDTI (Serial Data Transport Interface) is defined in SMPTE 305M. SDTI transmits packets in a signal structure comprising frames of television lines. Ancillary data is carried in the horizontal blanking area of lines and data is carried in a payload area of each line. The payload area is in the active line interval. SDTI allows TS packets to be routed wherever SDI connections are available and also allows TS packets from more than one source to be transmitted. However, the carriage of TS packets over SDTI requires buffering to ensure that the packets are confined to the payload area of the SDTI and to allow multiple packets on each line for efficiency. The buffering process introduces delay and jitter (i.e. variation in the timing of the packets relative to each other) to the packets but, for accurate decoding of an MPEG 2 signal, the packets of that signal must be provided to the MPEG decoder with accurate timing relative to one another to allow correct decoding. Whilst absolute delay of packets is not a problem because it affects all packets equally, there is a need to correct jitter at or before the MPEG decoder.
Whilst the foregoing discussion describes the technical problem faced by the present invention with reference to the transmission of MPEG 2 TS packets via SDTI, similar problems may occur in the transmission of other types of time sensitive packets over other data transmission systems.