A New Method for Clock Recovery in MPEG Decoders

We propose and analyze a new method for reconstruction of the reference clock which is needed for correctly timing the decoding and presentation of video and audio streams in MPEG decoders. Clock recovery is possible by transmitting time stamps called Program Clock References (PCR’s) in the bit stream at the rate of at least 10 per second. The PCR’s are generated at the encoder by sampling the System Time Clock (STC) which runs at ppm MHz 30 27 ± .Since the decoder’s free-running system clock frequency doesn’t exactly match the encoder’s STC, the reference time is reconstructed by means of a Phase Locked Loop (PLL) and the received PCR’s. Always, there’s a difference between the values of the incoming PCR’s and the values they should have when received by the decoder. This difference (Jitter) in input at the receiver’s PLL to a LPF whose output controls the instantaneous frequency of a VCO (Voltage Controlled Oscillator). The frequency variations of the VCO should be restricted according to the system’s clock specifications. This can be done by increasing the order of the LPF but at the expense of increasing the locking-time of the PLL which requires a high amount of buffering. In this paper, we take a new and different approach to solve the problem of clock recovery. In our proposed method, a fixed frequency oscillator is utilized to store the incoming times of the PCR’s and the reference clock is reconstructed by finding the LMS (Least Mean Square) Best Fit line for the last several hundred PCR values. The LMS Best Fit process is repeated by receiving every new PCR to keep the LMS line up-to-date. The proposed method is advantageous because it drastically reduces the required hardware for clock recovery. Significance of the new method is verified by computer simulations ١ Introduction MPEG-2 is the de-facto standard for audio and video compression. The MPEG-2 Systems Layer specifies two mechanisms to multiplex several audio, video or private-data elementary streams for storage or transmission purposes. The first which is called Program Stream is suitable for errorfree while the second, the Transport Stream, is the appropriate choice for data transmission in error-prone environments. An MPEG-2 Transport stream is a single packetized-format multiplex of several programs each composed of video, audio or private-data elementary streams which can be modulated and transmitted over communication networks. Several tables defined by MPEG-2 ∗ This work was supported by the Basamad Negar Company under Grant No. 123456.