Nowadays, most video signals have been recorded on tapes in an analog form. The quick uptake of digital video for playback, for example using DVD, has as a consequence that demand for recording video material on a digital recording medium also increases, for example using a recordable optical disc, like DVD+RW, or hard disc based recording devices. A commonly used digital encoding format is MPEG, in particular MPEG2. For recording an analog video signal, the user uses an analog reproducing device, such as a VCR and Camcorder to generate the analog video signal and uses an (MPEG) encoder to encode this signal to a digital format for storing on a digital storage medium. The MPEG encoder may be a stand-alone device, or integrated in another device, like the VCR, television, etc. Via a digital video communication system, such as the P1394 bus, the encoded signal can be supplied to a decoder/renderer or to a digital storage device for subsequent supply to a decoder/renderer. To enable decoding/rendering the encoded signal must comply with the encoding/transmission standard, such as MPEG. MPEG video encoding/transmission is, among others, described in International Standard ISO/IEC 13818-2: 1995(E) Information Technology; Generic coding of moving pictures and associated information: Video and International Standard ISO/IEC 13818-1: 1996(E) Information Technology; Generic coding of moving pictures and associated information: Systems. Especially timing of the stream of digital video is important. To this end, digital video encoding/transmission standards specify clock signals that enable clocks in the encoder and decoder to run in a synchronized manner. These clock signals will be referred to as program clock reference stamps (PCR). Additionally, timing signals are supplied for the individual frames to enable a rendering device to render the frames at the correct time. These timing signals will be referred to as presentation time stamps (PTS). Moreover timing signals can be present that indicate to the decoder when the MPEG decoding can start. These signals will be referred to as decoding time stamps (DTS). The analog or digital video signal supplied to an encoder in addition to the video signal also includes a frame signal (vsync) that indicates frame boundaries in the video signal. The frame signal is thus synchronous with the video frames. The PTS signals in the encoded stream are usually inserted in the encoded video stream synchronous with the arrival of the vsync signal.
For MPEG2 transmission as specified for DVB (Digital Video Broadcast), the timing for the PCR and PTS are based on a same clock with a defined frequency and tolerance. For example, the clock defined by MPEG runs at 27 MHz. and should deviate less than 30 ppm (parts per million). Usually, the frame rate of a video signal is 25 Hz. or 30 Hz. and the vsync pulse has the same frequency. A straightforward conversion of an analog video signal having a frame rate of exactly 25 Hz. will result in an MPEG stream with PTS values present once every 40 ms. However, the deviation of the frame rate of a video signal reproduced with analog reproducing devices may be up to 100 ppm. To be able to obtain a digital video signal which has a PTS once every frame period, the MPEG encoder could be locked to the vsync of the video input signal. The encoded digital video signal thus obtained will have a PCR which deviates 100 ppm and is therefore not MPEG compliant. Such an encoded video stream should not be supplied through a network to a decoder/renderer as the decoding device may not be able to decode the signal. On the other hand, directly using the MPEG clock and ignoring too large deviations in frame rate may result in rendering problems, such as having to skip or repeat frames.