Apparatus for the decoding of video data in first and second formats
View Patent ↗The invention relates to the generation of video displays on a display screen from video data which is received as an encoded MPEG stream of data and then decoded and processed to be displayed. In the invention the video display can be generated in a conventional format or at least one use selectable alternative format, known as trick mode stream, such as a fast forward or fast cue display. The method of the invention allows the amount of data to be placed into the buffer memory prior to decoding and display of the data in the alternate format, to be calculated without reference to the MPEG stream information which is unavailable in the trick mode stream of data. The method therefore minimizes delay in the commencement and end of the display of the trick mode stream display format when selected by the user.
1. A method for generating and processing data for the display of a stream of video data on a display screen connected to a data processing apparatus, said method comprising the steps of:
processing a motion picture expert group compliant data stream of video data selected to be viewed by a user in a first format via said apparatus, the largest frames of said video data known as I frames;
pre-filling a buffer memory in the apparatus with a first threshold level of said video data prior to decoding said video data, the user viewing the same in the first format;
said user selecting with selection means to view said video data in an altered format, and in response;
changing the required level of said video data to be held in said buffer memory for the altered format to a second threshold level;
wherein at the second threshold level the buffer memory substantially accommodates no more video data than that corresponding to a single I frame, plus a small tolerance percentage value;
filling the buffer memory with said video data corresponding to a single I-frame plus a same tolerance percentage value; and
thereby generating an altered format for said video data, wherein the altered format is a fast cue or fast review video display.
2. A method according to claim 1 wherein the second threshold level is used in identifying a value of the separation of the encoded frames in the video data bitstream and this value is used as a substitute for various header field values of the motion picture expert group data stream which may be unavailable.
3. A method according to claim 1 wherein the second threshold level is set at a value to minimize delay in the transition between the generation of video from the normal and altered video formats.
4. A method according to claim 1 wherein the second threshold level of the buffer memory data is estimated by reference to time stamp data transmitted as part of the video data.
5. A method according to claim 4 wherein said time stamp data is carried as part of the systems layer and allows data in the other levels to be time synchronized by referring to and retrieving a common reference time from said time stamp data.
6. A method according to claim 4 including the use of said time stamp data to estimate the size of the I frame data and hence the second threshold level.
7. A method according to claim 1 wherein said video data having been transmitted from a location remote to the apparatus is received by the apparatus.
8. A method according to claim 7 wherein said apparatus is a broadcast data receiver connected to receive data from a broadcaster.
9. A method of generating a video display in a first standard motion picture expert group format and a second user selectable fast forward or fast cue format, said method comprising the steps of:
upon user selection of a fast forward or fast cue format during generation of the display in the first format, obtaining a value indicative of the separation of received encoded frames in a video data bitstream;
using said value as a replacement value to indicate a new threshold level of data to be held in a buffer memory device prior to the commencement of the decoding;
filling the buffer memory device with video data corresponding to a single I-frame;
displaying the frames of data for the fast forward or fast cue display; and
wherein said new threshold level of data is substantially no more than that corresponding to the single largest frame in said video data bitstream plus a small tolerance percentage value.
10. A method of generating a video display as set forth in claim 9 including the additional step of referring to time stamp data transmitted as part of said video data to estimate said new threshold level of data.