Digitally encoded video has become a desirable alternative to analog video for transmitting a visual representation for display. The International Organization for Standards Moving Picture Experts Group has approved an audio/visual digital compression standard designated MPEG-2. Digitization can cause bandwidth expansion, as the transmission of digital bits may require more bandwidth space than would the nondigital analog signal. The digital encoding of a video camera output can produce millions of bits per second, the transmission of which would be prohibitively expensive. Therefore, data rate reduction to reduce transmission costs is desirable, and methods for digital compression have been developed to reduce the average number of bits per page by eliminating redundant or superfluous information.
In normal MPEG video encoding, the quantization matrices force many higher frequency discrete cosine transform (DCT) coefficients to zero as a way to limit the bandwidth of the signal. MPEG-2 technology provides for both constant bit rate (CBR) and variable bit rate (VBR) signal delivery for differing applications. In general, broadcast applications require CBR signals while DVD or file based delivery can be VBR. Due to the highly variable number of bits required to encode the various picture types (both due to picture content and predictive coding), the way CBR delivery is achieved is by monitoring an outgoing picture buffer. The buffer""s fullness is used to provide negative feedback in the quantization step. When the buffer gets too full, the quantization scale factor is increased, thereby truncating more bits and lowering the bit rate flowing into the buffer. The bits which are siphoned off by this process may be discarded, thereby reducing the quality of the transmitted video. In some instances, these bits are requantized and transmitted as enhancement bits in a single stream with the basic lower quality video, for the conventional MPEG-2 system is designed to transmit compressed digital video data in a single stream. The problem with transmitting the basic digital video and a block of enhancement bits in a single stream is that all reception stations are required to receive the total stream, thereby incurring the costs involved in transmitting the complete stream to stations which are to receive only the basic, unenhanced signal. These stations operate to discard the transmitted enhancement bits and the transmission costs for these bits are lost.
It is a primary object of the present invention to provide a novel and improved video encoding/decoding method for subdividing a high quality video signal into a plurality of MPEG compressed transport streams (enhancement wedges) and a base transport stream (base wedge) which are separately transmitted and selectively recombined at decoders to form a series higher quality MPEG-2 compliant video streams.
Another object of the present invention is to provide a novel and improved video encoding method for creating from a high quality video signal a low bit rate MPEG-2 video base transport stream while allowing bits to be siphoned off into one or more separate transport streams (enhancement wedges).
Yet another object of the present invention is to provide a novel and improved video encoding method for subdividing a high quality video signal into a video base transport stream and one or more transport streams (enhancement wedges) and providing a parallel encoding process for MPEG encoding each transport stream including the base transport stream.
A further object of the present invention is to provide a novel and improved video encoding method for subdividing a high quality video signal into a video base transport stream (base wedge) and one or more enhancement transport streams (enhancement wedges) with each enhancement wedge carrying information on wedge bit allocations.
A still further object of the present invention is to provide a novel and improved video encoding/decoding method for subdividing a high quality video signal into a base transport stream (base wedge) and a plurality of enhancement transport streams (enhancement wedges) and selectively providing a subscriber with only the base transport stream or the base transport stream combined with one or more of the enhancement wedges to which the subscriber subscribes to provide varying quality video to different subscribers.
These and other objects are achieved by providing a novel and improved video encoding/ decoding method wherein a normal MPEG encoding process is employed. However, when the outgoing picture buffer becomes full, bits from the encoded video which are siphoned off by the quantizing step are divided out into separate enhancement wedges while the remaining encoded video is transmitted as a base wedge or stream. The prequantized signal is compared to the postquantized signal to determine if additional bits should be shifted from a lower enhancement wedge to the next highest enhancement wedge. The base wedge and each enhancement wedge are transmitted separately to selective receiving locations and are not transmitted in a single stream. Thus receiving stations which are to receive different video qualities receive only the video quality designated for that receiving station, and no bits need to be discarded at receiving stations which subscribe to lower quality video signals.