The present invention relates to variable picture rate coding/decoding and conversion of scanning type of a video signal from interlaced scanning to progressing scanning, for example, in highly efficient coding of moving pictures into a bitstream at a small code amount for efficient video data transfer, storage and displaying, such as, MPEG-coding with inter-picture predictive coding.
Moving-picture coding at a low transfer bit rate, for example, 60 kbps, decimates several pictures from an incoming moving picture and encodes the remaining pictures according to need. An incoming moving picture carrying 30 frames per second (30 fps) is decimated, for instance, to 15, 10 or 5 fps. Decimation of pictures decreases the number of pictures, or frames, thus decreasing the amount of generated codes, although, motion smoothness on screen will be degraded a little bit.
Pictures under MPEG-coding are divided into three different types I-, P- and B-pictures. I-pictures (intra-coded pictures) are coded independently, entirely without reference to other pictures. P-pictures (unidirectionally predictive-coded pictures) are compressed by coding the differences the pictures and reference preceding I- or P-pictures. B-pictures (bidirectionally predictive-coded pictures) are also compressed by coding the differences the pictures and reference preceding or upcoming I- or P-pictures.
B-pictures can be removed from a coded bitstream for changing a picture rate because B-pictures are not used as reference pictures. A bitstream of 30 fps with P-pictures for every 3 frames, for example, can be converted into a bitstream of 10 fps by removing B-picture streams only.
A well-known variable picture rate coding performs predictive-coding for P- and B-pictures after decimation. A changed picture rate due to decimation varies a distance for prediction between a picture to be coded and a reference picture. The lower the picture rate, the longer the prediction distance, thus the amount generated codes being not so decreased.
Particularly, in MPEG, B-picture decimation causes lowering of picture rate too much, and can not change picture rate at several stages.
Moreover, for interlaced moving picture, decimation in unit of field lowers vertical resolution and decimation in unit of frame causes time reversal due to frame interpolation.
Decimation in unit of field or frame for varying picture rate is thus not applicable to an interlaced moving picture.
Although it is applicable to progressive moving picture, several scanning lines are decimated in reproduction by interlaced scanning.
This decimation processing causes redundancy in scanning lines and decoding processing. It further causes change in picture rate that depends on the amount of generated codes. Particularly, a picture rate tends to be lowered for moving picture of a big movement on screen by decimation in progress scanning, thus un-smoothness in reproduced picture being noticeable.
Moving picture is composed of interlaced or progressive moving pictures, as discussed above. An interlaced moving picture has been decimated half the scanning lines, however, has resolution about 70% of that in progressive scanning on stationary scenes. The progressive scanning produces almost no line flicker or crawling.
TV broadcast usually employs interlaced scanning, however, digital TV broadcast employs both interlaced and progressive scanning. Cinema films and animation carry progress moving pictures of about 24 frames per second as an interlaced scanning signal. Such an interlaced scanning signal carries 60 fields per second with the same picture for two or three successive fields.
Encoding of moving pictures by MPEG inter-picture predictive coding requires the same bit rate for both interlaced and progressive scanning when the picture rate and the number of scanning lines is the same. Progressive scanning is, however, superior to interlaced scanning on coding efficiency because the former carries scanning lines twice the latter.
A picture rate converted to half in progressive scanning lowers frame correlation, thus a required bit rate being lowered to 60 to 80% of the original rate. The bit rate in progressive scanning is lowered drastically compared to that in encoding in interlaced scanning at the same number of scanning lines. A picture rate is, however, half in progressive scanning, thus loosing smoothness in motion on screen a little bit when reproduced.
A well-known scanning line conversion performs conversion of interlaced pictures into progressive pictures at the same picture rate. Progressive pictures converted to half the picture rate lowers a coding bit rate compared to interlaced pictures having the same number of scanning lines, however, being reproduced rough on screen for rapidly moving pictures.