Information Technology Reference
In-Depth Information
known as 3:2 pulldown. In this way two frames produce five fields and so the correct 60 Hz field rate results. The
motion portrayal of telecine is not very good as moving objects judder, especially in 60 Hz systems. Figure 1.10
shows how the optic flow is portrayed in film-originated video.
When film-originated video is input to a compression system, the disturbed optic flow will play havoc with the
motion-compensation system. In a 50 Hz system there appears to be no motion between the two fields which have
originated from the same film frame, whereas between the next two fields large motions will exist. In 60 Hz
systems, the motion will be zero for three fields out of five.
With such inputs, it is more efficient to adopt a different processing mode which is based upon the characteristics of
the original film. Instead of attempting to manipulate fields of video, the system de-interlaces pairs of fields in order
to reconstruct the original film frames. This can be done by a fairly simple motion detector. When substantial motion
is measured between successive fields in the output of a telecine, this is taken to mean that the fields have come
from different film frames. When negligible motion is detected between fields, this is taken to indicate that the fields
have come from the same film frame.
In 50 Hz video it is quite simple to find the sequence and produce deinterlaced frames at 25 Hz. In 60 Hz 3:2
pulldown video the problem is slightly more complex because it is necessary to locate the frames in which three
fields are output so that the third field can be discarded, leaving, once more, de-interlaced frames at 25 Hz. Whilst
it is relatively straightforward to lock-on to the 3:2 sequence with direct telecine output signals, if the telecine
material has been edited on videotape the 3:2 sequence may contain discontinuities. In this case it is necessary to
provide a number of field stores in the de-interlace unit so that a series of fields can be examined to locate the
edits. Once telecine video has been de-interlaced back to frames, intra- and inter-coded compression can be
employed using frame-based motion compensation.
MPEG transmissions include flags which tell the decoder the origin of the material. Material originating at 24 Hz but
converted to interlaced video does not have the motion attributes of interlace because the lines in two fields have
come from the same point on the time axis. Two fields can be combined to create a progressively scanned frame.
In the case of 3:2 pulldown material, the third field need not be sent at all as the decoder can easily repeat a field
from memory. As a result the same compressed film material can be output at 50 or 60 Hz as required.
Recently conventional telecine machines have been superseded by the datacine which scans each film frame into
a pixel array which can be made directly available to the MPEG encoder without passing through an intermediate
digital video standard. Datacines are used extensively for mastering DVDs from film stock.
[ 6 ] ISO Joint Photographic Experts Group standard JPEG-8-R8
[ 7 ] Wallace, G.K., Overview of the JPEG (ISO/CCITT) still image compression standard. ISO/JTC1/SC2/WG8 N932
(1989)
1.8 Introduction to MPEG-1
As mentioned above, the intention of MPEG-1 is to deliver video and audio at the same bit rate as a conventional
audio CD. As the bit rate was a given, this was achieved by subsampling to half the definition of conventional
television. In order to have a constant input bit rate irrespective of the frame rate, 25 Hz systems have a picture
size of 352 × 288 pixels whereas 30 Hz systems have a picture size of 352 × 240 pixels. This is known as common
intermediate format (CIF). If the input is conventional interlaced video, CIF can be obtained by discarding alternate
fields and downsampling the remaining active lines by a factor of two.
As interlaced systems have very poor vertical resolution, down- sampling to CIF actually does little damage to still
images, although the very low picture rates damage motion portrayal.
Although MPEG-1 appeared rather rough on screen, this was due to the very low bit rate. It is more important to
appreciate that MPEG-1 introduced the great majority of the coding tools which would continue to be used in
MPEG-2 and MPEG-4. These included an elementary stream syntax, bidirectional motion-compensated coding,
buffering and rate control. Many of the spatial coding principles of MPEG-1 were taken from JPEG. MPEG-1 also
specified audio compression of up to two channels.
1.9 MPEG-2: Profiles and Levels
MPEG-2 builds upon MPEG-1 by adding interlace capability as well as a greatly expanded range of picture sizes
and bit rates. The use of scaleable systems is also addressed, along with definitions of how multiple MPEG
bitstreams can be multiplexed. As MPEG-2 is an extension of MPEG-1, it is easy for MPEG-2 decoders to handle
 
Search WWH ::




Custom Search