Digital Image Formats - A Practical Guide to Video and Audio Compression - page 125

The form factor is either square or rectangular just like the PDA devices and the
phones support viewing in landscape or portrait modes.
The different sizes seem to be used on a per-manufacturer basis. The bulk purchase
of display modules ensures that there is a generic Motorola screen size vs. a generic Nokia
screen size.
Preparing your content for mobile phones will be quite an exacting challenge to pre-
serve any kind of quality at these image sizes. Successful trials have proven that it is pos-
sible, however. One possible technique that saves significant bit-rate capacity is to reduce
the frame rate. While the number of differences between frames will increase, the amount
of data stored for each frame remains roughly the same. This is because of the way that
compression systems reduce the image to a series of smaller tiles. These tiles are called
macroblocks, and we will look at them more closely in Chapters 8 to 14 when we discuss
how video compression actually works.

6.8 Physical and Visible Raster Areas

In the earlier discussion of how analog TV works, we saw that the picture was carried on
a physical raster signal that had synchronization pulses and a defined area within which
the picture is placed. Not all of the physical area is used for visible picture information,
however. A little margin is provided at each end of the line scan, and a few lines are kept
at the top and bottom as well, as those lines happen while the scanning circuits move the
vertical position back to the top of the screen. That vertical repositioning is called flyback,
and the stream of electrons to the CRT tube face is turned off while it happens. The time it
takes for the flyback to happen is called the vertical blanking time.
So, for example, we have a physical raster of 480 lines with 720 samples. Converting
an analog signal to a digital version sometimes leaves a small portion of black on either
side of the picture. There are often some black lines at the top and bottom as well.
It is not necessary to compress these, but they must be there at the analog output if we
are feeding a TV set or video recorder with our decompressed video. If we crop at the input
stage, the playback system must ensure that the output physical raster is correctly sized.
If that is not done correctly, we will find our picture cropped more than we expect at
the output stage. This is not fatal, but if we know about it, a safe margin placed around the
image will prevent problems later.
Figure 6-6 illustrates how the physical raster is derived from the scanned TV signal.

6.9 D-Cinema

Digital cinema (D-cinema) is becoming more sophisticated and operationally easier to
deploy. The IBC and NAB conferences in 2003 and 2004 featured more coverage of this
topic than ever before, with special screenings of full-length feature movies. This included
a digitally processed version of the old Robin Hood movie starring Errol Flynn. The pro-
cessing worked from the original Technicolor separations and combined them digitally.