Appendix 1

The Filmmaker’s Guide to Final Cut Pro Workfl ow
value, like the sheep’s feet, let’s just record one and use it for all four. If the four pixels on the next
line are more or less the same value, the fi rst value can be used for all eight pixels in both lines.
This can also be used from one frame to the next. If the fi rst pixel has the same value as that pixel
in the fi rst four frames, this value can be recorded once and used in all four frames. Compression
within a single frame is called spatial compression. Compression over time from one frame to the
next is called temporal compression.
There is also a system where the actual bit depth is reduced at times in certain parts of the picture.
If your image is white titles over black, you don’t need eight bits to record black. In can even be
recorded with one.
The compression scheme of several popular video formats is based on the MPEG system, which is
an offshoot of Apples QuickTime. There are many forms of MPEG, but all use these systems for
compression.
In MPEG the screen is divided spatially and temporally. The block is 8 pixels wide, 8 pixels high
and 8 frames deep. All of these values are compared and, if similar, one value is recorded and
used in the entire block. If only some are similar, these are combined in smaller blocks until all
pixels are being recorded individually. The amount of compression is controlled by the tolerance
of the compared values. If the compression will only accept an exact match, the compression is
lossless. If it rounds off values that are close but not a match, there is some loss but much more
compression.
Most compression schemes constantly change the tolerance of the pixel matching up and down to
keep the data stream within a specifi c bandwidth. At times, on a highly compressed image, the picture
can look perfect; at times, it can break up into square chunks of image that extend over several
frames. It depends on the amount of compression being used at that moment, which is controlled by
the picture content and the limits of the bandwidth. A complicated image with many colors and
lots of movement will cause a spike in the amount of compression and loss of quality. Conversely,
a very static shot with broad fi elds of color will cause the compression to reduce and image quality
to go up.
Color Compression
Often, the color information is compressed more than the black-and-white information. In the earliest
color video, the color was added to an existing black-and-white signal. The color signal (chrominance)
was always a much lower quality than the black-and-white signal (luminance). But as long
as the luminance was sharp, the entire image looked, more or less, sharp.
RGB color can’t use this trick because the three signals contain only color components. The luminance
is the sum of all three. Compression of one or two of the colors while keeping the third color
sharp would only add color shifts and look a mess. In order to selectively compress color while
maintaining a sharp luminance requires a different type of component video.
The R−Y/B−Y/Y system was developed just for this purpose. In this case, the RGB is combined into
a luminance signal called the Y. If the red information is subtracted from the Y, (R−Y) and the blue
is subtracted from the Y, (B−Y) the remainder is the green information. If the R−Y and B−Y are
160