What Are We Trying to Compress? - A Practical Guide to Video and Audio Compression - page 38

shipping a range of digital camcorders. Sony, for instance, manufactures the DCR PC105E
model, which will play back DVCAM tapes recorded in low-end studio digital VCRs such
as the DSR 11. DVCAM is widely used in news-gathering operations, and the video is
often manipulated using the 25-Mbps DV25 format. A range of video formats is discussed
in Chapter 5 and tabulated in Appendix E.

3.5 Problems You May Encounter

If the video you are trying to compress is of particularly poor quality, you will find the
results of compressing it to be disappointing. The encoder interprets any significant noise
in the original as frame-to-frame changes. Even non-moving background content will
compress badly if there is significant noise present. Either your compressor will be unable
to reach the target bit rate, or if you cap the bit rate, quality of the video when played back
will be very bad indeed.
Motion artifacts are particularly hard to deal with when your source is interlaced
video. Removing the interlacing is a significant challenge.
Digitizing home movie film may be subject to unstable positioning of the film within
the gate of the projector or telecine unit unless you are using a professional system. This
is called gate weave, and it leads to tracking errors and unintended frame-to-frame differ-
ences that must be motion compensated. Since the camera introduced some gate weave as
well, you have two film-weave artifacts to cope with.
Tools that track the frames and stabilize the video before compression are worth
investing in. Stabilization is not vital to the compression process, of course, but the output
quality will be improved. The bit rate is consumed by compressing movement within the
frame and not by the movement of the frame.
This last point is very subtle, isn't it? This is very typical of the sort of complexity you
will encounter in the compression process.

3.5.1 Dealing With Difficult Content

Preprocessing your video through a noise filter will help the compression process. At the
outset, you are unlikely to purchase products such as Final Cut Pro or Premier as a prior-
ity. Their superior color-correction and noise-reduction plug-ins will rescue footage that
was unusable, and you can add them to your system when you can afford to.
Tracking errors due to gate movement could be corrected by using the motion-track-
ing capabilities of Adobe After Effects to gently coerce the frames back to their optimum
position with respect to one another. A slight loss of resolution will happen because you
must frame a portion of the screen that always encloses a picture. If the film shifts signif-
icantly in the gate, you will lose some of the picture around the edges. A neatly cropped
final output will remedy this, and as long as you only crop the over-scanned part of the
picture, it is unlikely to seriously affect your viewing enjoyment. There is always the
option to scale or composite into a frame of some kind if the movement is significant and
the necessary cropping becomes extreme.