Video

No such established relationship presently exists for image or video quality. We also
need to consider that compression ratios increase as processor bandwidth increases.
As a rule of thumb, you can expect video compression ratios to increase by an order of
magnitude every 5 years. In 1992, a data rate of 20 Mbps was required for broadcastquality
video. By 1997, this had reduced to 2 Mbps. However, as compression ratios
increase, the quality of the source-coded material decreases. Digital TV provides an
example, as shown in Table 7.1. Acompression ratio of 100:1 yields VHS quality; a compression
ratio of 10:1 yields high-definition TV.
Inconveniently, higher compression ratios also mean the data stream becomes more
sensitive to errors and error distribution (burst errors) on the channel. These can be
coded out by block coding, convolutional coding, and interleaving, but this introduces
delay, and, of course, time is money.
If we take the historical trend forward, by 2007 we could have compression ratios of
500 to 1. These will work very well over low BER consistent physical channels—for
example, an ADSL line specified at 1 in 1010 BER or optical fiber specified at 1 in 1012
BER (1 in 10,000,000,000,000 bits errored—effectively an errorless channel). These
highly compressed media files will work less well over inconsistent, relatively high
BER radio channels.
This brings us to the issue of differential encoding. In JPEG, we compare one pixel
block with another and produce a difference figure. In MPEG, we do the same, but in
addition, we look for similarities from image to image and express these as a difference
coefficient. The problem with differential encoding is that it does not like delivery
bandwidth discontinuity—for instance, burst errors on the radio channel or nonisochronous
packets in the network.
The problem is partially overcome by using periodic refresh pictures. This is known
as intracoding. The refresh pictures are only spatially, not temporally, compressed. Even
using intracoding, differentially encoded video streams can be very jerky when sent
over a wireless network (particularly, as we discuss later, over a wireless IP network).
An alternative is to use JPEG for video. Individual still images become moving images
by simple virtue of being sent at a suitable frame rate per second. JPEG does not use
differencing and therefore avoids the problem, but it does not provide the same level
of compression efficiency.
The better answer is to improve radio and network bandwidth quality. Better radio
bandwidth quality means avoiding burst errors in the radio channel, better network
bandwidth quality means avoiding transmission re-tries and minimizing delay and
delay variability. This then allows the efficiency benefits of differential encoding to be
realized. 172