Image Processing Reference
In-Depth Information
nally designed to deliver only limited speech service using a narrow band of analog fre-
It was only as recently as 1975 that figures of 1200-baud (approximately 1 Kbps)
download and 75-baud upload were provided by British Telecom for the Prestel Teletex
system. Since then, modem speeds have increased to 56 Kbaud. Dialup modems were dis-
placed by ISDN services with dual 64-Kbps lines. Today, asymmetric DSL ( ADSL ) com-
municates down that same original analog copper wire, achieving speeds in the region of
500 Kbps. In optimal conditions, broadband connections deliver potentially as much as
8 Mbps to the home.
Coupling a 500-Kbps connection with the improvements in codec design that yield
good-quality video at 800 Kbps brings us close to an interesting threshold. That critical
point where TV can be delivered via broadband infrastructures is not very far off.
Table 20-2 illustrates some data rates to which content should be compressed when
being delivered over various Internet connections.
With video that can be encoded at a lower bit rate and achieve the same quality, it is pos-
sible to reduce the footprint of that service on the cable connecting the subscriber's home
to the exchange. This reduced bandwidth means that a usable signal can be delivered over
a longer distance. Therefore, your signal reaches more people because the radius of your
catchment circle is increased. Only people within your catchment circle can subscribe to
your service. Therefore, making it bigger for little or no cost will improve your revenues
without requiring you to massively re-engineer your infrastructure. In fact, for an increase
in reach distance of 33%, your catchment expands by 44% due to the effects of
R 2 , which
is a very good return.
Table 20-2 Bit Rate for Encoding Based on Connection Type
Target encode bit rate
28 Kbaud modem
20 Kbps
56 Kbaud modem
32-28 Kbps
64 Kbps ISDN
40 Kbps
128 Kbps dual ISDN
80 Kbps
Corporate LAN
300 Kbps
UK Domestic Broadband (e.g., Demon Internet)
350-500 Kbps
Fast Home Broadband (e.g., Wanadoo)
1 Mbps
Wanadoo Broadband: http:///
Demon Internet:
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