Digital Broadcast Re-Transmission Over Cable Networks

Digital satellite broadcasting is to begin in December, 2000, followed by digital terrestrial broadcasting. In order to make the new services that digital broadcasting will bring about, such as data broadcasts and EPG, available to cable TV network subscribers as well, the digitization of cable TV network broadcasting facilities and equipment is indispensable.

1. Present Conditions of Cable Television and Its Digitization Trends
Figure 1: Reception condition of terrestrial/satellite broadcasting
The diffusion ratio of cable TV networks in Japan (Figure 1) shows that the households subscribing to cable TV networks, including larger apartment buildings, accounted for 50% of terrestrial broadcast reception and 30% of satellite services. It is expected that diffusion will reach up to 30 million households (60 % of the total number of households) by the year 2010.
Larger scale providers are extending their businesses toward communication services such as the Internet for their survival. Some have started giving serious consideration to the expansion of their service area and its digitization. Cable TV service providers seem to be setting goals for facility upgrades to correspond with the digitization of BS and terrestrial broadcasting.

2. Re-transmission systems for digital satellite broadcasting
The maximum transmission capacity of a transponder for one digital BS channel is 52Mbps. 6MHz digital cable TV has a capacity of 39Mbps, which is not capable of efficiently and directly re-transmitting digital BS services. The transmission system for digital BS broadcasting is designed with the capacity to transmit data for one channel on multiple TSs (Transport Stream).
Figure 2: Digital BS broadcasting re-transmission schemes over cable TV networks
As Figure 2(a) shows, this will enable digital cable networks to transmit data for one channel utilizing several individual TSs.
Figure 2(b) shows a possible future system that uses a 12MHz band, which is same as two 6MHz channels, and re-transmits the entire data steam of a digital BS transponder.

In order to diffuse a common digital broadcast receiver, NHK has proposed re-transmission plans corresponding to cable TV networks.