[This is preliminary documentation and subject to change.]
The Program Guide may have to combine information about multiple broadcast sources. For example, data from the local cable company may have to reside in the Program Guide along with data from a satellite service provider. This potential requirement can be problematic when channels from different systems use the same channel numbers.
To deal with issues arising from overlapping channel numbers, Broadcast Architecture implements the idea of tuning spaces. A tuning space is a set of nonoverlapping channels that are all available through the same type of physical channel tuner, such as an analog cable tuner. A broadcast client with multiple tuning devices may provide channels from multiple tuning spaces.
A physical device may support more than one tuning space. For example, if someone moves an analog receiver from San Francisco to Seattle, that person finds that channel 3 on broadcast television is different in the two cities. A viewer with both a cable connection and a conventional broadcast antenna, using a switch box to select between providers, has inputs that represent two tuning spaces. The Program Guide must be able to handle and display information for more than one tuning space.
In addition, a channel in a single tuning space can be shared by multiple broadcast content providers. For example, channel 21 might broadcast network 1 from 9 P.M. to 9 A.M., and network 2 from 9 A.M. to 9 P.M. The Program Guide must also be able to handle and display such shared channels.