What is Multicasting? What Does it Mean for HDTV?

Written by Clearly Resolved
Sunday, 20 May 2007

[ Q ] I understand that one of my local digital TV stations is using its digital bandwidth to broadcast both HDTV and multiple "standard definition" channels. Can they really do this? It seems like it makes HD programs look inferior to stations that aren't doing HD and "standard def" at the same time.

[ A ] Digital multicasting, a new development that's part of the ATSC digital broadcast system in the US, allows local stations to optionally provide multiple channels of programming over the same frequency, instead of just a single channel.

For example, many of the local affiliates for the Public Broadcasting System provide viewers with multiple PBS channels as part of its digital TV service -- an HD program stream channel that typically carries the PBS national HD feed, and one or more standard-definition streams that provide a combination of local and network PBS channels.

Another example of multicasting: many NBC local affiliates use their digital TV service to provide an HD channel, along with the 24-hour "Weather Plus" service that is jointly owned by the network and its participating affiliates.

One way to tell if a particular channel in your market is multicasting is that you'll see channel designations such as "7-1" for the HD channel and "7-2," "7-3," or "7-4" for the subchannels (depending on how many subchannels are operating).

Whenever a digital TV station uses multicasting, a portion of the station's bandwidth that would otherwise be solely dedicated to HD video and digital sound is reallocated to enable delivery of "subchannels." Although broadcasters will normally claim otherwise, any time "bits" are diverted from HD to create a subchannel, HD picture quality will suffer. This likely explains the difference that you're seeing between stations in your market that are multicasting and those that are not.

One of the key issues facing broadcasters is to figure out how to make money from digital television and HDTV, especially in light of the equipment required, the cost of HD program production and the fact that Congress and the Federal Communications Commission (FCC) are mandating that broadcasters make these investments.

Although viewers who wish to see high-definition television at its best are likely to be disappointed, multicasting is likely to be a key element in how broadcasters realize a return on the investments they've made in digital television.

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