Originally Posted by action_jackson
What does the TA do? I know they keep talking about all the clear qam channels being fased out soon and that I need a box for each television. Does the TA allow me to get the digital channels or something?
In short, the TA provides the two-way communications with the headend that are required to receive channels that are transmitted as Switched Digital Video (SDV). SDV channels are not streamed continuously on a fixed channel & subchannel. They are only streamed from the headend to your SDV group when at least one customer in that SDV group has requested (tuned to) that channel.
When you tune to an SDV channel, the tuning adapter sends an upstream request for that service to the headend. If you are the first subscriber in your group to request that channel, the headend routes the transport stream for that channel to an available RF channel/subchannel, and then reports to your TA what that channel/subchannel is. The TA then supplies that information to your CableCard tuner so that it can tune to the temporary RF channel/subchannel for that service.
If you are NOT the first subscriber to request a particular SDV service in your group, then the transport stream routing would already be in place at the headend and the headend only has to inform your TA of the existing channel assignment. Typically, a list of the currently running stream assignments (called a carousel) is regularly broadcast to and stored by all STBs/TAs. This gives them a chance of tuning an SDV channel even if upstream communications needed to request a particular stream setup is temporarily interrupted.
If you are the only subscriber watching a particular SDV service and you tune away from it, the streaming of that service will be discontinued and the channel/subchannel it was occupying will be returned to the SDV pool, available for reassignment to a new SDV channel request.
The whole point of SDV is to be able to offer more services in the same RF bandwidth since services not currently being viewed can be turned off instead of occupying bandwidth with a transport stream that nobody is watching. Heavily viewed services like ESPN, USA, TNT are typically not run as SDV services since the likelihood is very high that somebody will always be watching them. Cable ops need to carefully monitor and manage several parameters to insure that an SDV system operates seamlessly from the perspective of the subscriber, including: the specific services run under SDV, the amount of bandwidth allocated to SDV, and the number of subscribers in an SDV group must be carefully balanced to avoid exhausting the pool of SDV slots available. When that happens, you get the dreaded "channel temporarily unavailable, please try again" message. (Note: that message can also be caused by interruptions in the two-way communications with the headend... often caused by things a subscriber might do within his own home beyond the control of the cable op... loose connections, added splitters, adding an amplifier without a(n) (active) return path, etc.).