Consider user-friendliness. When you change channels, do you have to take out one card and plug in a different one? That's a loser. How about a TV set with several slots? But maybe the TV set manufacturers won't put more than one slot in a TV set. Then the broadcasters in town will have to agree on a single scrambling system that uses the same card for all broadcasters in town. There are ways to do this, with each broadcaster transmitting its own entitlement and authorization messages that are addressed to individual subscribers, but all using the same decryption circuitry.
So all the broadcasters in town would have to agree on a decryption system. Broadcasters compete vigorously with one another; they'd have to cooperate on this. And they would, so long as each station broadcasts its own entitlement messages.
But suppose you live in Princeton, N.J. and can receive both New York and Philadelphia TV stations. You might still need to swap cards, unless all the New York and all the Philadelphia stations agree on the same system. All up and down the East Coast, there will be daisy chains of TV stations that would all have to agree on a single decryption system. And that is all feasible, because they could each control their own subscribers' access, because they each transmit only their own entitlement messages.Security risks
How many recall the initial attack on the VideoCipher scrambling system? Pirates exploited a weakness in the system that allowed access to all scrambled programming if the decoder was legally authorized for any scrambled programming. Customers signed up for the cheapest service, then used illegally modified circuitry to watch all programming.
Of course, that was in 1986. Security technology has improved since then. But so have the pirates' capabilities, as evidenced by the successful attacks on the DirecTV security system.
Moreover, because each TV broadcaster could presumably procure its security cards from any manufacturers, there would be multiple card designs, some of which might be more susceptible to hacking than others. Liability is another issue. If I buy a security card to watch channel 7's scrambled programming, but then use it to steal channel 4's programming, does channel 7 have to pay channel 4 for the lost revenue?
With the above scenario, each broadcaster transmits its own entitlement messages on its own channel. That differs from a cable system, where all entitlement messages are transmitted in a separate out-of-band data channel. And it differs from satellite security systems, where all messages are transmitted on all channels. So no matter what channel you are watching, your security card knows what scrambled channels you are entitled to receive.
But broadcasters don't want to carry entitlement messages for other broadcasters, for obvious competitive reasons. Not only that, but entitlement messages eat up channel capacity.
More importantly, when you change channels, your security card may have to wait for the next transmission of entitlement messages in order to determine whether it is authorized to descramble that programming. The channel change problem is serious. It can be ameliorated by lengthening the "epochs;" that is, the period of time over which an entitlement message is valid. If you tuned to a channel anytime during the past month, then you've received the entitlement message that's valid now. Of course, longer epochs mean reduced security.
Even if broadcasters were willing to carry each others' entitlement messages, which ones would they carry? Would Philadelphia stations carry New York messages in order to serve viewers who live in Princeton? With TV broadcasting, the laws of radio propagation determine which stations you can receive, and the TV stations themselves don't know because it depends on your location, intervening terrain, antenna height, etc.
Some broadcasters, Fox in particular, are keen on delivering scrambled programming. Fox's interest comes partly from having a sister corporation under the Murdoch family umbrella which is a leading player in video security, having supplied both the now-hacked DirecTV scrambling system and similarly-hacked systems for Europe. But apart from Fox, broadcasters don't appear to have given a great deal of consideration to the security risks and channel change problems. And TV set manufacturers won't start building TV sets with security card slots until they're convinced there's a business there. It will take some real effort to get this started. Maybe subsidized TV sets, which I wrote about last month, will do the trick.