Two differences between the original "MultiPort" and the current Decoder Interface are: 1) an IF output (and removal of the video output) from the TV, and 2) expanded two-way communications between the plug-in module and the TV or VCR.
There were two reasons for going to an IF output. The first was to accommodate digital signals in 6 MHz to be decompressed in the external module. The second was to accommodate the Zenith PM scrambling system and the GI audio masking approach. Together, these both form a small fraction of the total number of descramblers deployed. The addition of an IF input in the Decoder Interface module significantly increases the cost of the Decoder Interface module. One of the compromises out of the negotiations is the elimination of a required video output from the TV. This means that even those scrambling systems which can operate at baseband will not be able to. They will have to also include IF processing. This is a serious limitation, but a consequence of the negotiation process. In the event that the Decoder Interface did not accommodate digital plug-ins, this compromise would be even more odious. Under those circumstances, it should be revisited.
The expanded two-way communications between the TV or VCR and the plug-in module are to accommodate the greater variety in cable services now available. On March 29, 1994, cable operators and set-top suppliers completed a series of meetings during which the functions of all then-currently supplied set-top boxes were reviewed and a list of "commands" issued by remote controls to set-top boxes was compiled. To accommodate future needs, we added to that list 10 "soft keys" which we called f1 through f10. There were a total of 56 commands from the remote control to plug-in modules, and 13 commands from the modules back to the TV or VCR. To our disappointment, the consumer side rejected this list of already-in-use commands and demanded we get by with only 23.
More seriously, the consumer side insisted that these "commands" are just the electrical signals on the plug on the back of the TV or VCR. For a full nine months, each time the subject of communication from the consumer's remote control was brought up, the consumer side refused to discuss it in any of the engineering committees. In desperation, the cable side went to the FCC with two extensive demonstrations proving that the IR pass-through concept worked and was necessary for full consumer benefit from the Decoder Interface. The consumer side has rejected this and adamantly refused to consider the IR pass-through and any compromises.
In March 1995, the cable side made an FCC ex parte filing in which it proposed several compromise alternatives to IR pass-through which would yield some measure of unedited, open, near real-time communication from the couch to the plug-in module.
The alternate proposals included a remote control "twin"; remote control standards and "addressable ASCII." The remote control twin was intended to deal with the major objection of the consumer electronics side to IR pass-through: if these undefined signals are allowed to pass through the microcomputer in the TV or VCR, they will confuse it. The "twin" proposes a less-than-$1 dollar addition of a second IR receiver which is totally unconnected to the electronics of the TV or VCR. It goes directly to the plug-in modules.
The addressable ASCII approach involved sending an eight-bit data word to any of the individual plug-in modules. The last alternative was to recognize that enough is known about remote control technology to standardize it.The "addressable half-ASCII" compromise
During a meeting of the EIA and NCTA staff and the co-chairs of the C3AG, the need for open, unedited, near real-time communications from the consumer to the plug-in Decoder Interface modules was again discussed. The consumer side again mentioned that nearly any command could be constructed out of a concatenated sequence of f-codes. These f-codes could be created by macros in the remote control so that the consumer does not have to understand the process; it happens automatically in the remote control designed by the manufacturer of the Decoder Interface plug-in module.
It then occurred to me that instead of having f1 through f12, the addition of just four more f-codes would allow f0 through f15. These 16 codes would each represent four bits of data-half of an eight-bit ASCII word. If two f-codes were concatenated, an eight-bit word would be created. If three f-codes were concatenated, a four-bit address would direct the eight-bit data word to one of 16 devices, yielding the "addressable half-ASCII" approach.
As long as the total time for the three concatenated f-codes was less than 100 milliseconds, it is estimated that this should implement most functions needed. In addition, it was agreed that manufacturers of "cable ready" products will publish their codes so that the manufacturers of Decoder Interface plug-in modules can supply remote controls if necessary. This compromise constituted the first formal recognition that there is a need for communications from the remote control through the TV or VCR to the plug-in module.
There still is a serious issue of latency, i.e., the time it takes from button push to signal at the plug-in module. This compromise also requires the consumer to have a more expensive "universal" remote control.
Perhaps the best thing that can be said about the "addressable half-ASCII" approach is that it appears acceptable to the majority on both sides.