Will I be able to test the device's video processing features by feeding in a test signal - a transport stream generated according to ITU-T J.83.

Is the authorized STB configured for a particular content from the provider?

Some vital components within the generated test stream may be missing or suffer from a lack of SI (system information) or other tables (SI/PSI/PSIP , etc).

The cable box only displays the channels it's told to by the head end system. If you disconnect it from the cable, I don't know how long it will stay authorized. If you're talking about an MPEG transport stream with MPEG-2 video and DD audio, if you know a QAM channel and A/V PID set that's being used by an existing channel it *might* work temporarily, but I wouldn't count on it.

...and even if you could, what good would that do?

...and even if you could, what good would that do?
+1

+1

Testing an implementation of video decompression (MPEG-2,-4, H.264, etc) and video signal post-processing methods is a part of my professional activities.

So, aside from your helpful comments, I figure that the "consumer" unit cannot be tested (see the above post by demonfoo).

I think it can be tested as demonfoo suggested -- feed the box from the cable company (you probably just need their "out of band" channel), then inject your own stream on an existing unencrypted channel and PID set.