Why do I need special equipment to receive High Definition TV?

This is a question I recently posed to Doug Finck from WPME, WPXT.

I have a High Def television (tuner). I use a VHF/UHF antenna to receive over the air HD signals. I don’t have cable TV or satellite. A cable company essentially uses a big antenna to receive signals from satellites and send those signals down the wire to a cable subscriber. If that signal being picked up via the satellite is in HD, what stops it from being in HD when it gets to my house over the cable wire. In other words, if there was no cable box in between the signal and my TV, why would I not receive the signal in HD. Or better yet, does the cable box prevent me from receiving HD or is there something going on in that box that is doing the upconverstion. If the cable box is doing the upconversion, why does the signal need to be upconverted if it originated in HD to begin with. Is the cable company stopping the HD signal from showing up in HD format at your house over the wire so that they can charge you money to upconvert it?

Here is his response. (Warning: Contains some technical information but worth the read)

Good question! Our signal is modulated using an 8VSB modulation system. That is the only approved modulation system for digital TV broadcasting in the US. This modulation system is completely different than the old analog system of NTSC (regular old TV signals) which is why the two systems are completely incompatible. But cable does not send their signals to your house using the 8VSB (or NTSC) modulation system. Instead, cable receives an 8VSB digital signal and converts it to a QAM modulated signal. QAM compresses the 8 VSB signal and makes it smaller. Smaller signals use less bandwidth. The less bandwidth cable uses for each signal they send, the more signals they can send. Your set top box converts the QAM signal back to either an 8VSB for your digital TV or an NTSC signal for your analog TV.

This isn’t the only thing cable does to reduce the bandwidth. They also strip out the sub-channels from digital TV broadcasts. By stripping the digital broadcast signal of its sub-channels and then using QAM modulation to compress the remaining signal, cable is able to squeeze out additional bandwidth in the cable/fiber which can be used for more video channels, telephony, internet, etc. At this point the government has endorsed the practice of stripping and compressing signals as long as the primary signal is delivered and is not “materially degraded” in transit. Satellite uses some sort of compression algorithm as well as a different modulation system but I don’t know which ones. Again, they want to compress all signals and increase the number of signals they can offer. The result to the end user is that a pristine signal coming from a broadcaster is compressed or reduced in size to fit through the cable, then expanded to be recognizable by the TV. These two steps do not improve the quality of the signal and can degrade the quality. For the best picture, use an antenna and watch over-the-air HD. You’ll get the entire signal, uncompressed and just the way it was sent out!