What about a blu ray disc? I thought that is the only true “HD” available, meaning 1080p capability.
However, we have HD tv channels, upconverted dvd’s which play on my HD tv, etc.
I believe there are few true 1080p HD formats (I thought only blu ray), but we still view plenty of what is called “HD”. There is a vast difference in the “HD” tv channels viewed on my HD Sony, although I also understand no broadcast is in tue 1080p HD.
If I have that right?
There is no "true" HD. HD is anything over 480p standard definition (SD). 720 and 1080, interlaced or progressive, are the most common in consumer use. 1080p is the highest-level of HD on the common consumer market, but there are others that go much higher. Blu-ray is simply the winner of the format war for the optical disk successor to DVD. It's just one way of distributing HD content.
There are three factors here: resolution, bitrate and compression. Up the resolution, other two stay the same, picture quality will suffer. Up the bit rate with the same resolution, you can lower compression for better picture quality. Up the compression, you can afford a lower bitrate or a higher resolution, but picture quality will suffer. Obviously what we want is high resolution, high bitrate and low compression. That requires a LOT of storage and a HIGH bandwidth to transmit it.
Blu-ray can store 50 GB dual layer and support movie bitrates up to 48 megabits per second. In practice most movies range around 20-40 megabits per second including audio (that's 18 gigabytes per hour on the high end). This bitrate can support 1080p at 24 frames per second with a very high quality. This is what makes people go "Oooooh" when watching the store demo. But you can get that same quality in a 1080p movie stored on your hard drive, or any modern storage medium for that matter, if your computer is powerful enough to play it.
Over the air to your antenna can't support the bitrate to offer any quality in 1080p, so they just don't do it. Cable doesn't have the infrastructure to support high bitrates for dozens of channels at 1080p, so they don't do it either (as it is, many compress their 720p broadcasts so much that the poor picture quality is quite obvious). Some Internet services offer 1080p streaming, but usually at bitrates much lower than Blu-ray, like 5 Mpbs, meaning quality will suffer. I'd rather see high-quality 720p than poor-quality 1080p.