Broadcast HDTV and ATSC recording primer

When people hear about recording HDTV for a hard disk personal video recorder like MythTV, they get interested but often make some fundamental wrong assumptions that give them the wrong impression.

This primer will try to clear some of those up.

HDTV is always transmitted (be it for cable, satellite or antenna) as a highly compressed digital file. It starts out as an uncompressed signal in a camera or editing studio, but to broadcast it, it's always turned into a compressed digital file in the MPEG 2 standard.

This is also true for other digital TV, including EDTV (DVD quality, known as 480p) and SDTV -- regular old quality, also known as 480i. The 480 is the number of lines in the picture. The "i/p" we'll get into later, but one way to think about it is that the p (progressive) modes run at twice the number of frames per second, giving smoother motion.

Compressing video like this is a pretty complex task. It takes a lot of computer power or special hardware. You can get fairly cheap hardware to do it for SDTV, but the HDTV pictures have up to 2 megapixels per frame, while SDTV has 1/3 of a megapixel. It's a big jump.

Once the compressed signal arrives at your house, it has to be decompressed (decoded) to put on your TV/Monitor/Display. For over the air broadcast TV, (known as ATSC) there is sometimes a tuner and MPEG decoder built into the TV. Otherwise an external box, often called a set top box though it never sits on top of the typical HD set these days, decompresses the signal, and sends it decompressed to the TV.

Decompressing takes work too, but it's not nearly so hard as compressing. A cheap chip in your box, or a modern 3ghz processor, can do it. The box sends the signal to the TV uncompressed, either over analog (component video) or digital (DVI/HDMI). The uncompressed signal is huge. The digital form (DVI/HDMI) uses about 1.5 gigabits of data per second. That's a thousand times faster than the average DSL connection, 15 times faster than most people's ethernets. Faster than USB 2 or Firewire, faster than your hard drive can write data full blast! Faster than older computers could even store things in their random-access memory. (Modern RAM can go about 25 gbit/s.) It's a lot. It's why nobody sends HDTV around except as compressed digital files.

If you have a hard disk video recorder for HDTV, it records the pre-compressed stream directly. That's different from some recorders for SDTV, like the early Tivo, which would read the uncompressed (analog) signal and compress it in real time, using special hardware chips. The pre-compressed stream is the only thing your HDTV recorder can possibly record.

But your cable box puts out uncompressed video, as does your satellite box, so you can't record them on any consumer HD recorder. Some boxes however, put out the compressed stream over firewire, and you can get that and record it. In fact, they're required to let you get the firewire output of at least the same stations you can pull off the air, but they usually encrypt the rest. They are also allowed to shrink down the image to SDTV, but that's actually hard to do so as a plus they often don't do that today. Some people also have hacked boxes to get the firewire out.

However, there is equipment which the studios use which can record the uncompressed HD video. Today it costs thousands of dollars. In a few years it will cost much less and you'll be able to get one.

(In theory, I can imagine today producing an affordable card that might do 720p at 30fps to something you can record on a very big disk and then compress further, but nobody makes it yet.)

The HD signal comes pre-compressed and set at a given resolution. So you can only record it at that resolution. What you're recording is just bits -- any changes you make will be done later. You can play back at another resolution, and indeed most HDTVs sold today only can display 720 lines so they rescale any 1080 line signal you try to send them. Your computer video card also is capable of doing this rescale in hardware, which is good because it would be expensive to do in software.

ATSC is the broadcast standard in the USA, Canada and some other countries. DVB is the standard used in many other nations, and it has forms for over-the-air and satellite. Cable companies use a different scheme called QAM for digital cable, and there are tuner cards that can receive it, but it's functionally very similar to ASTC and DVB, except they usually encrypt the stream.

ATSC signals in the USA support the "broadcast flag" -- a bit which, if present, commands a compliant tuner card to not let the unencrypted video out. (Even though that's just what's in the air.) In the USA, cards must obey this flag or they can't be made after June of 2005. This is being fought.