Ten years ago when the first version of this article appeared, digital TV was an exotic future technology. Today everyone can stream TV shows to their cell phone. A mature technology doesn’t need to be introduced, but when the magic breaks down and you have a problem it is still useful to understand how things really work.
Today your TV signal is a high speed stream of computer data. It is like a LAN where you can receive but not send information. Broadcast over the air to an antenna, each TV “channel” frequency carries 20 million bits per second. A Cable TV channel frequency carries 40 million bits per second. Each TV program is a stream of compressed video and one or more streams of compressed audio transmitted as 188 byte packets of data. Since even HD TV does not require 20Mbps, a TV station transmits on HD program and one or more Standard Definition programs over a single channel frequency, and cable can carry two HD and several SD programs over each of its 135 frequencies. Since it is just computer data, the stream can be recorded to a hard disk and played back later with no loss of picture or sound quality.
High Definition TV (HDTV) transmits pictures in 720p (720 lines of 1280 dots) or 1080i (1080 lines of 1920 dots). Old TVs are “Standard Definition” (SD) and support 480i (480 lines of 720 dots). However, LCD flat panels can be used for either TV or computer display, and computer monitors add a few additional resolutions that are widely used, such as 1920 by 1200. Theatrical movies are wider than HDTV, so Blu-Ray movies maintain the shape of the wider screen by reducing the number of lines of data from 1080 down to 800 leaving a black margin at the top and bottom of the screen. Be careful because some TV sets are widely advertised as supporting a 1080 signal but in the fine print you discover that they have a native resolution of 1366 by 768. Read the specs carefully.
In the old days, the term “channel” meant both a TV station broadcasting a particular programming (“Channel 3” was a CBS affiliate) and it meant a particular TV frequency on which the signal was transmitted. In the modern world of digital TV, the two meanings have separated. In New Haven, the ABC affiliate is called “Channel 8” but when it switched over to digital broadcast it had to abandon the frequency that old TV sets associated with position “8” on the dial and moved to the frequencies previously associated with “10” on the dial. They still call themselves “Channel 8” because a change would be expensive and confusing. The labelling that a digital TV set applies to digital programming is dynamic. The TV set or tuner device scans all the frequencies and discovers what programming is available and what label each station wants to apply to its programming. So it reports a HD program for Channel “8” (or sometimes “8.1”) without regard for the actual frequency on which it is being broadcast. Over the air broadcast does not change over time, but cable TV moves its programming around when it needs to, so you may periodically have to rerun the cable setup.
Video and audio data has to be compressed before it can be stored on disk or transmitted. Compression takes advantage of the fact that the background does not change much in a scene and that the side of a building is all one color. Over time programmers have come up with more sophisticated tricks to either squeeze the data down into smaller files or else improve the level of detail in the same sized file. Every new compression technique (called a “CODEC”) requires more processing power. Twenty years ago when the DVD first appeared and the FCC was standardizing digital TV, the best option available was MPEG 2. Obviously a lot of new options appeared in two decades, but none of them was widely enough used to be important, until Apple selected AVC for the iPad and iTunes and the same choice was made for most Blu-Ray disks. Advance Video Code (AVC) is a nickname for a technique that has two official standards, H.264 when used in telecommunications and “MPEG 4 Part 10” when used for movies. Since AVC is now used in tablets, smart phones, browsers, and disks, it has become the first really important standard after MPEG 2. Broadcast TV still sends MPEG 2 data (there are a lot of TV sets out there that can’t do anything else) but if you want to record a TV program and view it on your Android tablet, it has to be converted (transcoded) from MPEG 4 to AVC.
Most computers and set top boxes connect to modern TV sets and monitors using an HDMI cable. However, not all equipment is modern. If you have some older stuff, you may want to know about component cables and the other legacy options.
Over the air broadcast gives you network programming for free. If you are close enough to the station, you get a perfect program. However, digital TV doesn’t degrade gracefully like the old TV signal. The picture is perfect, or else you get nothing. Cable and satellite dishes handle the case where you do not get a good signal, and they add additional non-broadcast programming.
With free programs your computer can display TV programs on your monitor. For as little as $40 you can get a USB device that receives TV programs from an antenna or unencrypted programs from cable. However, you probably need to spend $100 to get a fully usable system. That is still a lot less than Tivo or other consumer product solutions.