Broadcast or Internet
You can view TV shows from Netflix or Hulu over the Internet. This is an individual stream of bytes sent from a server somewhere on the Internet to your personal computer, so you can choose any program or movie and start or stop it any time. The problem is that every packet of data has to be individual transmitted from server, through routers and fiber cables, to your machine.
Broadcast Digital TV transmits “NCIS” to every household in America each Tuesday at 8:00 PM. The data is sent once through the interconnected networks and arrives at every home simultaneously. You can watch one of these programs live, or set up your Digital Video Recorder (DVR) to record one or both of them to view later on.
It is said that 20% of the Internet is used streaming Netflix video to viewers. Up to this point, the national network infrastructure has been able to handle the load. However, if tomorrow every household decided to cancel its cable subscription and depend on the Internet for video delivery, the entire system could collapse. Yet the current system of pricing and incentives charges nothing for extra Internet usage while charging a lot for more efficient cable TV delivery.
Packets and Frequencies
Broadcast TV began by assigning frequencies to channels 2 through 12, and later the FCC added additonal blocks of frequencies for higher numbered channels. Between the blocks of frequencies assigned to TV there are frequencies assigned to other users. The entire FM radio dial, for example, is between the frequencies assigned to Channel 6 and the one for Channel 7. Cable also carries FM Radio, but it has no need for any of the other assigned frequencies. So it compresses its channels using all of the available frequencies and has room for 150 channels in the frequencies up to 1 GHz which is the traditional frequency range of cable amplifiers and splitters.
Each frequency that previously carried a single analog TV channel can, with a different modulation technique, carry a digital signal with several high definition and standard definition programs.
The network broadcast TV stations that you receive with an antenna use modulation called 8VSB that is more robust to signals that bounce off nearby buildings or hills. The digital data stream is 20M bps (million bits per second). In modern TV jargon you will find this type of signal called OTA (“Over the Air”) or ATSC.
Cable has no such interference and uses a modulation called QAM that carries twice as much data (40M bps). Cable companies encrypt all the digital channels. There used to be unencrypted HD versions of ABC, CBS, NBC, PBS, CW, and so on called Clear QAM that could be received by TV sets and tuner devices without a cable box. In Oct. 2012 the FCC changed its rules and Clear QAM has disappeared from cable systems.
Traditionally each cable TV company decrypted signals with a “set top box” (STB). The FCC required them to separate out the decryption function in a CableCard device that rents for less than the full STB, and today there are finally a full range of devices that use CableCard capability. This leaves you with three alternatives:
- All Cable Company: Rent a Set Top Box (with optional digital recording or DVR capability) from the cable company and do everything with a remote control and the big screen TV. There is a monthly fee that adds up. If you have a show you really like and want to keep, buy old programs a season at a time on DVD or BluRay or stream them from Netflix.
- Consumer Product: Buy a Tivo Premiere for $100 and install a CableCard in it. There is a monthly subscription charge that adds up over time.
- Computer Do It Yourself: Buy a device from SiliconDust or Ceton and install a CableCard into it. Connect a Windows 7 or 8 computer running Windows Media Center. You get programming updates over the internet for free, without the Tivo monthly subscription charge. TV shows are recorded to your computer hard drive. Premium content can only be viewed on screens attached to the computer, but network shows and regular cable programming can be shared over the network or copied to DVD-R.
Digital cable or broadcast TV transmits data in 188 byte packets. Each 20M bps OTA or 40M bps cable channel frequency carries a continuous stream of these packets. Each packet has an ID number that identifies it as control data or part of one of the logical programs. Packets with the same ID number contain the same type of data, which may be the picture or one of the audio streams (a program may be simultaneously broadcast with English and Spanish audio streams).
The FCC licenses specific broadcast frequencies to individual channels. However, each cable channel frequency can carry twice as much data, so the cable company can combine programming from several sources in the same channel. For example, the Comcast system in New Haven used to carry both CBS and Fox HD network programming on the frequency traditionally referred to as Cable Channel 134 (855 MHz). The program ID number 17 was associated with the High Definition version of WTIC (Fox), while ID number 18 was the HD stream of WFSB (CBS). Two secondary program ID numbers (4 and 19) were associated with standard definition TV streams from both stations carrying local non-network programming.
In the old analog days, a TV or device simply needed to switch to a particular frequency. Since digital TV can carry many different program streams on a single old frequency, a digital TV or device has to know both the frequency and the program ID number. This is complicated, and the cable systems change them when they need to. So digital TVs and devices have a setup mode where they scan all the frequencies, determine which program numbers are in use and then they try to pretend to the end user that nothing has changed. Since CBS used to be old analog Channel 3, and that is how people thought of it for 50 years, the TV set presents it as Channel 3 even if it is really Channel 134 Program ID 18 under the covers.
What is in the packets? Basically network TV is a version of the MPEG 2 standard used for movie DVDs. A newer MPEG 4 format is used on BluRay disks, but the digital TV standards were set almost 20 years ago and in order for all the equipment to work together they cannot change frequently. Since the original standards were set in the 1940’s and this was the first revision, we should not expect anything new for a while.
Picture Quality
The picture and sound are compressed at the TV station. If you pick up all the packets broadcast, then you will get the complete best available picture. If packets are lost, however, this will break the compression algorithm for some area of the screen. Every three seconds a complete screen image is retransmitted, and that resets everything.
As a result, there are three possibilities. You can get a perfect picture. You can get occasional screen freezes with infrequent packet drops, or when the reception gets bad you get nothing. There is no graceful loss of picture as there used to be with analog TV.
For as little as $30 you can buy a small USB stick that receives a digital TV signal from antenna or cable and generates a stream of data to your computer. On cable it will only receive the “clear QAM” signals of network broadcast stations. Bundled software can tune the various network TV stations and can record the program on computer disk or display it on your screen.
It used to be that more expensive TVs used higher quality components and could receive a better picture. Better quality tape recording devices could record a sharper playback, but the copy was never as good as the original. Those were the analog days. The data from the TV networks is a string of numbers. You either receive them or you don’t. If you receive them and write them to a computer disk, then you have made an exact copy of the network transmission. Pass them on later to a TV and you get a picture identical to the original network broadcast.
Bandwidth
While today we may use this technology only to broadcast what seems like 90 repeat episodes of Law and Order every day, there are other possibilities. The TV cable has at least 135 channels each transmitting 40 million bits per second. That is a total of 500 megabytes of data per second transmitted constantly into every home. The entire Encyclopedia Britannica could be transmitted in less than a second (although since a single device only receives one channel at a time it would take a minute to receive it all on just one of the 130 TV channels). One has to assume that there is some data more valuable to broadcast than reruns of Family Feud. The network and technology are in place. We just have to decide to use it for something better.
Even if the programmers decide to fill every available channel with Reality TV shows, there will still be a considerable amount of unused bandwidth. Because TV sets have very little memory and no disk storage, the stream of packets carrying one program’s video and audio has to be transmitted in “realtime”. That is, every second of transmission has to carry about one second of every TV program being transmitted. In a football game, for example, when the two sides line up and everyone stops moving the compression is very efficient and very little data is required to maintain the picture, but the TV system has to maintain enough bandwidth to handle the second after the ball is snapped when everyone starts to move and the compression generates a lot more data to follow all the action. During the periods of inaction (when the network doesn’t fill in with reruns of the last play) the compression has less data to send than the bandwidth reserved, so the unused capability is wasted with “fill” packets that contain no data but just occupy space. These fill packets could be replaced with data if the devices were prepared to record and use it.