Digital radio

You're driving along the freeway and your favorite song comes on the radio. You go under a bridge and—buzz, hiss, crackle, pop—the song disappears in a burst of static. Just as people have got used to such niggles, inventors have come up with a new type of radio that promises almost perfect sound. Digital radio, as it's called, sends speech and songs through the air as strings of numbers. No matter what comes between your radio and the transmitter, the signal almost always gets through. That's why digital radio sounds better. But digital technology also brings many more stations and displays information about the program you're listening to (such as the names of music tracks or programs).

Photo: A typical Roberts DAB digital radio. The big orange button in the middle lets you pause a live radio broadcast and restart it later.

Last updated: May 5, 2009.

The basic idea of radio

Radio is a way of sending electrical energy between two places without using wires. That's why it's often called wireless. The piece of equipment that sends a radio wave is called a transmitter; the radio wave ends its journey at another piece of equipment called a receiver.

It's easiest to understand digital radio if you know something about ordinary (non-digital) radios first. When you pull up the antenna (aerial) on an ordinary radio receiver, it catches some of the electromagnetic energy rushing by. Tune the radio into a station and an electronic circuit inside the radio selects only the program you want from all those that are broadcasting.

How does this happen? The electromagnetic energy, which is a mixture of electricity and magnetism, travels past you in waves like those on the surface of the ocean. These are called radio waves. Like ocean waves, radio waves have a certain speed, length, and frequency. The speed is simply how fast the wave travels between two places. The wavelength is the distance between one crest (wave peak) and the next, while the frequency is the number of waves that arrive each second. Frequency is measured with a unit called hertz, so if seven waves arrive in a second, we call that seven hertz (7 Hz). If you've ever watched ocean waves rolling in to the beach, you'll know they travel with a speed of maybe one meter (three feet) per second or so. The wavelength of ocean waves tends to be tens of meters or feet, and the frequency is about one wave every few seconds.

When your radio sits on a bookshelf trying to catch waves coming into your home, it's a bit like you standing by the beach watching the breakers rolling in. Radio waves are much faster, longer, and more frequent than ocean waves, however. Their wavelength is typically hundreds of meters—so that's the distance between one wave crest and the next. But their frequency can be in the millions of hertz—so millions of these waves arrive each second. If the waves are hundreds of meters long, how can millions of them arrive so often? It's simple. Radio waves travel unbelievably fast—at the speed of light (300,000 km or 186,000 miles per second).

How old-style (analog) radio works

Ocean waves carry energy by making the water move up and down. In much the same way, radio waves carry energy as an invisible, up-and-down movement of electricity and magnetism. This carries program signals from huge transmitter antennas, which are connected to the radio station, to the smaller antenna on your radio set. A program is transmitted by adding it to a radio wave called a carrier. This process is called modulation. Sometimes a radio program is added to the carrier in such a way that the program signal causes fluctuations in the carrier's frequency. This is called frequency modulation (FM). Another way of sending a radio signal is to make the peaks of the carrier wave bigger or smaller. Since the size of a wave is called its amplitude, this process is known as amplitude modulation (AM). Frequency modulation is how FM radio is broadcast; amplitude modulation is the technique used by AM radio stations.

Photo: Digital radios need antennas to pick up signals, just like old-style analog radios.

An example makes this clearer. Suppose I'm on a rowboat in the ocean pretending to be a radio transmitter and you're on the shore pretending to be a radio receiver. Let's say I want to send a distress signal to you. I could rock the boat up and down quickly in the water to send big waves to you. If there are already waves traveling past my boat, from the distant ocean to the shore, my movements are going to make those existing waves much bigger. In other words, I will be using the waves passing by as a carrier to send my signal and, because I'll be changing the height of the waves, I'll be transmitting my signal by amplitude modulation. Alternatively, instead of moving my boat up and down, I could put my hand in the water and move it quickly back and forth. Now I'll make the waves travel more quickly—increasing their frequency. So, in this case, my signal will travel to you by frequency modulation.

Sending information by changing the shapes of waves is an example of an analog process. This means the information you are trying to send is represented by a direct physical change (the water moving up and down or back and forth more quickly).

How is digital radio different from analog?

The trouble with AM and FM is that the program signal becomes part of the wave that carries it. So, if something happens to the wave en-route, part of the signal is likely to get lost. And if it gets lost, there's no way to get it back again. Imagine I'm sending my distress signal from the boat to the shore and a speedboat races in between. The waves it creates will quickly overwhelm the ones I've made and obliterate the message I'm trying to send.

How could we get around this problem? We could arrange in advance that we will communicate by a special code. To this end, I could store hundreds of plastic ducks on my boat, each one carrying a number. If I get into trouble, I could send you an emergency coded message “12345” by releasing just the ducks with those numbers. Suppose I do hit a problem. I release ducks with the numbers 1, 2, 3, 4, and 5—but I send maybe 10 or 20 of each duck to increase the chances of the message arriving. Now, even if a speedboat cuts through the water, there's still a high chance enough of the ducks will get through. Eventually, waves will carry ducks with the numbers 1, 2, 3, 4, and 5 ashore. You collect the ducks together and work out what I'm trying to say.

This is more or less how digital radio works:

• The transmitter sends program signals broken into fragments and coded in numbers (digits).
• The transmitter sends each fragment many times to increase the chances of it getting through.
• Even when things interrupt or delay some of the fragments, the receiver can still piece together fragments arriving from other places and put them together to make an uninterrupted program signal.

To help avoid interference, a digital radio signal travels on a huge, broad band of radio frequencies about 1500 times wider than those used in analog radio. To return to our rowboat example, if I could send a wave 1500 times wider, it would bypass any speedboats that got in the way and get to the shore more easily. This wide band allows a single digital signal to carry six stereo music programs or 20 speech programs in one go. Blending signals together in this way is called multiplexing. Part of the signal might be music, while another part could be a stream of text information that tells you what the music is, the name of the DJ, which radio station you're listening to, and so on.

A brief history of radio

• 1888: German physicist Heinrich Hertz (1857-1894) made the first electromagnetic radio waves in his lab.
  
• 1894: British physicist Sir Oliver Lodge (1851-1940) sent the first message using radio waves in Oxford, England.
  
• 1899: Italian inventor Guglielmo Marconi (1874-1937) sent radio waves across the English Channel. By 1901. Marconi had sent radio waves across the Atlantic, from Cornwall in England to Newfoundland.
• 1906: American engineer Reginald Fessenden (1866-1932) became the first person to transmit the human voice using radio waves. He sent a message 11 miles from a transmitter at Brant Rock, Massachusetts to ships with radio receivers in the Atlantic Ocean.
• 1906: American engineer Lee De Forest (1873-1961) invented the triode (audion) valve, an electronic component that makes radios smaller and more practical. This invention earned De Forest the nicknamed "the father of radio."
• 1910: First public radio broadcast made from the Metropolitan Opera, New York City.
• 1920s: Radio began to evolve into television.

Further reading

Books you can read

• Signor Marconi's Magic Box by Gavin Weightman. Cambridge, MA: Da Capo Press, 2003.

Websites

• US Marconi Museum: The early history of radio.