RF Modulation and Demodulation of Audio and Video Signals

Any information that is broadcast over the airwaves is broadcast via what are known as radio waves. Radio frequency (RF) is the rate of oscillation or frequency at which a particular type of information is transmitted over the airwaves. It doesn’t matter if we’re talking about radio, television, a Bluetooth mouse, your cell phone or the WiFi your computer is connected to; it’s all RF. Both analog and digital data must be converted to RF for transmission.

 

 

Specific RF frequencies or bands of frequencies are assigned for specific uses by the FCC (Federal Communications Commission). Most other countries adopt the FCC’s assignments, although not all do. This allows for some standardization worldwide, allowing the same equipment to be used in different countries.

 

The frequency that a particular that a particular type of data is transmitted with is referred to as its carrier frequency. Regardless of the type of data being transmitted, it has to be overlaid onto the carrier frequency. At the other end, the receiving device will be able to identify the specific data that it is looking for by looking for that carrier frequency. Whatever data is on it is the data that the device wants. When we tune a radio or change channels on at television, we are telling the device what carrier frequency to look for.

 

With much of today’s communications being digital, including television, wireless transmission of this data has become much simpler. Digital data can be transmitted at any frequency. The higher the frequency, the more digital data that can be transmitted per second. Since digital signals are merely a series of ones and zeroes, all that has to happen is to make the ones and zeroes at the frequency to be transmitted.

 

Analog data is another thing entirely. An analog audio or video signal is a severely modified sign wave. With audio, the frequency denotes the note while the amplitude of the sign wave denotes the volume. With video, the frequency denotes the color, while the amplitude denotes the brightness. That makes it much harder to turn audio and video signals into RF for transmission.

 

Before the introduction of digital television, all television and radio signals were analog. Older televisions only had a RF connector on the back, where the antenna was connected; not the plethora of different connections we see on the back of today’s TVs. Video from videocassette tapes and DVDs had to be output by the device as RF, so that it could be transmitted over wire to the TV and accepted by an input in the antenna connection.

 

This process, called RF modulation, consists of converting the incoming radio or television signal to a standard frequency, called an “intermediate frequency” (IF). This made it easier for the next step in the process, combining the audio and video signals with the carrier wave, producing the final RF signal. Even if this signal was not broadcast over the airwaves, the same process had to occur.

 

At the opposite end, in the receiving radio or television, the signal is then demodulated. The first part of this process is to strip off the carrier wave. This is actually a very simple process. Since the carrier wave is a sine wave, it can be easily reproduced by any device.

 

Going back to what I was saying when I talked about a radio tuner or TV channel selector, what is actually happening is selecting the frequency of carrier wave or sine wave to be generated. This sine wave is produced 180 degrees out of phase with the original RF signal. When combined with the RF signal, the carrier wave and the produced sine wave cancel each other out, making it effectively disappear. What is left is the IF.

 

The IF doesn’t need to be converted back to the original frequency to use, as the radio or TVs electronics can work with it the way it is, producing audio and video that is as good as the original.

 

The same basic process can happen with computers as well. A “modem” is really a “modulator/demodulator;” a device that does both modulation and demodulation. When the computer’s data needs to be sent over a telephone line, it needs to be converted to audio signals, because that’s what the phone system is designed to work with.

 

The modulator portion of the computer’s modem does the same thing as what we’ve already discussed for radio and television, combining the computer’s data with a carrier wave. It does one more thing as well, it turns that into an audio signal. That is the rasping sound you hear when a modem is trying to connect to another modem at the other end.

 

Upon receiving the data, the modem strips off the carrier signal in the same way we’ve already seen, with the exception that there is no manual selection of the frequency. That is an automatic function done by the modem. The resulting data is an digital data stream which the computer can use.

 


R.A.M. Rich Murphy