Modulation.

What is modulation?
Modulation is where one signal is varied in some way by another signal. The frequency of the signal being modulated should be several times higher than the signal being used to vary it.
The type of variation, determines the type of modulation. For example, Amplitude Modulation (AM)varies the amplitude of the main signal (known as the carrier).

Why modulate?
Low frequency signals, such as speech and music will not travel far, at least not without making everyone between you and your contact deaf.
On the other hand, radio waves can travel great distance and can be filtered (so that you only pick up the one you want). Unfortunately the radio wave, on it's own, is pretty useless.
So, what is needed is some way to hook the sound waves onto the radio waves, so the radio waves will carry the sound further. This is where modulation comes in - the radio signal acts as a carrier for whatever you really want to get across - rather like the box that your Corn Flakes come in. Just like the box, it is thrown away.

What are the different types of modulation?
There are many different types of modulation, but some of the main ones that you may hear about are listed below. For more info about one of the methods, just follow the link.

Amplitude Modulation: The main carrier signal (27MHz for CB) has it's amplitude varied by the signal from your microphone.

Frequency Modulation: The frequency of the main carrier is varied by the audio signal from your microphone.

Sideband: The carrier is usually filtered out, and the signal transmitted will just be either carrier + audio, carrier - audio, or both. The carrier is added again at the other end, by the receiver.

SSB: Single Side Band. Only one of the sidebands is transmitted - either upper or lower.

USB: Upper Side Band - SSB, where the signal transmitted is carrier + audio frequency.

LSB: Lower Sideband - SSB, where the transmitted signal is carrier - audio frequency.

DSB: Double Side Band - refers to signals containing both upper and lower sidebands. Where the carrier has been removed, it may be referred to as "suppressed carrier" DSB.

Which is best?
There is no simple answer for this - each type of modulation has it's good and bad points. Which is best often depends upon the use. Some of the features of the different modes are:

AM:
- Fairly quiet when no signal coming in.
- Weaker signals get quiet, rather than getting blocked out by receiver noise.
- Bandwidth equal to twice the highest modulating frequency. About 6KHz (3 above and 3 below carrier) for speech on CB.
- Prone to sound fading - weaker signals will get quieter.
- Prone to electrical interference on reception.
- Prone to cause interference - AM can be detected easily, by all kinds of electrical circuits.

FM:
- High immunity to electrical interference.
- FM sound should not interfere with stereos, etc.
- Not prone to audio fading - sound level is fairly constant as signal gets weaker.
- Capture effect means that the strongest signal will be heard - others will be blocked out.
- Receiver noise means that the speaker needs to be muted when there is no signal.
- Weak signals disappear into the noise, which gets louder as a signal get weaker. Turning the volume up will not help to hear a weak FM signal.
- FM bandwidth is NOT equal to twice the modulating frequency - it can be much higher.
- Fitting FM in the spacing between CB channels is not easy, and increasing modulation will soon cause interference.
- Interference from FM CBs causes steady symptoms while transmitting. Your sound is unlikely to break through, but you may cause buzzing.

SSB:
- Very quiet when no signal present.
- Weaker signals get quiet, rather than getting blocked out by receiver noise. Increasing the volume may help with weaker signals.
- No capture effect. You may be listening to a strong signal, yet still hear a weaker signal between the speech. This can be useful, as people can still "break in" to a conversation easily.
- Efficient on bandwidth, which is equal to the highest modulating frequency, about 3KHz for speech.
- Efficient use of power - all power is used for audio, no power transmitted until you speak.
- More efficient receiver, as bandwidth is smaller.
- Biasing requirement. An SSB transmitter output must be biased to class B. Although this is less efficient than the class C used for FM, the extra efficiency of SSB far outweighs any losses.
- Any RF amp used MUST be designed for SSB use, including biasing and delay.
- Prone to electrical interference on reception.
- Prone to cause interference - Your voice may break through on other equipment, although it may not be clear.

All types of transmission can cause interference, although the symptoms may be different and FM is unlikely to cause your voice to break through... a buzzing is more likely.
If the precautions listed in the interference section on this site are taken, then the interference problems are much less likely.