Modulation. (continued)

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.