Modulation is the process of facilitating the transfer of information over a medium. Sound transmission in air has limited range for the amount of power your lungs can generate. To extend the range your voice can reach, we need to transmit it through a medium other than air, such as a phone line or radio. The process of converting information (ex. voice) so that it can be successfully sent through a medium (wire or radio waves) is called modulation. There are three basic types of digital modulation techniques.
- Amplitude-Shift Keying (ASK)
- Frequency-Shift Keying (FSK)
- Phase-Shift Keying (PSK)
All of these techniques vary a parameter of sinusoid to represent the information which we wish to send. A sinusoid has three different parameters that can be varied. These are amplitude, phase and frequency. Modulation is a process of mapping such that it takes your voice (as an example of signal) converts it into some aspect of sine wave and then transmits the sine wave, leaving the actual voice behind. The sine wave on the other side is remapped back to a near copy of your sound.
The medium is the thing through which the sine wave travels. So wire is a medium and so are air, water and space. The sine wave is called carrier. The information to be sent, which can be voice or data is called the information signal. Once the carrier is mapped with the information to be sent, it is no longer a sine wave and we call it the signal. The signal has the unfortunate luck of getting corrupted by noise as it travels.
In ASK, the amplitude of the carrier is changed in response to information and all else is kept fixed. Bit 1 is transmitted by a carrier of one particular amplitude. To transmit 0, we change the amplitude keeping the frequency constant. On-Off Keying (OOK) is a special form of ASK, where one of the amplitude is zero.
In FSK, we change the frequency in response to information, one particular frequency for a 1 and another frequency for a 0.
In PSK, we change the phase of the sinusoidal carrier to indicate information. Phase in this context is the starting angle at which the sinusoid starts. To transmit 0, we shift the phase of the sinusoid by 180°. Phase shift represents the change in the state of the information in this case.
ASK techniques are the most susceptible to the effects of non-linear devices which compress and distort signal amplitude. To avoid such distortion, the system must be operated in the linear range, away from the point of maximum power where most of the non-linear behavior occurs. Despite this problem in high frequency carrier systems, Amplitude Shift Keying is often used in wire-based radio signaling, both with or without a carrier.
ASK is also combined with PSK to create hybrid systems such as a Quadrature Amplitude Modulation (QAM) where both the amplitude and the phase are changed at the same time.
