Introduction to Pulse Modulation
A system of modulation where the duration, amplitude, position might be so controlled like to present the message to be communicated.
Changing the amplitude, polarity, existence or nonexistence, duration of the pulses provides increase to the four basic types of pulse modulation:
In pulse modulation a significant concept is analog - to - digital (A/D) conversion. An opposite digital- to- analog (D/A) process is employed at the receiver to reconstruct an approximation of the original form. Theoretically, analog - to- digital conversion includes two steps.
1st, the range of amplitudes is spit into a finite number of predetermined levels, and every such level is presented through a pulse of fixed amplitude.
2nd, the amplitude of s(t) is periodically computed and replaced through the pulse presenting the level which corresponds to the measurement
In several practical systems the pulse duration, termed as the duty cycle is inadequate to a fraction of the sampling interval. Such type of a restriction makes the probability of interleaving through one sample interval one or more pulses derived from another PAM systems in a process termed as time - division multiplexing (TDM).
Figure: forms of pulse modulation
PULSE TIME MODULATION
Pulse time modulation (PTM) is a category of signalling technique which encodes the sample values of an analog signal on the time axis of a digital signal.
The two major categorizes of pulse time modulation are:
1. Pulse width modulation (PWM)
2. Pulse position modulation (PPM)
PULSE WIDTH MODULATION
In PWD (Pulse width modulation) the pulses presenting successive sample values constant amplitudes but change in time duration in direct proportion to the sample value. The pulse duration can be altered that relative to fixed leading or trailing time edges or a fixed pulse centre. To permit for time- division multiplexing, the maximum pulse duration might be limited to a fraction of the time among samples.
PULSE - POSITION MODULATION
PPM (Pulse position modulation) encodes the sample values through changing the position of a pulse of constant duration relative to nominal time of occurrence of it. Like in PAM and PWM, the duration of the pulses is usually a fraction of the sampling interval. Additionally, the maximum time excursion of the pulses might be limited.
PULSE DIGITAL MODULATION
The modulation technique that converts the analog signal into digital is known as Pulse digital modulation. PCM comes within pulse digital modulation in which we discretize both amplitude and time. Discretizing time, we know before is termed as sampling. Discretizing amplitude is known as 'quantizing'. Quantizing includes conversion of analog signal amplitude to discrete amplitude.
Several modern communication systems are intended to transmit and receive only pulses of two different amplitudes. Within these so called binary digital systems, the analog- to-digital conversion procedure is expanded through the additional step of coding, where the amplitude of each pulse presenting a quantized sample is converted into a distinctive sequence of one or more pulses along with just two probable amplitudes.
The whole conversation process is termed as pulse-code modulation.
Figure: Pulse-code modulation. 2 (a) three successive quantized samples of an analog signal.(b) With pulses of amplitude V or O. (c) with pulses of amplitude V or -V.
PCM enjoys several significant benefits over other forms of pulse modulation because of the fact that information is presented through a two-state variable.
First, the design parameters of a PCM transmission system rely vitally upon the bandwidth of the original signal.
A benefit inherent in all pulse modulation systems is transmitting signals over a common transmission system by the process of time-division multiplexing.
Pulse modulation systems might acquire a significant bandwidth penalty compared to the transmission of a signal in analog form of it. An instance is the standard transmission of analog voice signal band that limited to 4000hertz over a TI carrier.
PAM, PWM and PPM found important application early on in the development of digital communications.
Transmission systems make use of several forms of time-division multiplexing. The networks that have high speed like the integrated service digital network (ISDN) have also depended heavily on PCM technology. PCM and several modified forms like delta modulation (DM) and pulse code modulation (ADPCM) have also located important application in satellite transmission systems.
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