Pulse Code Modulation (PCM)Analog voice data must be translated into a series of binary digits before they can be transmitted.With Pulse Code Modulation (PCM), the amplitude of the sound wave is sampled at regular intervals and translated into a binary number.The difference between the original analog signal and the translated digital signal is called quantizing error.
PCMPCM uses a sampling rate of 8000 samples per second.Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000).
Converting Samples to Bits Quantizing Similar concept to pixelization Breaks wave into pieces, assigns a value in a particular range 8-bit range allows for 256 possible sample levels More bits means greater detail, fewer bits means less detail
TDMThe process where a transmissionmedium is shared by a number of circuitsin time domain by establishing asequence of time slots during whichindividual channels can be transmitted…Thus the entire bandwidth is periodicallyavailable to each channel
SAMPLING THEOREM “ If a band limited signal is sampled at regular intervals of time and at a rate equal to or more than twice the highest signal frequency in the band, then the sample contains all the information of the original signal” Fs= >2fH
PULSE CODE MODULATION• Voice Frequency range 0- 4 Khz• Sampling the Voice Signal @ 8 Khz (Double the Max. Frequency as per sampling theorem) i.e. 8000s/sec• Sampling time period Ts=1sec/8000• Ts= 125 microsec• Time available for sampling each channel, when we have N total channels=125/N• In PCM, Time frame=125microsec ;time available per chl=125/32 =3.9microsec.
QUANTIZING The process of measuring the numerical values of the samples and giving them a table value in a suitable scale The finite number of amplitude intervals is called the ‘quantizing interval’ like quantizing interval no.1 is 10-20mV; 2 is 20-30mV etc. in a case of 1V signal. Linear quantizing is where the quantizing intervals are of the same size
QUANTIZING Quantization intervals are coded in binary form, and so the quantization intervals will be in powers of 2. In PCM, 8 bit code is used and so we have 256 intervals for quantizing (128 levels in the positive direction and 128 levels in negative direction)
QUANTIZATION DISTORTION The deviation between the amplitude of samples at the transmitter and receiving ends In linear quantization, the distortion is more and to decrease the distortion, the no. of steps in the given amplitude range has to be increased. Due to BW limitations, more quantum levels in small amplitude region are planned results to Non linear (uniform) quantization
COMPANDING Is the process where non uniform quantization is achieved using segmented quantization In companding, to specify the location of sample value, the following are necessary… sign of the sample, the segment no., the quantum level within the segment.
FRAME STRUCTURE In PCM we have 32 Ts and Ts 0 (FAW) carries the synchronization signals and FAW digit value is X 0 0 1 1 0 1 1 . FAW transmitted in alternate frame. In FAW unused frames, supervisory and alarm signals are transmitted Ts 16 carries the signalling information (for 2 channels)
FRAME STRUCTURE For carrying the signalling for all 30 chls and for carrying sync. Data for all frames, in PCM 16 frame pattern is used and it is known as multi frame Duration of multi frame is 2msecs.
PCM Standards THERE ARE TWO STANDARDS OF PCM NAMELY 1) THE EUROPEAN 2 ) THE AMERICAN. THEY DIFFER SLIGHTLY IN THE DETAIL OF THEIR WORKING BUT THE PRINCIPLES ARE THE SAME. EUROPEAN PCM = 30 CHANNELS NORTH AMERICAN PCM = 24 CHANNELS JAPANESE PCM = 24 CHANNELS IN INDIA WE FOLLOW THE EUROPEAN PCM OF 30 CHANNELS SYSTEM WORKING.
JUSTIFICATION TYPES Positive justification: Common synchronization bit rate offered at each tributary is higher than the bit rate of individual tributary. Positive-negative justification Negative justification