Pulse Code Modulation (PCM) converts analog signals to a digital format
(signal). This process has four major steps.
• STEP ONE:- FILTERING
Frequencies below 300Hz and above 3400Hz (Voice Frequency range) are
filtered from the analog signal.. The lower frequencies are filtered out to remove
electrical noise induced from the power lines. The upper frequencies are filtered
out because they require additional bits and add to the cost of a digital
transmission system. The actual bandwidth of the filtered signal is 3100Hz
(3400-300). It is often referred to as 4kHz.
• STEP TWO:- SAMPLING
The analog signal is sampled 8000 times per second. The rate at which the analog
signal is sampled is related to the highest frequency present in the signal. This is
based on Nyquist Sampling Theorem. In his calculations, Nyquist used a voice
frequency range of 4000Hz (which represents the voice frequency range that
contains “intelligent” speech). Thus, the standard became a sampling rate of
8000Hz, or twice the bandwidth. The signal that is the result of the sampling
process contains sufficient information to accurately represent the information
contained in the original signal. The output of this sampling procedure is a Pulse
Amplitude Modulated, or, PAM signal.
• STEP THREE:- QUANTIZING
In the third step of the A/D conversion process, we quantize the amplitude of the
incoming samples to one of 225 amplitudes on quantizing scale (figure 3.13).
Thus, in this step the sampled signal is matchrd to the segmented scale. The
purpose of step three is to measure the amplitude (or height) of the PAM signal
and assign a decimal value that defines the amplitude. Based on the quantizing
scale, each sampled signal is assigned a number between 0 and +127 to define its
amplitude.
• STEP FOUR:- ENCODING
In the fourth step of A/D conversion process, the quantized samples are encoded
into a digital bit stream (series of electrical pulses).
A DIGITAL ENCODER
It recognizes the 255 different voltage levels of the quantized samples. Converts
each into a specific string of 8 bits (1s and 0s) that represent a particular voltage
value. Fig.3.14 is helpful for understanding the binary code used in the encoding
step. Each bit position in the 8-bit word (byte) iis given a decimal weight (2 to
some power ), except for the first bit position. Using this coding scheme, we can
code any number between +127 and –127 and zero.
For example:- If the PAM signal measures +45 on the quantizing scale, the output
of the encoding step is 10101101 (fig 3.15). This binary number (or 8 bit word) is
transmitted over the network as a series of electrical or optical pulses. This series
of pulses is called a digital bit stream. The PCM process requires a 64000bps
channel to encode a 4kHz audio input signal because 8000samples/sec.*8
bits/word=64000bps. This is known as the DS0 (Digital Signal 0) or VF (Voice
Frequency) in the digital hierarchy. It is the basic building block of the digital
network.
(signal). This process has four major steps.
• STEP ONE:- FILTERING
Frequencies below 300Hz and above 3400Hz (Voice Frequency range) are
filtered from the analog signal.. The lower frequencies are filtered out to remove
electrical noise induced from the power lines. The upper frequencies are filtered
out because they require additional bits and add to the cost of a digital
transmission system. The actual bandwidth of the filtered signal is 3100Hz
(3400-300). It is often referred to as 4kHz.
• STEP TWO:- SAMPLING
The analog signal is sampled 8000 times per second. The rate at which the analog
signal is sampled is related to the highest frequency present in the signal. This is
based on Nyquist Sampling Theorem. In his calculations, Nyquist used a voice
frequency range of 4000Hz (which represents the voice frequency range that
contains “intelligent” speech). Thus, the standard became a sampling rate of
8000Hz, or twice the bandwidth. The signal that is the result of the sampling
process contains sufficient information to accurately represent the information
contained in the original signal. The output of this sampling procedure is a Pulse
Amplitude Modulated, or, PAM signal.
• STEP THREE:- QUANTIZING
In the third step of the A/D conversion process, we quantize the amplitude of the
incoming samples to one of 225 amplitudes on quantizing scale (figure 3.13).
Thus, in this step the sampled signal is matchrd to the segmented scale. The
purpose of step three is to measure the amplitude (or height) of the PAM signal
and assign a decimal value that defines the amplitude. Based on the quantizing
scale, each sampled signal is assigned a number between 0 and +127 to define its
amplitude.
• STEP FOUR:- ENCODING
In the fourth step of A/D conversion process, the quantized samples are encoded
into a digital bit stream (series of electrical pulses).
A DIGITAL ENCODER
It recognizes the 255 different voltage levels of the quantized samples. Converts
each into a specific string of 8 bits (1s and 0s) that represent a particular voltage
value. Fig.3.14 is helpful for understanding the binary code used in the encoding
step. Each bit position in the 8-bit word (byte) iis given a decimal weight (2 to
some power ), except for the first bit position. Using this coding scheme, we can
code any number between +127 and –127 and zero.
For example:- If the PAM signal measures +45 on the quantizing scale, the output
of the encoding step is 10101101 (fig 3.15). This binary number (or 8 bit word) is
transmitted over the network as a series of electrical or optical pulses. This series
of pulses is called a digital bit stream. The PCM process requires a 64000bps
channel to encode a 4kHz audio input signal because 8000samples/sec.*8
bits/word=64000bps. This is known as the DS0 (Digital Signal 0) or VF (Voice
Frequency) in the digital hierarchy. It is the basic building block of the digital
network.
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