Pulse Code Modulation
[1] What Is PCM ?
- Pulse Code Modulation (PCM) Is A Method Of Digitally
Representing Analog Signals, Commonly Used In
Telecommunications, Audio Recording, And Many Other
Applications.
- A Signal Is Pulse Code Modulated To Convert Its Analog
Information Into A Binary Sequence, I.E., 1s And 0s.
- The Output Of A PCM Will Resemble A Binary Sequence.
- Instead Of A Pulse Train, PCM Produces A Series Of Numbers Or
Digits, And Hence This Process Is Called As Digital.
- In Pulse Code Modulation, The Message Signal Is Represented
By A Sequence Of Coded Pulses.
- This Message Signal Is Achieved By Representing The Signal In
Discrete Form In Both Time And Amplitude.
[2] Basic Elements Of PCM
(A) Transmitter Section :
- Low Pass Filter
- Sampler
- Quantizer
- Encoder
(B) Receiver Section :
- Regenerative Repeater
- Decoder
- Reconstruction Filter
(1) Low Pass Filter :
- A Low Pass Filter Helps In Removing The High-Frequency
Components Included In The Input Of The Analog Signal.
- These Frequency Components Are Higher Than The Highest
Frequency Of The Message Signal.
- Hence, A Low Pass Filter Is Added In The Pulse Code
Modulation Technique To Avoid Aliasing Of The Message Signal.
(2) Sampler :
- This Is The Technique Which Helps To Collect The Sample Data
At Instantaneous Values Of Message Signal, So As To
Reconstruct The
Original Signal.
- The Sampling Rate Must Be Greater Than Twice The Highest
Frequency Component W Of The Message Signal, In Accordance
With The Sampling
Theorem.
(3) Quantizer :
- Quantizing Is A Process Of Reducing The Excessive Bits And
Confining The Data.
- The Sampled Output When Passed Through A Quantizer,
Reduces The Unnecessary Bits And Also Helps In Compressing
The Obtained Values.
(4) Encoder :
- The Encoder Is Used For Digitising The Analog Signal.
- Encoder Helps To Allot Each Quantised Level Through A Binary
Code.
- The Sample-And-Hold Process Is Adopted In This. Low Pass
Filter, Sampler, And Quantiser Aids To Convert Analog To Digital
Forms.
- For Example, In An 8-Bit PCM System, Each Sample Is
Represented By An 8-Bit Binary Number.
- Encoding Minimizes The Bandwidth Used.
(5) Regenerative Repeater :
- This Section Increases The Signal Strength.
- It Also Helps To Increase Signal Strength. Hence, The Output Of
The Channel Is Equipped With One Regenerative Repeater
Circuit.
(6) Decoder :
- The Decoder Helps To Form The Original Signal By Decoding The
Pulse Coded Waveform.
- Decoder Acts As The Demodulator.
(7) Reconstruction Filter :
- The Reconstruction Filter Helps To Obtain The Original
Signal.
- In The Pulse Code Modulator Circuit, The Given Analog Signal Is
Digitized, Coded And Sampled.
- The Resultant Signal Is Transmitted In An Analog Form.
- In Order To Obtain The Original Signal, The Whole Process Is
Repeated In A Reverse Pattern.
[3] Advantages And Disadvantages :
Advantages :
- Pulse Code Modulation Is Used In Lon Distance Communication.
- The Efficiency Of The Transmitter In PCM Is High.
- Higher Noise Immunity Is Seen.
- Efficient Method.
- It Provides Secure Data Transmission.
Disadvantages
- The Bandwidth Requirement Is High.
- PCM Is A Complex Process, Since It Involves Encoding,
Decoding And Quantisation Of The Circuit.
- Noise And Crosstalk Leaves Low But Rises Attenuation.
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