PCM (Pulse-Code Modulation)
Pulse code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals and each sample is quantized to the nearest value within a range of digital steps.
Characteristics of PCM
- It can use 4, 8 or 16 bits per sample.
- The number of levels depends on number of bits. Level size is fixed.
- Quantization error depends on the number of levels.
- Highest bandwidth is required since the number of bits is high.
- There is no feedback in transmitter or receiver.
- Complex system to implement.
DM (Delta Modulation)
Delta modulation (DM) is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. Delta modulation is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples is encoded into n-bit data streams. In delta modulation, the transmitted data are reduced to a 1-bit data stream.
Characteristics Of DM
- It uses one bit for one sample.
- Step size is kept fixed and cannot be varied.
- Slope-overload distortion is present.
- Lowest bandwidth is required.
- Feedback exists in the transmitter.
- Simple to implement.
ADM (Adaptive Delta Modulation)
Adaptive delta modulation also referred to as Continuously variable slope delta modulation (CVSD) is a modification of Delta modulation (DM) in which the step size is not fixed, rather, when several bits have the same direction value, the encoder and decoder assume that slope overload is occurring and step size becomes progressively larger. Otherwise, the step size becomes gradually smaller over time.
ADM reduces slope error, at the expense of increasing quantizing error. This error can be reduced by using a low-pass filter. ADM provides robust performance in the presence of bit errors meaning error detection and correction are not typically used in an ADM radio design; it is this very useful technique that allows for adaptive-delta modulation.
Characteristics Of ADM
- It uses only one bit for one sample.
- Step size varies according to the signal variation.
- Quantization noise is present but no other errors.
- Lowest bandwidth is required.
- Feedback exists in the transmitter.
- Simple to implement.
DCPM
Differential Pulse Code Modulation (DPCM) is a procedure of converting an analog into a digital signal in which an analog signal is sampled and then the difference between the actual sample value and its predicted value (predicted value is based on previous sample or samples) is quantized and then encoded forming a digital value.
If the input is a continuous-time analog signal, it needs to be sampled first so that a discrete-time signal is the input to the DCPM encoder.
Characteristics Of DCPM
- Bits can be more than one but are less than PCM.
- Number of levels is fixed.
- Slope overload distortion and quantization noise is present.
- Bandwidth required is less than PCM.
- Feedback exists in the transmitter.
- Simple to implement.
Also Read: Difference Between Delta Modulation And Differential Pulse Code Modulation
PCM Vs DM Vs ADM Vs DPCM In Tabular Form
BASIS OF COMPARISON | PCM | DM | ADM | DPCM |
Number of Bits | It can use 4,8 or 16 bits per sample. | It uses one bit for one sample. | It uses only one bit for one sample. | Bits can be more than one but are less than PCM. |
Levels And Step Size | The number of levels depends on number of bits. Level size is fixed. | Step size is kept fixed and cannot be varied. | Step size varies according to the signal variation. | Number of levels is fixed. |
Quantization Error & Distortion | Quantization error depends on the number of levels. | Slope-overload distortion is present. | Quantization noise is present but no other errors. | Slope overload distortion and quantization noise is present. |
Bandwidth | Highest bandwidth is required since the number of bits is high. | Lowest bandwidth is required. | Lowest bandwidth is required. | Bandwidth required is less than PCM. |
Feedback | There is no feedback in transmitter or receiver. | Feedback exists in the transmitter. | Feedback exists in the transmitter. | Feedback exists in the transmitter. |
Complexity | Complex system to implement. | Simple to implement | Simple to implement. | Simple to implement. |