PCM and DPCM are the procedures used to transform the analog signal into digital. These methods are different in that the PCM represents an example value for code words while in DPCM the original and example values ??depend on previous samples.
Analog-to-digital conversion is advantageous for many applications because digital signals are less susceptible to noise. The digital communication system offers better performance, reliability, security, efficiency and system integration. PCM and DPCM are the distinct source coding techniques, we understand the difference between them with the comparison table.
|Number of bits involved||4, 8 or 16 bits per sample.||More than one but less than PCM.|
|Quantization and distortion error||It depends on the number of levels.||Slope overload distortion and quantization noise may occur.|
|Bandwidth of the transmission channel||Requires high bandwidth.||You need less bandwidth than PCM.|
|Reply||It does not provide any feedback.||The feedback provided.|
|Complexity of the notation||Complex||Simple|
|Signal to noise ratio||Good||Average|
|Application area||Audio, video and telephony.||Speech and video.|
|Bits / sample||7/8||4/6|
Definition of PCM
PCM (Pulse Code Modulation) a source coding strategy in which the coded pulse sequence is used to represent the message signal with the help that it tracks the signal over time and amplitude in the discrete form. It involves two basic operations: time discretization and amplitude discretization. There time discretization is performed by sampling and the amplitude discretization is obtained by quantization. It also includes an additional coding step where quantized amplitudes generate simple impulse patterns.
The PCM process divided into three parts, the first transmission at the end of the source, the second regeneration on the transmission path and the receiving part.
The operations performed at the end of origin transmission –
- Sampling – Sampling is a process of measuring the signal at equal intervals in which the message signal (baseband) is sampled with the line of rectangular pulses. These pulses are extremely small to extract the instant sampling process closely. Accurate reconstruction of the baseband signal is obtained when the sampling frequency must be greater than twice the higher frequency component, known as Nyquist speed .
- quantization – After sampling, the message signal undergoes a quantization which provides a discrete representation in time and amplitude. In the quantization process, the sampled instances are values ??integrated into a particular range.
- Encoding – The transmitted signal made stronger against the interference and noise of the quantized signal by translating it into a more appropriate form of signal and this translation known as coding.
Operations performed at the time of regeneration along the transmission path –
The signals are regenerated by positioning the regenerative repeaters on the transmission route. Performs operations such as equalization, decision making and timing.
Operations performed at the end of reception
- Decoding and expansion – After regeneration, the clean pulses of the signal are then combined into a code word. Then the code word is decoded into a quantized Pulse Amplitude Modulation (PAM) signal. These decoded signals represent the projected sequence of compressed samples.
- Reconstruction – In this operation, the original signal is restored to the receiving end.
Definition of DPCM
Differential Pulse Code Modulation (DPCM) nothing but a variant of PCM. PCM is not efficient as it generates a lot of bits and consumes more bandwidth. Therefore, to overcome the problem described above, DPCM was devised. Similar to PCM, DPCM includes sampling, quantization and coding processes. But DPCM differs from PCM in that it quantifies the difference between the actual sample and the expected value. This is why it is called differential PCM.
DPCM uses the common property of PCM in which the high degree of is used correlation between adjacent samples. This correlation is generated when the signal is sampled at the speed greater than the Nyquist frequency. Correlation means that the signal does not adapt quickly to the change from one sample to another.
As a result, the difference between adjacent samples consists of an average power which is less than the average power of the original signal.
Encoding an extremely correlated signal in the standard PCM system produces redundant information. Eliminating redundancy can produce a more efficient signal.
The future value of the redundant signal is deduced by analyzing the past behavior of the signal. This prediction of future value gives rise to a differential quantization technique. When the coded quantizer output, the differential pulse code modulation is obtained.
Key differences between PCM and DPCM
- The number of bits included in the PCM of 4, 8 or 16 bits per sample. On the other hand, DPCM involves more than one bit, but less than the number of bits used in PCM
- Both PCM and DPCM techniques suffer from quantization error and distortion, but to varying degrees.
- DPCM requires less bandwidth while PCM works on higher bandwidth.
- PCM does not provide any feedback. In contrast, DPCM provides feedback.
- PCM composed of complex notation. By cons, DPCM has a simple notation.
- DPCM has an average signal-to-noise ratio. In contrast, PCM has a better signal-to-noise ratio.
- PCM used in audio, video and telephony applications. In contrast, DPCM is used in voice and video applications.
- If we talk about efficiency, DPCM is one step ahead of PCM.
The PCM procedure samples and converts the analog waveform into digital code directly with the help of an analog-to-digital converter. On the other hand, the DPCM does the similar job but uses the multibit difference value.