[Multimedia] understanding of PCM, DPCM, APCM, and ADPCM

The multimedia is suspended. Let's talk about a few irrelevant theme words. Although it can behave like nothing, it is quite lost. I have never believed that scores can be used to measure a person's level or something, but I still have a bottom line on my requirements. The bottom line is that I stick to my own style of capital. Over the bottom line, all justified reasons will turn into the arrogant requirements of the 80 s. Why have I fallen to the point where I am not involved? How can I spend the next six months? Should I reflect on myself? Should I stick to my own bottom line and be gentle and gentle?

However, logtail does have positive significance. A book that has never been read, and finally has a chance to read it. Recently, I will take some study notes on multimedia technology, which makes up for my own little wish.

PCM (pulse code modulation -- pulse Coding Modulation)

Pulse Modulation refersContinuous Time, continuous ValueThe analog signal is transformedDiscrete Time, discrete valueAfter the digital signal is transmitted in the channel.
I personally think the above sentence is quite classic.

Pulse Coding ModulationIt is the process of first sampling the analog signal, then quantifying the sample value amplitude and coding.

So-called samplingThat is, periodic scanning of analog signals converts consecutive time signals into discrete time signals. after sampling, the analog signal should also contain all information in the original signal, that is, the original analog signal can be restored without distortion. the lower limit of the sampling rate is determined by the sampling theorem. in this experiment, the sampling rate is 8 kbit/s.

QuantizationThat is, the instantaneous values obtained after sampling are discrete, that is, the instantaneous sampling value is represented by the nearest level value using a set of defined levels.
After a simulated signal is sampled and quantified, a quantified pulse amplitude modulation signal is obtained, which is only a finite number.

So-called EncodingA group of binary code groups is used to represent each quantified value with a fixed level. however, quantization is actually completed in the encoding process at the same time. Therefore, the encoding process is also called a modulo/number transformation, which can be recorded as A/D.

DPCM (differential pulse code modulation -- Differential Pulse Coding Modulation)
In PCM, each sample of the waveform is independently encoded. However, the vast majority of signals (including voice signals) sampled at a given rate or a higher rateThere is a significant correlation between adjacent samples.In other words, the mean variation between adjacent sampling ranges is small. Therefore, the bit rate of the voice signal will be reduced by the redundant encoding scheme in the sampling.
A simple solution isAdjacent sample difference encoding instead of coding the sample itselfBecause the difference between adjacent samples is smaller than the actual sample range, it indicates that the difference signal requires a small number of digits. An improvement solution for this common method is to use the N samples to predict the current sample according to certain rules, and then quantify the error between the predicted value and the actual value, based on the error signal, the original signal is restored using the same prediction method as the sending end. This is DPCM.

APCM (Adaptive pulse code modulation -- adaptive pulse Coding Modulation)
Adaptive pulse coding modulation isChange the quantization level based on the input signal amplitudeWaveform encoding technology. This kind of adaptability can beInstantaneous adaptiveThat is, the quantization order size changes every few samples. It can also beSyllable adaptiveThat is, the quantization order size changes over a long period of time.
There are two ways to change the quantization order: Forward adaptive (forward adaptation) and backward adaptive (backward adaptation ).

ADPCM (adaptive differential pulse code modulation -- adaptive Differential Pulse Coding Modulation)
ADPCMCombining the adaptive feature of APCM and the difference feature of DPCM, APCM is a waveform code with better performance.. Its core idea is: 1. Use adaptive changes to the size of the quantization order, that is, use a small quantization order to encode a small difference, and use a large quantization order to encode a large difference. 2. Use the previous sample value to estimate the predicted values of the next input sample, so that the difference between the actual and predicted values is always the smallest.