Pulse Code Modulation

With pulse-code modulation, the analog transmitted signal is converted into digital form by three operations: time sampling, amplitude quantization, and encoding ^[1] .

To convert an analog signal to digital, an analog-to-digital converter (ADC) is used. The ADC at regular intervals measures the amplitude of the analog signal - receives instantaneous values ​​or samples of the signal, then converts the reports into binary words ^[2] .

The instantaneous measured value (report) of the analog signal is quantized by levels (rounded off from the nearest integer). The number of quantization levels is usually equal to or a multiple of the integer power of 2, for example, 2 ³ = 8 , 2 ⁴ = 16 , 2 ⁵ = 32 , etc. The level number is encoded in binary words of length 3, 4, 5, etc. . bit ^[2] .

Then, the output words of the ADC in the parallel code are encoded by transferring to the shift register , clocked by the auxiliaryshear generator . Packs are formed at the output of the shift register ^{[ clarify ]} coded pulses in a serial code . Then the pulse packets are transmitted to the communication channel ^[2] .

The signal sampling frequency (or sampling rate, sampling rate ) to exclude information loss in accordance with the Kotelnikov theorem should be at least twice the maximum frequency in the spectrum of an analog signal.

There are specialized integrated circuits that combine ADCs, shift registers, clocks and other devices.

A demodulator is installed at the receiving end of the communication channel. In the demodulator, bursts of pulses are fed to the serial input ^{[ clear ]} shift register. After pushing all the bits of the pulse train into the shift register, the word from the shift register in the parallel code is written into the input register of the digital-to-analog converter (DAC) . The DAC converts the coded samples of the transmitted analog signal back to analog form. A step analog signal is generated at the output of the DAC. The steps are smoothed out by a low-pass filter (low-pass filter) , at the output of which a transmitted analog signal is generated. The cut-off frequency of the low-pass filter is chosen to be less than or equal to twice the sampling frequency.

A variety of binary codes are used to encode samples in PCM, for example:

• the usual representation of numbers in the binary system , and the sequential transfer of bits of a binary number can be carried out as the least significant bits forward, and high-order bits forward;
• various codes with the detection and elimination of errors in the transmission channel, for example, the Hamming code, the Reed-Solomon code , etc. The simplest of them is the redundant code with the transmission of the parity bit ;
• codes eliminating a constant component in a coded pulse two-level signal, for example, a self-synchronizing Manchester code .

• Differential (or delta ) pulse-code modulation ( DPCM , DPCM ) - PCM, in which the signal is encoded as the difference between the current and previous measured values. For audio data, this type of modulation reduces the required number of bits per sample by about 25%.
  • Adaptive DPCM ( ADPCM ) is a type of DPCM with a variable quantization step. Changing the pitch reduces the bandwidth requirements for a given signal to noise ratio .
• LPCM ( Linear pulse code modulation )

• In digital and IP telephony, PCM is used to convert a voice audio signal into a digital stream transmitted at a speed of 64 Kbps ( main digital channel ).
• PCM is used to convert analog audio to digital in order to store the signal on digital devices and media ( digital audio recording ). A list of some common file formats used to store audio (music, voice, etc.): WAV , MP3 , WMA , OGG , FLAC , APE .
• PCM was used previously in the ITU V.90 modem communication protocols (only the signal incoming to the client) and V.92 (incoming and outgoing signal) to ensure a maximum connection speed of 56 Kbps.

• Delta coding
• Adaptive modulation

• Panfilov I.P., Dyrda V.E. Theory of electrical communication. - M .: Radio and communications, 1991 .-- 344 p. - ISBN 5-256-00869-2 .