Pulse-code modulation

The pulse - code modulation ( PCM abbreviation ) is a pulse modulation method, which converts a time-and value continuous analog signal into a time and value discrete digital signal. It is used for example in audio technology as part of the G.711 standards and in video technology for digital video signals according to the ITU -R BT 601 standard and is the basis for digital audio applications, the most famous of the Compact Disc.

General

The reaction takes place in the following steps:

In electric circuits, the first step in the form of a sample-and- hold circuit (SH ), and the second and third steps in the form of analog-to- digital converters (ADC) is realized. In some analogue-digital converters of the SH is already integrated as a functional unit.

The number of quantization levels in the binary code is obtained from the number of bits having a codeword. The number of quantization levels largely determine the quantization noise. The larger the quantization levels, the greater the resulting error. The right figure shows a PCM is shown with a dynamic range of only 4 bits, the error is clearly visible. In many applications, a dynamic range of 8 is chosen up to 24 bits for quantization.

The quantization can be done linearly, with equally large ranges of values ​​, or non-linear. In the non-linear quantization larger signal excursions are summarized in a wider range of values ​​and therefore resolved coarser. Small signal excursions however, are quantized with a higher resolution. The advantage is that with fewer bits per sample less quantization noise than linear quantization can be achieved. Application of non-linear quantization in PCM will be that used in the switching technology PCM30 in the processes known as A-law and μ -law.

The advantage of the PCM with binary coding is the fault tolerance of the transmission. There must be at the receiver through the binary encoding only one can be distinguished between high and low signal ( 0 and 1 ). The disadvantage of the PCM is a high data transfer rate, so adapted and extended in various applications PCM methods are used, and the digital information is reduced by source coding.

Extensions

In the differential pulse code modulation ( DPCM) is not saved in each case the whole binary coded value, but in the simplest case only the difference from the previous value. This approach allows lower word lengths and thus higher compression. The so-called delta modulation thus represents a special case of DPCM where the sampling rate is increased to the extent, to the quantization is reduced to only 1 bit and the difference of a sample represents only 1 bit. The delta modulation is a precursor to the delta -sigma modulation, which is the noise shaping and minimize the quantization noise application, for example, that of higher quality AD converters.

In the Adaptive Differential Pulse Code Modulation (ADPCM ) for data reduction scaling the quantization is flexibly designed and adapted depending on the waveform ( adapted). Here, the algorithm estimates for encoding, such as the next value could appear (this is also called prediction), and so adjusts the scaling. Will transfer the difference to the estimated value. Depending on the process can be used a forward or backward prediction, which is the basis for the linear predictive coding (LPC).

History

Decisive contributions to the development of pulse code modulation were the publication of Claude Shannon channel capacity over the news channels and disturbed by Karl Küpfmüller on the system theory of electrical communications.

PCM was developed in the 1930s, among others, by Bell Labs as well as by Alec Reeves, who in 1938 received a patent on a PCM system with sampling rate of 8000 bits per second. The first application was made after 1943 in an encrypted telephone system called SIGSALY. In the 1960s, engineers developed the Japanese broadcasting company NHK recording equipment PCM-based with video tape as the carrier medium. The Japanese record label Nippon Columbia has sought to improve the quality of analog tape recordings and rented a recording device by NHK to perform test shots and then developed his own recording device. Also at the BBC in the early 1970s PCM devices have been developed.

In 1971, released the first recording under the label Denon, which was recorded digitally with the PCM method, as of 1972 were followed by works of classical music with European artists ( Mozart String Quartets K 421 and 458 with the Smetana Quartet ). In 1974 with Bach's Musical Offering ( Paillard Chamber Orchestra ), the first PCM production in Europe. When in 1982 was the launch of the CD, Denon had 400 digital recordings.