Of course, digital errors can occur during transmission due to channel noise and distortion that the design of digital transmission systems must take into account. This expression makes the important point that although digital transmission is not without noise, it has the great advantage that this noise level is set in advance, as small as possible consistent with the capacity of the transmission channel, and is not increased during transmission or storage. Where σ x 2 is the signal variance, x max 2 is the square of the maximum signal amplitude, and b is the number of bits per digital word, eight in the usual case for voice. (1.6) S N R = Signal variance / Quantization noise variance = 3 ( σ x 2 / x max 2 ) 2 2 b , For the waveform shown, the six quantized samples result in the digital data stream 110 011 001 100 001 101. 1.3 shows a simplified PCM coding into eight quantization levels, each represented by a 3-bit digital word. Each sample is rounded off to 1 of 256 quantization levels, each of which is represented by an 8-bit digital word, that with 8000 samples/s results in a 64 kbps data stream. He demonstrated that good quality speech could be reproduced from a sequence of these approximate sample values rather than the actual ones. Reeves’ big additional step was to define a fixed set of discrete quantization levels over the dynamic range of the signal and to represent each level by a fixed-length digital word, normally 8 bits long. PCM started with the sampling theorem, presuming samples taken at (typically) 8000 samples/s to represent a telephone voice signal bandlimited to 4 kHz. PCM saw limited use during World War II, notably for encrypted communication between President Roosevelt and Prime Minister Churchill.
Analog transmission of digital data creator code#
Digital transmission was not seriously implemented in the telephone network until the 1960s although the foundation for it was the invention of pulse code modulation (PCM) by Alec Reeves in the UK in 1938.
0 kommentar(er)
