Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1989-03-16
1991-10-29
Crosland, Donnie L.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
375 25, 341 56, 341 58, 341 59, 341107, G02F 100, H03M 500
Patent
active
050621521
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to Pulse Code Modulation. (PCM) transmission techniques. It is particularly concerned with the problem of minimising the demands such techniques put upon the transmitter in the context of a system such as a passive optical network used for telephony.
In standard forms of coding for Pulse Code Modulation, such as the coding scheme recommended by CCITT for telephony, an analogue signal is divided into a number of quantization levels (256 in the case of 8-bit coding) and each quantization level assigned a binary codeword. The quantization levels may be uniformly distributed over the amplitude range of the signal or alternatively a logarithmic distribution may be used. In either case the binary codewords are assigned sequentially to the quantization levels so that, for example, in the CCITT 8-bit coding scheme quantization levels 130, 131 and 132 are assigned binary codewords 1000 0001, 1000 00010 and 1000 0011. The first (i.e. most significant) bit of the codeword is used to denote the sign of the quantization level.
According to a first aspect of the present invention a method of processing a signal having a non-uniform probability density for transmission in pulse code modulated form comprises quantizing the signal and coding the quantized signal using a non-sequential coding scheme in which binary codewords for the quantization levels are chosen in accordance with the probability of the quantization levels and the number of ON bits in the codeword so that quantization levels of higher probability are assigned code words with fewer ON bits.
The present invention provides a coding scheme which minimises the power required to transmit signals such as speech or music which have non-uniform probability densities. The amplitude probability distribution of speech, for example, peaks around zero amplitude and decreases with increasing amplitude. Similarly after quantization the most probable quantization levels are those corresponding to the lowest amplitudes and the quantization levels corresponding to increasing amplitudes have decreasing probabilities. The codeword which consumes the least power is all zeros, 0000 0000 in 8-bit coding, and that which consumes the most power is all ones, 1111 1111. Since the most likely quantization level, i.e. zero for speech, will over a period of time occur most frequently it is allocated the codeword 0000 0000. The nearest eight levels are the next most likely and are therefore allocated codewords having just a single ON bit, i.e. words taken from the set 0000 0001, 0000 0010, 0000 0100, . . . 1000 0000. Such a coding scheme by matching the codewords requiring least power to the most frequently occurring quantization levels effects a marked reduction in the time-averaged power required for transmission of the signal.
The advantages of using a method in accordance with the present invention are found to be particularly great for optical systems using sources such as semiconductor diode lasers. By enabling the transmitter in such a system to run cooler and place less demand on the power supply a significant increase in transmitter reliability is obtained. A further advantage is that with a method in accordance with the present invention near-end cross-talk levels are reduced. Intersymbol interference (ISI) is also reduced.
According to a second aspect of the present invention an optical network includes a central station having a master clock source and being connected to a remote station including signal processing means arranged to process a signal for return transmission to the central station by a method in accordance with the first aspect of the present invention.
In transmission from the central station to a remote station without its own clock it is necessary to code the signal using a conventional line code which allows a synchronous clock to be recovered at the remote station. However for the return direction of transmission there is no need to re-transmit the clock since the master clock, with an appropriate phase shift, can be used to s
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British Telecommunications public limited company
Crosland Donnie L.
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