Method and apparatus for spectral shaping in signal-point limited transmission systems

DETAILED DESCRIPTION OF THE INVENTION Methods and apparatus for spectral shaping in signal-point limited transmission systems are disclosed.
In the following description, various aspects of the present invention are presented in terms of process flows, functional block diagrams and even mathematical expressions, which are the ways those skilled in the art communicate with each other in conveying their ideas and designs.
FIG.
2 illustrates a block diagram showing one environment where the spectral shaping in accordance with one aspect of the present invention is applicable.
Source samples 200 from a data source, such as the ISP's server, are transmitted by a transmitter 210 (of an ISP modem) to generate transmitted samples 220.
The transmitted samples 220 are transmitted, through a channel 240, to the local loop and eventually the user 250.
Those skilled in the art will recognize that the channel 240 is PCM-based with a limited set, i.
e.
255 in .
mu.
-law or A-law, of signal levels, since it is specified by the telephone networks.
Those skilled in the art will also appreciate that the transmitter 210 must be one using lossless coding such that no information is lost between source samples 200 and transmitted samples 220.
To avoid sending the low-frequency energy to the channel 240, the low-frequency component must be eliminated (230) as the source samples 200 are coded by the transmitter 210 based on a set of characteristics of the frequency spectrum.
In other words, the running digital sum ("RDS") of any block of source samples must be minimized, which means that the low frequency energy is partially offset or entirely eliminated.
Also, there must be redundancy added to ensure a lossless coding by the transmitter.
The conventional way of making RDS bounded is to use an Alternate Marking Input ("AMI") method, which generates additional, but opposite, voltage levels for the samples to bring the RDS to within a symbol size.
The AMI, however, is unacceptable for PCM channels, since there is a fixed set of signal points, or voltage levels for the PCM channel as dictated by the telephone networks