Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1998-01-06
2001-02-27
Tse, Young T. (Department: 2734)
Pulse or digital communications
Transceivers
Modems
C375S242000
Reexamination Certificate
active
06195384
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the resolution of errors caused by digital loss insertion in transmissions between high speed modems over a digital network.
In a co-pending application entitled “A High Speed Modem Synchronized To A Remote Codec” Ser. No. 07/963539, filed Oct. 20, 1992 and assigned to the assignee hereof, an invention was disclosed for employing modems that are synchronized to the A/D and D/A converter clocks of the digital network over which signals are communicated. Specifically, a “&mgr;-law modem” was described, which is a modem synchronized both in time and in quantization levels to the A/D converters in the network, and which further takes into account the compressions introduced in the telephone network. The synchronizing of such modems to the telephone network permits one to greatly reduce the noise that is introduced into the signals and also allows user modems to operate at the network's clock rate.
One characteristic of the improved modem disclosed in the above-referenced patent application is that the signals generated by the modem take into account the &mgr;-law compressions and expansions in the telephone network that must be performed to achieve a constant signal/distortion performance over the wide dynamic range required for the transmission of voice signals. These &mgr;-law compression and expansion operations are accomplished through the use of PCM &mgr;-law encoders and decoders implemented on the digital side of the telephone network as shown in
FIGS. 1 & 2
.
With reference to
FIG. 1
, two modems are shown coupled together via a digital network. Between each modem and the network is a Local-Exchange Carrier (LEC) subscriber loop coupled to a LEC Central Office or another equivalent A/D conversion element. Referring hereinafter to an analog type subscriber loop, a hybrid is provided within the Central Office for separating the bi-directional analog signals received from a transmitting modem into unidirectional signals sent to and received from the digital network via the &mgr;-law encoders and decoders in addition to respective A/D and D/A converters. On the transmitting side, an original, analog signal A
n
input to the network is converted by the A/D converter into a digital signal B
n
typically consisting of 14 bits, which signal is then &mgr;-law encoded into a digital &mgr;-law code word N typically consisting of 8 bits. To arrive at the code word N, the 14 bit digital signal B
n
undergoes a quantization since it must be matched to the nearest 8 bit &mgr;-law signal level corresponding to the resultant code word N. Hence, the signal B
n
is converted into a quantized signal C
n
which is subsequently mapped to its corresponding &mgr;-law code word N. After transmission over the digital network, the digital code word N is decoded by a &mgr;-law decoder back into the quantized signal C
n
on the receiving side and subsequently converted to a corresponding analog signal level A
n
′ for transmission through the analog LEC portion of the network.
One drawback with the performance of &mgr;-law encoding and decoding is that the quantizing of the digital signal B
n
to a predetermined &mgr;-law signal level C
n
for mapping into a digital code word N produces an inherent quantization error. This error arises from the fact that the amplitude of the analog signal A
n
′ regenerated from the quantized signal C
n
(during decoding on the receiving side) does not exactly match the analog signal level of the original signal A
n
. More importantly, however, this mismatch between the transmitted and received analog signal levels becomes significantly worse when the common technique of loss insertion is used to mitigate the adverse effects of echo produced at the hybrids.
Loss insertion is used in the Public Switched Telephone Network to control echo impairment during speech calls through a reduction in the signal amplitude of the transmitted analog signals, and hence, a corresponding reduction in the distortion amplitude. When a signal is to be transmitted over the network, a standardized transmission loss (typically 6 dB for most networks, and 3 dB for the rest) is inserted into the signal path generally before transmission over the analog LEC subscriber loop. The signal amplitude of the received signal is then recovered in the called party's modem through a well-known equalization process which scales the signal amplitude back up to its expected level.
When performed in a digital network, loss insertion can be accomplished by either analog or digital means. Referring first to analog loss insertion, the &mgr;-law code word N transmitted across the digital network is decoded and converted into its corresponding analog signal level A
n
′ and the scaled down by a factor of 2 (for a 6 dB loss) before it is transmitted to the receiving analog subscriber loop. Digital loss insertion, on the other hand, is accomplished by means of mapping the first &mgr;-law code word N into a second &mgr;-law code word M representing a digital signal having approximately one half the amplitude of the digital signal represented by the first code word N. However, this itself gives rise to a secondary quantization error since, in effect, the mapping from code word N to code word M requires that the quantized signal C
n
be divided by a factor of 2 (for a 6 dB loss), which loss inserted signal level is then again quantized to another &mgr;-law level C
m
for mapping to the corresponding (second) code word M. The total quantization error then incurred by the two mappings cumulatively yields an error that is on the average twice as large as the first quantization error when considering that the modem's equalization process will bring the two errors to comparative levels.
Although digital loss insertion as compared to analog loss insertion over a digital network obviously introduces a significantly larger quantization error, the digital method is preferred because of the simplicity of its implementation, and hence, its lower cost. Implementation of an analog loss insertion means would require the adaptation of a significant number of potentiometers to the switches of each LEC Central Office, whereas digital loss insertion can be achieved through the use of a code word N to code word M mapping table implemented within either the IEC digital network or the &mgr;-law decoder on the receiving side of the network.
Hence, it would be desirable to provide a means for reducing the over-all quantization error made worse by use of the digital loss insertion technique in modem transmissions over a digital network.
SUMMARY OF THE INVENTION
A reduction in the total quantization error resulting from the performance of digital loss insertion in transmissions between modems is achieved by an inversion mapping of &mgr;-law code words prior to the digital loss insertion mapping performed at the &mgr;-law decoder. The inversion mapping, performed in one embodiment through a direct code word-to-code word table mapping implemented within the digital portion of the network, scales the amplitude of the already quantized and &mgr;-law encoded signal up by the predetermined loss insertion factor such that when the loss is later digitally inserted at the terminating central office, the secondary quantization error normally introduced by the loss insertion mapping is canceled by the effect of the preceding inversion mapping. That is, if the quantization error produced by &mgr;-law mapping the digital signal B
n
into a &mgr;-law code word N is represented by Q
1
, the quantization error introduced by the loss insertion mapping is represented by Q
2
, and the quantization error introduced by the inversion mapping is represented by Q
3
=−Q
2
, then the total quantization error turns out to be Qt=Q
1
+Q
2
+Q
3
=Q
1
+Q
2
−Q
2
=Q
1
.
In an alternative embodiment, the method and apparatus of the invention is preferably implemented within a transmitting digital modem having digital access to the digital
Lucent Technologies - Inc.
Tse Young T.
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