Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission
Reexamination Certificate
1997-10-10
2002-03-26
Smits, Talivaldis Ivars (Department: 2641)
Data processing: speech signal processing, linguistics, language
Speech signal processing
For storage or transmission
C379S068000, C379S088160
Reexamination Certificate
active
06363339
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and an apparatus for transmitting digitized voice signals, in a telecommunication environment in which the compression and decompression of voice signals is involved. More specifically, it relates to a method and an apparatus for improving the quality of an audio signal, which has been compressed or encoded with a digital signal processing technique, when the signal is stored or retrieved to or from a store-and-forward (e.g. voicemail) system in a telecommunication network.
BACKGROUND OF THE INVENTION
In recent years, the telecommunications industry has witnessed the proliferation of a variety of digital vocoders in order to meet bandwidth demands of different wireline and wireless communication systems. Vocoders are usually integrated in wireless telephones and the base stations of the communication network or a link therefrom. They provide speech compression of a digitized voice signal as well as the reverse transformation. Typically, a voice signal is digitized through one of many quantization techniques. An example of this technique is Pulse Code Modulation (PCM). For the purposes of this description, we will refer to PCM as the input format for the vocoder. Thus a vocoder includes an encoder stage that will accept as input a digitized voice signal and that will output a compressed signal, a possible compression ratio being 8:1. As for the reverse transformation the vocoder is provided with a decoder stage that will accept the compressed speech signal and that will output a digitized signal, such as PCM samples.
The main advantage of compressing speech is that it uses less of the limited available channel bandwidth for transmission. In voice messaging applications, it also minimizes storage memory requirements thereby increasing the number of subscribers that can be served. The main disadvantage is loss of speech quality specially when speech is subjected to multiple instances of vocoders.
The rapid growth in the diversity of networks and the number of users of such networks is increasing the number of instances where two vocoders are placed in tandem to serve a single connection. An example of such a situation is when a wireless user records a message to a voice storage and forwarding device and a wireline terminal then retrieves the message. In such a case, a first encoder is used to compress the speech at the terminal of the wireless user. The compressed speech is transmitted to a base station serving the local wireless terminal where it is decompressed (converted to PCM format samples). The resulting PCM samples are routed to the voice storage and forwarding device where a second encoder is used to compress the input signal for storage in a database. If the person who has access to the voice storage and forwarding device retrieves the message from a wireline terminal, a speech decoder in the voice storage and forwarding device decompresses the stored compressed speech data into PCM format for transmission to the wireline terminal over the PSTN.
The situation is even more complex when the user uses a wireless terminal to retrieve, from a voice storage and forwarding device, a message that was recorded by a wireless terminal user. In such a situation, the compression and decompression of speech occurs three times: once for the wireless terminal/base station combination, a second time in storing and retrieving the message in the voice storage and forwarding device, and finally, a third time in the last base station/wireless terminal combination. Yet another expanding area of concern is Internet telephony. Indeed, Internet telephony uses vocoders as well.
In an attempt to eliminate the condition of vocoder tandeming, a method called <<bypass>> has been proposed in the past. The basic idea behind this approach is the provision of a digital signal processor including a vocoder and a bypass mechanism that is invoked when the incoming signal is in a format compatible with the vocoder. In use, the digital signal processor associated with the first base station that receives the RF signal from a first wireless terminal determines, through signaling and control that an identical digital signal processor exists at the second base station associated with the wireless terminal at which the call is directed. The digital signal processor associated with the first base station rather than converting the compressed speech signals into PCM samples invokes the bypass mechanism and outputs the compressed speech in the transport network. The compressed speech signal, when arriving at the digital signal processor associated with the second base station is routed such as to bypass the local vocoder. Decompression of the signal occurs only at the second wireless terminal. The “bypass” approach is described in the international application serial number PCT/95CA/00704 dated Dec. 13, 1995. The contents of this disclosure are incorporated herein by reference.
However, the “bypass” solution described above was designed for the base station portion of the telecommunication network. This implies that it is useful only for the tandem situations in which a telecommunication link is established between two wireless users. Also, this solution is only valid for identical vocoders. With the diversity of vocoders quickly increasing, the bypass solution is only valid for a small portion of connections involving tandem vocoding.
Thus, there exists a need in the industry for devices capable of improving voice quality in other situations that involve vocoder tandeming such as in digital voice store-and-forward systems.
OBJECTS AND STATEMENT OF THE INVENTION
An object of the invention is to provide an apparatus for storage and forwarding of voice data that allows to reduce signal degradations as a result of successive signal compression/decompression cycles, particularly when non-compatible vocoders are used to effect the compression/decompression cycles.
Another object of the invention is to provide a method for storage and forwarding of voice data that allows to reduce signal degradations as a result of successive signal compression/decompression cycles, particularly when non-compatible vocoders are used to effect the compression/decompression cycles.
Another object of the invention is a communication system with voice storage and forwarding capability, designed to reduce signal degradations as a result of successive signal compression/decompression cycles, particularly when non-compatible vocoders are used to effect the compression/decompression cycles.
As embodied and broadly described herein, the invention provides a voice storage and forwarding device, comprising:
an input for receiving a digital signal that conveys audio information;
a group of vocoders;
a switch capable of acquiring a plurality of vocoder selection positions, in each vocoder selection position said switch directing the digital signal that conveys audio information received at said input to a selected one of said vocoders of said group of vocoders;
a storage medium in a data communicative relationship with said vocoders, said storage medium capable to store data that conveys audio information issued by a vocoder from said group of vocoders and transmit stored data that conveys audio information toward a selected one of said vocoders.
In this specification, the term “wireless terminal” is intended to include both mobile terminals and fixed wireless terminals. The term “wireless terminal” is part of a larger family of terminals that we call “speech compression terminals”. These terminals usually include vocoders that are capable of converting speech from a digitized format to a compressed format and vice versa. Other examples of these terminals are those used for Internet telecommunications, Integrated Services Digital Network (ISDN) terminals, etc. This disclosure is therefore meant to include all terminals capable of converting speech from a digitized format to a compressed format and vice versa.
The expression “data frame” will refer to a group of bits organized in a
Patel Girish
Rabipour Rafi
Nortel Networks Limited
Smits Talivaldis Ivars
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