Telephonic communications – Audio message storage – retrieval – or synthesis – Digital signal processing
Reexamination Certificate
1998-08-25
2001-03-27
Tsang, Fan (Department: 2645)
Telephonic communications
Audio message storage, retrieval, or synthesis
Digital signal processing
C379S088080, C379S093080, C379S100150, C379S100170
Reexamination Certificate
active
06208715
ABSTRACT:
This application is the national phase of international application PCT/F196/00579 filed Oct. 30, 1996 which designated the U.S.
FIELD OF THE INVENTION
Background of the Invention
The invention relates to a method and an apparatus which allow a transmitter of a digital telecommunication system transmitting speech to transmit predetermined messages to a receiver on a speech channel. In many digital telecommunication systems it is necessary to transmit not only encoded speech but also messages containing other information, e.g. messages pertaining to the control of a speech connection or messages containing data completely independent of speech. Such messages are often called signalling.
In telecommunication systems transmitting speech, a speech signal is usually subjected to two coding operations: speech coding and channel coding. Speech coding comprises speech encoding performed in a transmitter by a speech encoder, and speech decoding performed in a receiver by a speech decoder.
With reference to
FIG. 1
, a speech encoder
106
located in a transmitter
100
compresses a speech signal so that the number of bits used to represent it per unit of time is reduced. The speech encoder
106
typically processes speech as speech frames containing a certain amount of speech samples. On the basis of sampled speech, the speech encoder
106
calculates speech parameters, each of which is encoded as a separate binary code word. The speech parameters produced by the RPE-LTP speech encoder used in the full-rate channel of the pan-European GSM mobile telephone system are described in ETSI GSM Recommendation 06.10. These parameters are also disclosed in Table 1 of Appendix 1. The RPE-LTP (Regular Pulse Excitation—Long Term Prediction) produces 76 speech parameters from one speech frame of 20 ms (corresponding to 160 speech samples at a sampling frequency of 8 kHz). Recommendation GSM 06.10 also discloses the length of the binary code word assigned for each parameter.
Very often speech encoders also group speech parameters together, in which case each group—instead of a single speech parameter—is encoded to a separate code word. Encoding parameters in groups is called vector quantization. Modern speech encoders usually encode some speech parameters separately and some in groups (the RPE-LTP speech encoder of the example does not employ vector quantization). The result produced by the speech encoder is thus a constant-speed bit stream. The RPE-LTP speech encoder of the invention produces 260 speech coding bits per each speech frame of 20 ms.
The speech decoder
110
of a receiver
102
performs a reverse operation and synthesizes a speech signal
112
from the bits produced by the speech encoder. The decoder
110
receives binary code words and generates corresponding speech parameters on the basis of them. The synthesization is performed by the use of the decoded speech parameters. The speech synthesized in the receiver is, however, not identical with the original speech compressed by the speech encoder, but it has changed more or less as a result of the speech coding. The higher the degree of compression used in the speech coding, the more the quality of speech usually deteriorates in the coding process.
The RPE-LTP speech encoder, for example, compresses a speech signal to a rate of 13 000 bits per second (13 kbps). The compression is performed in such a way that it affects the intelligibility of speech as little as possible. In special cases, such as identification of tone pairs used in tone dialling, the compression may detrimentally affect or even completely obstruct the process.
The above-mentioned channel coding comprises channel encoding performed in the transmitter by a channel encoder, and channel decoding performed in the receiver by a channel decoder. The purpose of channel coding is to protect speech coding bits to be transmitted against errors occurring on the transmission channel. Channel coding may either allow transmission errors to be detected without being able to correct them or it may allow transmission errors to be corrected, provided that the number of errors is smaller than a certain maximum number, which is dependent on the channel coding method.
The channel coding method to be used is selected according to the quality of the transmission channel. In fixed transmission methods, the error probability is often very small, and there is not much need for channel coding. In wireless networks such as mobile telephone networks, however, the error probability is often extremely high, and the channel coding method employed has a significant effect on the quality of speech. In mobile telephone networks, both error-detecting and error-correcting channel coding methods are usually employed simultaneously.
In telecommunication systems transmitting speech, speech coding and channel coding are closely connected. The importance of bits produced by a speech encoder for the quality of speech usually varies such that, in some cases, an error in an important bit may cause an audible disturbance in synthesized speech, whereas several errors in less important bits may be almost imperceptible. How great the difference between the importance of speech coding bits is depends essentially on the speech coding method employed, but at least small differences can be found in most methods. When a speech transmission method is developed for a telecommunication system, channel coding is thus designed together with speech coding to allow the bits that are the most important for the quality of speech to be better protected than less important bits. In a full-rate channel of the GSM, for example, the bits produced by an RPE-LTP speech encoder have been divided into three different classes according to their importance to channel coding: the most important class is protected in channel coding with both an error-correcting and an error-detecting code; the second most important class is protected only with an error-correcting code; and the least important class is not protected in channel coding at all. Table 2 of Appendix 1 shows the classification of bits produced by an RPE-LTP encoder in two different ways: 6-parted subjective classification, and 3-parted classification used by channel coding.
Channel coding is not directly relevant to the principle of the invention. In view of speech coding, channel coding is part of the transmission channel. In view of the practical implementation, channel coding is, however, of essential significance to the transmission of messages as regards the selection of bits, as will be seen from the examples below.
The term “channel” can be interpreted in many ways in the field, wherefore the meaning of the term for the present invention can be specified as follows. When messages and speech are transmitted on separate channels, the receiver can distinguish between message bits and speech coding bits irrespective of the contents of the information transmitted on the channels. However, two channels are not necessarily physically separate channels. Separate channels can also be provided by dividing one physical transmission channel (e.g. a radio path or a transmission line) into a plurality of time slots and frequency ranges. When such a division is made unambiguously, the receiver can distinguish between the channels irrespective of the contents of the information transmitted on them.
The methods of
FIGS. 1
to
3
are employed for transmitting messages substantially simultaneously with speech. The methods will be considered from four points of view. 1: Is a separate transmission channel needed for transmitting messages, or can messages be transmitted on the same channel as speech? 2: How does the transmitter of the message have to communicate to the receiver that a message is on its way? 3: How does the transmission of a message affect the quality of speech transmitted simultaneously? 4: What happens in an old receiver if a new transmitter transmits a message and the message transmission method is not implemented in the old receiver?
FIG. 1
illustrate
Hoosain Allan
Nokia Telecommunications Oy
Pillsbury Winthrop LLP Intellectual Property
Tsang Fan
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