Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission
Reexamination Certificate
1998-03-31
2001-09-18
Korzuch, William (Department: 2641)
Data processing: speech signal processing, linguistics, language
Speech signal processing
For storage or transmission
Reexamination Certificate
active
06292774
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a transmission system comprising a transmitter with a speech encoder for deriving from frames of speech signal samples, data frames with coefficients representing said frames of speech signal samples, the speech encoder comprising frame assembling means for assembling complete data frames and incomplete data frames, said incomplete data frames comprising an incomplete set of coefficients representing their frame of speech signal samples, the transmitter further comprises transmit means to transmit said data frames via a transmission medium to a receiver, the receiver comprises a speech decoder, said speech decoder comprising completion means for completing the incomplete sets of coefficients with interpolated coefficients obtained from coefficients corresponding to frames of speech signal samples surrounding the frames of speech signal samples corresponding to said incomplete data frame
The present invention is also related to a transmitter, a receiver, an encoder, a decoder, a speech coding method and a coded speech signal.
2. Description of the Related Art
A transmission system according to the preamble is known from U.S. Pat. No. 4,379,949.
Such transmission systems are used in applications in which speech signals have to be transmitted over a transmission medium with a limited transmission capacity or have to be stored on storage media with a limited storage capacity. Examples of such applications are the transmission of speech signals over the Internet, the transmission of speech signals from a mobile phone to a base station and vice versa and storage of speech signals on a CD-ROM, in a solid state memory or on a hard disk drive.
A speech encoder derives from a frame of speech samples data frames comprising coefficients representing said frames of speech signal samples. These coefficients comprise analysis coefficients and excitation coefficients. A group of these analysis coefficients describe the short time spectrum of the speech signal. An other example of an analysis coefficient is a coefficient representing the pitch of a speech signal. The analysis coefficients are transmitted via the transmission medium to the receiver where these analysis coefficients are used as coefficients for a synthesis filter.
Besides the analysis parameters, the speech encoder also determines a number of excitation sequences (e.g. 4) per frame of speech samples. The interval of time covered by such excitation sequence is called a sub-frame. The speech encoder is arranged for finding the excitation signal resulting in the best speech quality when the synthesis filter, using the above mentioned analysis coefficients, is excited with said excitation sequences. A representation of said excitation sequences is transmitted as coefficients in the data frames via the transmission channel to the receiver. In the receiver, the excitation sequences are recovered from the received signal and applied to an input of the synthesis filter. At the output of the synthesis filter a synthetic speech signal is available.
The bitrate required to describe a speech signal with a certain quality depends on the speech content. It is possible that some of the coefficients carried by the data frames are substantially constant over a prolonged period of time, e.g. in sustained vowels. This property can be exploited by transmitting in such cases incomplete data frames comprising an incomplete set of coefficients.
This possibility is used in the transmission system according to the above mentioned U.S. patent. This patent describes a transmission system with a speech encoder in which the analysis coefficients are not transmitted every frame. These analysis coefficients are only transmitted if the difference between at least one of the actual analysis coefficients in a data frame and a corresponding analysis coefficient obtained by interpolation of the analysis coefficients from neighboring data frames exceeds a predetermined threshold value. This results in a reduction of the bitrate required for transmitting the speech signal.
A disadvantage of the transmission system according to the above mentioned U.S. patent is that the speech signal is always delayed over several frames due to the interpolation to be performed.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a transmission system in which the delay of the speech signal has been reduced.
Therefor the transmission system according to the invention is characterized in that said assembling means being arranged for introducing into at least one of said incomplete data frames, additional coefficients representing frames of speech signal samples being later in time than the frames of speech signal samples corresponding to said incomplete data frames, and in that the completion means are arranged for completing the incomplete sets of coefficients using said additional coefficients.
By transmitting the additional coefficients representing later frames of speech signal samples in the incomplete data frames, these additional coefficients are available at least one frame interval earlier in the decoder. Because these additional coefficients are used for completing the incomplete set of coefficients by interpolation, this interpolation can also be performed at least one frame interval earlier. Consequently the synthesis of the reconstructed speech signal can take place earlier and the signal delay is reduced with at least one frame interval.
An embodiment of the invention is characterized in that the frame assembling means are arranged for introducing into the data frames indicators for indicating whether or not the frame is an incomplete data frame, and whether or not the data frames carry coefficients representing frames of speech samples different from its corresponding frames of speech samples.
The introduction of the first and second indicator, enable a very easy decoding in the receiver. The completion means in the receiver can easily extract the incomplete frames from the input signal, and start with completion (by interpolation) as soon an incomplete frame carrying additional coefficients is available. If only one indicator is present, the speech decoder needs the indicators corresponding to previous data frame to be able to decode the signal. This requires a very reliable communication to prevent errors in or loss of data frames.
REFERENCES:
patent: 4379949 (1983-04-01), Chen et al.
patent: 5012518 (1991-04-01), Liu et al.
patent: 5479559 (1995-12-01), Fette et al.
patent: 5504834 (1996-04-01), Fette et al.
patent: 5623575 (1997-04-01), Fette et al.
Gerrits Andreas J.
Taori Rakesh
Korzuch William
Storm Donald L.
U.S. Philips Corporation
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