Speech interpolation method and apparatus in a digitized speech transmission system.

p0001The invention relates to the transmission of digitized speech signals and more particularly to a method, and corresponding apparatus, interpolation of speech in a digital speech transmission system.

p0002Many telephony transmission needs by radio currently exist for both civil applications for military applications. The trend is to make these transmissions in digital form, this type of transmission to regenerate the signals, encodings, and also allowing a digital encryption or complex processing of the speech signal.

p0003In the frequency ranges used, VHF and UHF, coding of speech is the DELTA type, that is to say a coding result of a differential quantification of the analog signal on two levels giving rise directly to the digital message. Several variants of this coding are used; the most commonly used rate is 16 kbits / sec.

p0004In radio transmission, many phenomena disrupt the transmission. In conventional radio transmissions type transmissions are fixed rate and are subject to error packets or impulsive noise, for example those created by vehicle engines. In civil or military radio transmissions using frequency hopping, a set of frequencies is used, each of these frequencies used as transmission media for a brief time called landing. Some levels can be disrupted or jammed by involuntary or voluntary presence of other programs or on the corresponding frequencies.

p0005In the presence of such. disruptions, the coded speech signal DELTA renders with very unpleasant transients for listening comfort and making the transmission unintelligible when the probability of occurrence of these phenomena increases.

p0006A device for reducing errors in digital transmission, is described in patent application GB 2 025 176. This device comprises a transmission line code error detector which controls the transmission of a signal from a pulse generator in place of the received signal in case of error detection.

p0007The present invention is intended to detect the presence of disturbances and to achieve an adequate treatment for improving the listening comfort, given probability of interference, or to increase the permissible interference level at a level listening comfort acceptable to a detection device for implementing a detection of the energy outside the signal modulation band.

p0008According to the invention, a method of interpolation of speech in a digital speech transmission system using a narrow-band FSK modulation method prefiltered, is characterized in that it consists:<ul><li>- Detecting the energy received outside of the modulation band by high-pass filter cut-off frequency at least equal to the highest frequency of the prefiltered modulation band to form a disturbance detection signal;</li><li>- Order the disturbance detection signal transmission to the input of the speech decoding circuit or the digital data sequence received either a predetermined sequence to remove noise in the speech signal recovered during disturbance.</li></ul>

p0009The invention also relates to the speech interpolation device implementing this method.

p0010The invention will be better understood and other features will become apparent with the following description with reference to the attached figures.

p0011<ul><li>- Figure 1 is a diagram of one embodiment of the speech interpolation device according to the invention.</li><li>- Figure 2 is a signal diagram illustrating the method according to the invention.</li></ul>

p0012In digital transmission of voice by radio, the type of modulation used is often a frequency shift keying, FSK, prefiltered issue to mitigate abrupt transitions. Indeed, this type of modulation combines simplicity an interesting property that is the containment of the spectrum in a narrow frequency band; which allows to work in channels of contiguous frequencies, closely spaced in frequency. In the presence of an audio signal of lower rate or equal to 16 Kbit / sec, there is very little energy beyond a cutoff frequency f; f is from about 1.3 times the flow rate. or 20 kHz to a rate of 16 Kbits / s. By cons, in the presence of interference-related disruption, erratic behavior of the signal leads to rapid transitions and consequently to greater than f frequency components.

p0013To detect the presence of interference, the method of the invention consists in detecting the components outside the useful signal band, by a high-pass filter of cutoff frequency f. When the disturbance is detected, the processing performed in the device of the invention is to remove noise by replacing the disturbance by improving listening comfort signal.

p0014The device of the invention, hereinafter described with reference to Figure 1, comprises a conventional chain for receiving and demodulation of a digitized speech signal, encoded in DELTA and transmitted FSK modulation prefiltered. The input E, which receives the intermediate frequency signal is connected to the input of an intermediate frequency filter 1. The output of this filter is connected to the input of an intermediate frequency amplifier 2 whose output is connected to the input of a limiter 3. the output of the limiter is connected to the input of the frequency discriminator 4 in series with a smoothing low-pass filter whose cutoff frequency is f. The output of this low-pass filter 5 is connected to the input of the bit synchronization detecting circuit, said circuit detecting the zero crossings of the filter output signal low-pass 5. The output of this pass filter out there is also connected to the input of a sample and decision circuit 7 further having a sampling control input connected to the output bit of the synchronization detection circuit 6. the circuit 7 shows the numerical value demodulated from the received bit. The output of the decision circuit 7 is coupled to the input of a decoder restoring DELTA 8, from digital following the release of the decision circuit 7, the speech signal in analog form.

p0015For carrying out the method according to the invention, the interpolation of the speech device further comprises a high-pass filter 10, of cutoff frequency f equal to the cutoff frequency of the low-pass filter 5. In the absence of noise or interference, the filter of this output signal is virtually zero. This output signal increases as a function of the noise level and in particular in case of jamming or noise spike. In this case a significant level of signal is present at the output of the high pass filter 10. The output of this filter is connected to the input of a threshold detector circuit 11, the threshold being the level at which it is considered there is interference. For example, this threshold can be the average level of an output signal corresponding to eight decibels of signal to noise ratio. The output of threshold detector circuit 11 is connected to the input of a monostable circuit 12 whose output is connected to the control input of a two-position switch 14. The monostable has the function of shaping the output of the detector circuit to make it suitable for controlling the switcher.

p0016The output of the decision circuit 7 is coupled to the input of the DELTA decoder 8 via a delay circuit 15 and the first switch port 14, first input-output, this output being connected to the input DELTA decoder 8. the delay circuit 15 has a clock input connected to the output of the bit synchronization detection circuit 6.

p0017The delay circuit 15 adjusts the transit time in the demodulation path to the time required to detect disturbances, so that the detection of defects does apply to recognized erroneous corresponding bits.

p0018In the illustrated embodiment, the treatment performed to remove noise during the disturbances is to replace the scrambled bits transmitted in the reception chain a sequence of "bearing" formed by an alternation of 0 and 1. For this, the output bit synchronization detecting circuit 6 is connected to the input of a rolling generator circuit 13, whose output is connected to the second input of the switch 14. Thus, upon detection of disturbances, interference is replaced by a ball much more acceptable to the hearing. Indeed, experience shows that the perception of speech disrupted during relatively long periods, 0.5 to 10 ms, for example, is little affected by these disruptions if it can remove noise for them, because the noise is noticeable and makes the unpleasant hearing well before intelligibility is affected.

p0019Figure 2 is a signal diagram for explaining this method.

p0020The first signal Sp shown is the voice analog signal before coding and modulation.

p0021Upon receiving the modulated signal is processed by the amplifier 2, the limiter 3, the discriminator 4 and the low-pass filter 5. The parallel processing by the high-pass filter, 10, and threshold detector 11, determines whether the received bit sequence is "estimated correct" or "blurred". The monostable 12 which controls the switch 14 associated to the running generator 13, is used to replace the bit sequence "estimated blurred" by a rolling sequence. The signal at the input of the DELTA Eg decoder and therefore consists of a succession of correct estimated sequences and rolling sequences.

p0022At the end of Sg DELTA decoder, being adaptive coding, the speech signal from the level reached at the end of the estimated correct sequence is gradually reduced to a zero signal. At the end of interference and when the shot circuit has returned to its initial state, the speech signal returns to its correct value without interruption.

p0023This gradual change does not introduce sudden change of phase. Indeed, upon receipt of a rolling sequence 1, 0, 1, 0, ..., DELTA decoder enslaves its slope to the minimum value since there is more than one detection and successive 0. Therefore, regardless of the amplitude of the signal prior to receiving the bearings, the signal amplitude tends to 0 with a time constant which is that of the adaptation loop of the DELTA decoder or about 4 ms. In the case of a decoding DELTA, working generator 13 can be a simple type flip-flop whose inverted output is connected to the signal input; the output of the bit synchronization detecting circuit, giving the clock rate is connected to the clock input of flip-flop 13.

p0024The invention is not limited to the embodiment specifically described and shown. In particular, the above description is illustrated by an example of the speech transmission system via radio. This example is not limiting and interpolation device is also applicable to a transmission system son or microwave. This interpolator is particularly suitable for packet transmission, the scrambled packets being replaced by rolling sequences.

p0025Similarly the interference detection can be done differently and possibly result from the application of separate criteria.

p0026In the case of digital speech transmission by radio using for the transmission of digital data a frequency shift keying prefiltered emission, noise detection outside the useful band is particularly effective, since the issue no components emitted outside the useful band.

p0027Anyway it is possible to decode the digitized signal DELTA in ternary form as follows. The decision of 1 given bit reflects the previous bit, two successive "1" decoded "1", two successive "0", the decoded "-1"; alternation of different successive bits, decoded "0". The implementation of the interpolation device is then facilitated.