WO1995026082A1

WO1995026082A1 - Method and receiver apparatus for reducing subjective sensitivity to interference in reception subject to interference when using digitally transmitted sound signals

Info

Publication number: WO1995026082A1
Application number: PCT/EP1995/001088
Authority: WO
Inventors: Robert Sedlmeyer; P.H.A. Dillen; Michel Lever; Pierre Urcun; Andreas Brefort
Original assignee: Institut für Rundfunktechnik GmbH; Philips International B.V.; France Telecom; Telediffusion De France S.A.
Priority date: 1994-03-23
Filing date: 1995-03-22
Publication date: 1995-09-28
Also published as: AU2072495A; DE4409960A1

Abstract

In order to attain subjectively pleasant listening even when reception is poor with frequent muting, in the case of digital radio transmission, the level of the sound signal received is gradually decreased. When reception is improved such that, only by using error correction and error masking, the reproduced sound signal is felt not to be unpleasant, the level of the sound signal is gradually increased again to the normal value.

Description

METHOD AND RECEIVER FOR REDUCING THE

SUBJECTIVE INTERFERENCE WITH DISTURBED

RECEPTION WHEN USING DIGITAL TRANSMISSION

SOUND SIGNALS

DESCRIPTION

When receiving radio broadcasts with portable receivers or with mobile reception in a motor vehicle, the reception is subject to constant changes in its field strength and in its content of interference signals.

With FM reception, this leads to an increasing number of audible disturbances in the reproduced audio signal when the reception field strength decreases or the interference signal increases, until the audio signal becomes ever quieter depending on the demodulator circuit and finally disappears completely, or, for example, when using PLL -Demodulators by means of a special circuit, which increasingly reduces the volume as the reception field strength decreases, for example above a certain threshold. When receiving AM transmitters, there is also audible interference and a loss of volume when the reception field strength drops. The use of control elements keeps the volume constant over a wide range in the event of fluctuations in the field strength. From a certain limit, however, the volume decreases with decreasing reception field strength in many practical receivers.

With the introduction of DAB (Digital Audio Broadcasting), the radio reception for portable receivers and in the moving vehicle is significantly improved compared to reception with FM modulation by improving the reception behavior in multi-path reception and by being able to correct transmission errors on the digital level.

However, since this is a digital transmission system, the exit behavior with a decreasing field strength or increasing interference components is naturally different than with analog systems. Uncorrectable errors in the received signal would lead to very loud interference in the reproduced audio signal without special precautions. It has therefore been provided the possibility of recognizing errors which can no longer be corrected, going beyond the error correction already mentioned. In the presence of such recognizable but no longer correctable errors, a method for error concealment can be used which replaces faulty signal passages, for example with estimated values or signal repetitions. However, if there are faulty sound signal passages for a long time, the sound signal must be muted. When receiving in a moving vehicle with a strongly fluctuating reception field strength on the edge of the coverage area, this sometimes leads to the presence of a sound signal for a second, which alternates with passages that are often as long or even longer, until finally the reception field strength is muted permanently if the reception field strength drops even further.

This effect is very annoying for the listener, especially if he is in such a reception area for a longer time.

The object of the invention is to offer a system behavior even with new digital transmission methods such as DAB, which uses the described advantages of digital technology, but exhibits a subjectively more pleasant behavior during poor reception situations where frequent muting occurs.

The object is achieved by the characterizing features of claim 1.

Advantageous further developments and refinements of the method according to the invention result from the subclaims. A receiving device for carrying out the method according to the invention is specified in claim 11.

The invention is illustrated by the single drawing. FIG. 1 shows a block diagram of a receiver which is suitable for receiving a digital broadcast signal, as is broadcast in the system known by the name DAB, together with the inventive method features. The idea of the invention consists in, in radio receivers, which are built to receive digitally transmitted radio signals - especially in the case of car receivers - in reception situations in which the reception quality becomes so poor that the methods of error correction and error concealment shown can no longer be used alone. Similar to FM receivers to gradually (either step-wise or steadily) reduce the sound signal level, and thus to obtain a subjectively more pleasant reception behavior when there are undetected errors and their audible disturbances in the sound signal, as well as when the muting occurs temporarily.

If the reception then improves again, a gradual increase in the volume is desirable in order to obtain a subjectively more pleasant reception behavior when re-entering a better reception area in the case of experience-based unrecognized errors and their audible disturbances in the audio signal, as well as when mutes occur.

The method should only be effective if error correction and concealment no longer prove to be meaningful, because the advantages of error correction and concealment by the method should in no way be reduced by unjustifiably already being used in the case of faults that are still subjective allow advantageous use of error correction and concealment.

To ensure this, the sole use of the received field strength to control a level reduction is no longer sufficient. Rather, it makes sense to use signals in the case of DAB reception, for example can be obtained in error correction in the receiver, or on signals to trigger the concealment of errors, as well as to trigger the temporary muting in the receiver. The former is the so-called "reliability signal" that can be obtained in the channel decoder, which provides information about the reliability of each individual bit or entire signal passages in the channel-decoded signal by indicating the probability with which individual bits or entire signal Passages are disturbed. The error concealment concerns the scale factor CRC signals specified in the ETSI standard ..., which provide information about errors in the scale factors due to violations in the so-called "Cyclic Redundancy Check". Muting is concerned with the error detection signal (ISO-CRC signals) standardized according to ISO / MPEG 11172-3, which normally triggers the audio signal to be muted when the ISO-CRC is disturbed.

The interaction of the individual method features in the execution of the method according to the invention is to be explained using an exemplary embodiment in FIG. 1:

In an input stage 2 (front end) of a digital radio receiver, the signal 101 received by the antenna 1 is amplified and demodulated. The channel decoding takes place in the following channel decoder 3 and the source decoding in the source decoder 4. The linearly coded digital audio signal 104 obtained is fed to a level control element 5, which changes the level of the digital audio signal 104 in accordance with the manipulated variable 108. The leveled output signal forms the linearly coded digital audio signal 105, which is converted analog / digital and via an A / D converter and amplifier is amplified in order to be supplied to the loudspeakers 7 as an analog signal 106. The manipulated variable 108, which causes a reduction in the sound signal level in the actuator 5 when the reception is severely disturbed, is formed either from the reliability signal 103, or from the ISO CRC signal 114 or from the scale factor CRC signal 124, or from several of these signals, with the optional addition of the field strength signal 102.

In order to make the method work suitably, it is advantageous to change the manipulated variable 108 in the event of frequent occurrence of ISO-CRC violations, which each trigger a temporary muting of the sound signal in the source decoder 4, in such a way that a slow reduction in the signal level in the level actuator 5 he follows. If, on the other hand, the number of ISO-CRC violations is reduced again, and with it the frequency of temporary muting, the manipulated variable 108 must be changed so that the level control element 5 slowly increases the signal level until the level control element 5 does not Level lowering does more.

When implementing the method, it has proven to be advantageous to count occurring ISO-CRC violations and the resulting error message within a time window, and to form the manipulated variable depending on the number within the window. A weighting of the individual error messages located within the time window has also proven to be favorable.

Likewise, it has proven to be advantageous in practical tests to count the scale factor CRC violations that occur, which in each case lead to suitable temporary error concealment measures, and within a time window if necessary, to weight, and to form the manipulated variable depending on the number within the window. It should be noted that a small number of error concealments do not lead to such a large subjective interference effect in the audio signal as muting the audio signal. It therefore makes sense, for example, to set the number n of scale factor CRC violations, which lead to a level reduction, higher than a number m of scale factor CRC violations, which also lead to a level reduction. If these relationships are taken into account, both criteria can be used very well together to obtain the manipulated variable for the level control element.

In a similar way to the CRC signals, the reliability signal 103 can be used to change the manipulated variable 108 in such a way that the level control element 5 slowly reduces the signal level when a cluster of bits that are likely to be disturbed occurs. If, on the other hand, the frequency of bits which are likely to be disturbed decreases again, the manipulated variable 108 is changed so that the level control element 5 gradually increases the signal level until the level control element 5 no longer lowers the level. A combination with the statements of the CRC signals is of course also possible.

In principle, the field strength signal 102 can also be used to obtain the manipulated variable 108 by changing it with a falling field strength 102 so that a gradual reduction in the signal level takes place in the level control element 5, and vice versa. However, the field strength signal 102 does not provide such precise information about the temporarily prevailing state of interference of the sound signal obtained in the receiver, since interference signals in the reception channel also have a high level Can generate field strength, but only disturb the reception of a transmitter. If the field strength signal 102 is nevertheless to be used to control the level reduction by the level control element 5, then possibly only together with the "reliability signal" 103, or the ISO CRC signal 114 or scale factor CRC signal 124 or more of these signals, which make clearer statements about the state of the received audio signal.

The embodiment shown in FIG. 1, in which the actuator 5 is located between the source decoder 4 and the A / D converter and amplifier 6, represents only one possible arrangement of the actuator 5. The actuator 5 can just as well in practical embodiments be arranged within the source decoder 5 or within the A / D converter and amplifier 6. If the actuator is arranged within the source decoder 5, the level of the sound signal can e.g. also before filtering back the audio signals from the subband level with the aid of a change in the scale factors.

Claims

PATENT CLAIMS

1. A method for reducing the subjective perception of interference when receiving digitally transmitted sound signals, characterized in that the level of the received sound signal in reception situations in which the reception quality becomes so poor that, despite the use of error correction and error concealment, the reproduced sound signal is perceived as unpleasant , the level of the audio signal is gradually reduced, and that the level of the audio signal is gradually raised back to the normal value if the reception improves so that the reproduced audio signal is not perceived as unpleasant simply by using error correction and error concealment.

2. The method according to claim 1, characterized in that the so-called "Reliability Information", which is obtained in the course of the channel decoding in the receiver, and a statement about the. Is used to determine the size of the reduction in the level of the received audio signal depending on the reception quality Reliability of each individual bit or entire signal passages in the channel decoded signal makes by indicating the probability with which each individual bit or entire signal passages are disturbed.

3. The method according to claim 1, characterized in that for determining the size of the dependent on the reception quality reduction in the level of the received audio signal, the scale factor error detection signals (scale factor CRC signals) used according to ETSI / ..., which are used for errors in the scale factors due to injuries in the so-called "Cyclic Redundancy Check".

4. The method according to claim 1, characterized in that an ISO / MPEG 11172-3 standardized error detection signal (ISO-CRC signal) is used to determine the size of the dependent on the reception quality reduction in the level of the received audio signal, which is about errors in ISO-CRC provides information due to violations in the so-called "Cyclic Redundancy Check".

5. The method according to any one of claims 1 to 4, characterized in that a combination of the ISO-CRC signal and the scale factor CRC signals, and optionally the. To determine the size of the reduction in the level of the received sound signal depending on the reception quality "Reliability Information" is formed and used.

6. The method according to any one of claims 1 to 5, characterized in that the field strength signal is additionally used to determine the size of the reduction in level dependent on the reception quality, which, however, with one or more of the following signals, the ISO CRC Signal, the scale factor CRC signals, and possibly the "Reliability Information", is combined.

7. The method according to claim 3, 5 or 6, characterized in that the individual error messages by the scale factor CRC signals, which provide information about errors in the scale factors due to injuries in the so-called "cyclic redundancy check", are counted within a time window , and based on this number, the size of the reduction in the level of the received audio signal, which is dependent on the reception quality, is formed.

8. The method according to claim 7, characterized in that the individual error messages by the scale factor CRC signals, which are counted within a time window, are weighted within this time window with a calibration function before they depend on the quality of reception to determine the size of the size Reduction of the level of the received audio signal can be used.

9. The method according to claim 4, 5 or 6, characterized in that the individual error messages are counted within a time window by the ISO-CRC signal, which provides information about errors in the ISO-CRC due to violations in the so-called "Cyclic Redundancy Check" and, based on this number, the size of the reduction in the level of the received audio signal which is dependent on the reception quality is formed.

10. The method according to claim 9, characterized in that the individual error messages by the ISO-CRC signal, which are counted within a time window, are weighted within this time window with a calibration function before they are used to determine the Size of the reduction in the level of the received audio signal depending on the reception quality can be used.

11. Receiving device for reducing the subjective perception of interference when receiving digitally transmitted audio signals with interference, for carrying out the method according to one of the preceding claims.