Abstract:
Apparatus for reproducing coded audio signals includes interpolation circuitry for substituting interpolated signal values for erred or lost audio signal. To minimize undesirable audible artifacts which may be introduced into the audio signal by virtue of the substitution process, the rate of substitution is monitored. When the rate of substitution exceeds a predetermined rate, the higher frequency components of the audio signal, including the substituted values, are attenuated.

Description:
This invention relates to audio reproduction apparatus and more particularly to apparatus for concealing segments of lost audio signal. 
     BACKGROUND OF THE INVENTION 
     During the reproduction of audio signals, in particular digital audio signals, isolated or long-lasting errors occur that cannot be corrected by a corresponding data correcting system. To conceal extended error bursts, some systems generate an estimate of the probable signal by performing an interpolation from nearby signals, which estimate, to a certain extent, will not nominally be perceivable by the ear. Interpolations of this type are performed, for example, in the integrated circuits CXD1167Q available from Sony or the SAA7220 available from Philips. A disadvantage of these devices is that under certain conditions the interpolations are perceivable by the user. It has been discovered that the audibility of such interpolations is heavily dependent on the frequency of interpolations and the audio frequency of surrounding data. In particular, at lower audio frequencies, a higher interpolation rate can be permitted without significant perception than for higher audio frequencies. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an apparatus or method for the reproduction of an audio signal includes circuitry for generating interpolated signal for substitution of incorrectable data. The decoded data, including substitute interpolated signal is applied to a selectively controlled attenuator. The rate of occurrence of interpolated signal is monitored, and if such rate exceeds a predetermined threshold over predetermined intervals, a control pulse is generated. The control pulse is applied to the selectively controlled attenuator to reduce the amplitude of ones of the frequency components of the decoded audio signal. 
    
    
     BRIEF SUMMARY OF THE DRAWINGS 
     FIG. 1 is a block diagram of a portion of an audio decoder including an error detection/correction stage and an interpolator stage. 
     FIG. 2 an illustration of the frequency dependent threshold of audibility of interpolations during the reproduction of sinusoidal signals. 
     FIG. 3 a block diagram of a known audio reproduction circuit including an error corrector and an interpolator. 
     FIGS. 4, 5, 7, 8 and 9 are block diagrams of alternative audio reproduction circuits including an interpolator and a selective attenuator embodying the present invention. 
     FIG. 6 a pulse diagram useful in describing the operation of the circuit of FIG. 5 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a basic circuit of an audio reproduction circuit having a linear interpolation stage 2 and an error correcting circuit 1 such as is known for example from Sony IC CXD1167Q or Philips SAA7220. The error correcting stage 1 has two outputs, the first of which is directly connected to the interpolation stage 2 of an audio channel and the second of which is connected to an input of a sample value checking circuit (sample check) 3. The first output of the error correcting circuit 1 is connected to an input of a first sample and hold circuit (n-1) 4a which, for its part, is connected to a further sample and hold circuit 4b via a data bus. Moreover, the first sample and hold circuit 4a is connected to a divider which divides the value of the sampling value by 2. One output of the sample and hold circuit 4b is coupled directly to a first input terminal of a switch 5 while another output of the sample and hold circuit 4b is connected to an input of a third sample and hold circuit 4c (n-1) whose output is connected to a second divider circuit 6b. The outputs of the two divider circuits 6a and 6b are-connected respectively to a summing circuit 7 whose output is coupled to a second input terminal of the switch 5. The output of the switch 5 forms the data output of the interpolation stage. 
     The sample value checking circuit 3 checks the sampling values for deficiencies i.e. including their lack of correctability by the error correcting stage 1, and generates a logical 1 for a faulty sampling value and a logical 0 for a correct sampling value. These values are supplied to a cascade connection of register circuits 4d, 4e and 4f. The outputs of the register circuits 4d and 4f are connected respectively to inverters 8a and 8b whose outputs are coupled to respective input terminals of an AND circuit 9. The output of the register circuit 4e is coupled to a third input terminal of the AND circuit 9. The output of the AND circuit is coupled to control the switching circuit 5 between positions of the outputs of the summing circuit 7 and the sample and hold circuit 4b. That is, when correct samples are available at the sample and hold circuit 4b, the switch 5 passes samples from the sample and hold circuit 4b. When uncorrectable erred samples are resident in the sample and hold circuit 4b, then the switch 5 is conditioned to pass interpolated samples from the summing circuit 7. Furthermore, the output of the AND circuit 9 is made available from the interpolation stage as a herein designated J-Flag output. 
     FIG. 2 shows the waveform of the experimentally determined threshold of audibility against the number of interpolations per second or the error rate in dependence on the audio frequency (drawn logarithmically). A higher interpolation rate can be allowed for lower frequencies than for higher frequencies. 
     FIG. 3 shows a block diagram of an audio reproduction circuit of a conventional type which has an error correcting circuit 1, an interpolation stage 2, a D/A converter 10, a circuit 11 for the processing of analog audio signals and a loudspeaker 12. 
     FIG. 4 shows a block circuit diagram of an audio reproduction circuit in accordance with the invention. The interpolation circuit 2, which may be similar to the one in FIG. 1, is connected to an output of a known error correcting circuit 1. The data output of the interpolation circuit 2 is connected to the input of a digital to analog converter 10 whose output is connected to an analog audio signal processing circuit 11. The J-Flag output of the interpolation stage 2 is connected to a threshold value evaluating and processing stage 13 which is connected to a physiological correcting and control stage 14. The physiological correcting stage 14 conditions the processing circuit 11 to reduce the amplitude of the higher frequencies of the analog audio signal in accordance with whether the interpolation rate exceeds a predetermined threshold value, S. 
     In practical experiments, it has been shown that the threshold value S should preferably be about 30 interpolations per second. The number of interpolations occurring in each second may be determined by means of a counter 15 which has a clock and reset inputs in addition to the J-Flag output as is depicted in FIG. 5. Upon exceeding the threshold value S, a low pass filter 17 in the audio section is activated in a simple manner via a mono flip flop 16 which provides a control pulse of predetermined width &#34;M&#34;. The pulse timing of the apparatus in accordance with FIG. 5 is shown in FIG. 6. Analysis of the number of interpolations per unit time is, in the FIG. 6 illustration performed over one second intervals. At the end of each interval, the output of the counter 15 is reset to zero by means of the reset signal. If the count value in the counter at the end of respective intervals is greater than the threshold, resetting the counter will cause a voltage transition at the counter output. The occurrence of such a transition will trigger the mono flip flop 16 to generate a pulse which activates the low pass filter 14 until such time as the number of interpolations per unit of time falls below the predetermined value. 
     FIG. 7 illustrates a block diagram of an audio reproduction circuit having a two stage physiological control means. A multi-step reduction of the higher frequencies is effected by such a circuit. In dependence on the magnitude of the interpolation rate, a plurality of low pass filters 17a, 17b in the analog audio reproduction circuit are activated by corresponding mono flip flops 16a, 16b. Assuming the counter provides binary output bits, different ones of these bits may be decoded to provide first and second trigger signals which are coupled to the respective mono flip flops. For example the count values 30 and 100 may be decoded to provide logic high values when these counts are exceeded. If the reset function causes a high to low transition on the respective decoded output the respective mono flip flop will be triggered. Thus if a count of 135 occurs in an analysis interval, both mono flip flops will be triggered, causing both low pass filters 17a and 17b to be switched into the signal path. Alternatively if the count value is 54, then only mono flip flop 16b will be triggered causing only low pass filter 17b to be switched into the signal path. 
     FIG. 8 shows a further development of the invention in accordance with FIG. 4. In FIG. 8, a buffer 18 is provided for the storage of the audio data in the digital signal path. This buffer, which may be constructed as a so-called shock memory, can be used to effect an improvement of the physiological control when interpolations occur, in such a way, that the control is turned on even before the start of interpolation interference. That is the audio signal is delayed for a period equal to an analysis interval such that the results of analysis may be applied to the signal interval analyzed. To this end the J flag output of the interpolation stage 2 is connected to the J Flag input of the threshold value stage 13. A timing member 19, which is connected to the physiological control stage 14 that is effective for its part on the analog output stage 11, is connected to an output of the threshold value stage 13. A digital to analog converter 10 is arranged between the buffer 18 and the analog output stage 11. 
     Up to this point, the examples have been shown only the analog side of physiological audio control of audio reproduction circuitry. 
     FIG. 9 shows an example for the physiological control of the audio signals at the digital side. To this end, upon the occurrence of interpolation flags, the data that was stored in the buffer 18 is assessed by means of digital filters in a digital signal processor 20 (DSP) as regards frequency and a corresponding reduction of the amplitude of the high frequencies is effected. The output of the digital signal processor 20 is connected to the D/A converter stage 10 to which the analog audio output stage 11 is then subsequently connected. 
     The invention may be used in digital audio and video systems of every kind, in particular in apparatus and circuits in the fields of compact discs (CD), magneto-optic discs (MOD), digital audio broadcast (DAB), digital satellite radio (DS), NICAM apparatus, digital compact cassettes (DCC), mini discs (MD) as well as in audio systems in which errors that can no longer be corrected in corresponding error correcting systems are interrupted by an interpolation.