GB2279542A - Testing a plural-channel signal processing system - Google Patents

Abstract

A plural-channel audio signal processing system (1), e.g. a stereophonic amplifier, is tested by means of a test apparatus (8) to determine whether the system inverts one or both channels and/or interchanges them. To this end the apparatus includes four cross-correlators (11, 12, 13, 14) which are connected between respective pairs of the inputs (2, 3) and the outputs (4, 5) of the system (1). The output of a particular cross-correlator is negative if there is inversion between the relevant system input and the relevant system output, this being indicated by means of sign-determining units (27, 28, 29, 30) and a look-up table (31). The magnitude of the output of a particular cross-correlator is higher if the relevant system input and the relevant system output carry the same channel signal then otherwise. Channel interchange is detected by comparing the sum of the magnitudes of the outputs of the two cross-correlators (11, 12) for which this is normally the case with the sum of the magnitudes of the outputs of the other two cross-correlators (13, 14). This is achieved by means of magnitude determining units (33, 34, 35, 36), adders (37, 38) and a comparator (39). <IMAGE>

Description

TESTING A PLURAL-CHANNEL AUDIO SIGNAL PROCESSING SYSTEM This invention relates to apparatus for testing a plural-channel audio signal processing system. Such a system may be, for example, a stereophonic signal amplifier or a duplication system for magnetic tapes.

With many such systems it is highly desirable that conformity exists between the phases of a first channel signal to be processed by the system and this first channel signal after processing by the system, and between the phases of a second channel signal to be processed by the system and this second channel signal after processing by the system. If the first and second channel signals are the left and right channel signals of a stereophonic pair, reversal of the phase of one relative to the other will result in the intolerable loss of a satisfactory stereophonic image upon reproduction. Thus, for example, if such a reversal occurs during production of a batch of stereophonic magnetic tapes from a master the result will be scrapping of the batch if the reversal is detected, or release of a faulty product otherwise.Such a fault may arise, for example, due to the use of differential circuits, inverting amplifiers, or buffers in the system.

Another fault which may arise with such systems is the accidental interchange of the first and second channel signals processed by the system, for example due to incorrect connection of plugs and sockets either at the system inputs or at the system outputs.

Conventional methods of testing for such faults include the use of test signals or trained listeners. The use of test signals has the disadvantage that the technique is invasive and requires the normal use of the system to be interrupted. Reliance on trained listeners is prone to error, particularly in terms of signal interchange. In fact some faults such as phase reversal of both signals cannot be detected by the most expert of listeners.

It is an object of the invention to render the use of test signals and trained listeners unnecessary.

The invention provides apparatus for testing a plural-channel audio signal processing system, said apparatus having first and second inputs for connection to outputs of the system for first and second channel signals processed by the system respectively, said apparatus comprising first cross-correlator means coupled to said first input for cross-correlating a signal presented to said first input or a filtered version thereof with the first channel signal to be processed by the system or a filtered version thereof, and second cross-correlator means coupled to said second input for cross-correlating a signal presented to said second input or a filtered version thereof with the second channel signal to be processed by the system or a filtered version thereof.

Provided that the system does not interchange the channel signals and that corresponding sections of the first channel signal to be processed by the system and the signal presented to the first input (normally the first channel signal processed by the system) are applied to the first cross-correlator means, and corresponding sections of the second channel signal to be processed by the system and the signal presented to the second input (normally the second channel signal processed by the system) are applied to the second cross-correlator means, the signs of the output signals of the first and second cross-correlator means will be indicative of whether the system inverts the first channel signal and/or the second channel signal respectively.Such a situation may be signalled, for example, by providing indicator means coupled to outputs of said first and second cross-correlator means for indicating whether any cross-correlation determined thereby is negative.

If on the other hand the system does interchange the channel signals and it so happens that there is no correlation between the two channel signals the outputs of the first and second cross-correlator means will be substantially zero, whether or not channel inversion occurs.

In order to cater for this situation the apparatus preferably includes third cross-correlator means coupled to said first input for cross-correlating a signal presented to said first input or a filtered version thereof with the second channel signal to be processed by the system or a filtered version thereof, and fourth cross-correlator means coupled to said second input for cross-correlating a signal presented to said second input or a filtered version thereof with the first channel signal to be processed by the system. The indicator means (if present) may then also be coupled to outputs of said third and fourth cross-correlator means for indicating whether any cross-correlation determined thereby is negative.

If the system includes the third and fourth cross-correlator means then it may also include magnitude comparison means having respective inputs coupled to outputs of the first, second, third and fourth cross-correlator means for comparing the sum of the magnitudes of the outputs of the first and second cross-correlator means with the sum of the magnitudes of the outputs of the third and fourth cross-correlator means.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing the single Figure of which is a block diagram of the embodiment.

In the drawing reference numeral 1 denotes a plural-channel audio signal processing system (shown in dashed lines), in the present example a stereophonic amplifier. The amplifier 1 has inputs 2 and 3 for first and second channel signals to be processed by the system respectively, in the present example the left and right stereophonic signals respectively, and outputs 4 and 5 for the first and second channel signals processed by the system respectively, in the present example the amplified left and right stereophonic signals respectively. The outputs 4 and 5 are connected to first and second inputs 6 and 7 respectively of a test apparatus 8. The inputs 2 and 3 are connected to further inputs 9 and 10 respectively of the apparatus 8.

The apparatus 8 includes first, second, third and fourth cross-correlator means 11, 12, 13 and 14 respectively (which may be implemented either in hardware or software). Means 11 has a first input 15 coupled to input 6, a second input 16 coupled to input 9 and an output 17. Means 12 has a first input 18 coupled to input 7, a second input 19 coupled to input 10 and an output 20. Means 13 has a first input 21 coupled to input 6, a second input 22 coupled to input 10 and an output 23. Means 14 has a first input 24 coupled to input 7, a second input 25 coupled to input 9 and an output 26.The outputs 17, 20, 23 and 26 are coupled to sign-determining means 27, 28 and 29 and 30 respectively (which may be implemented either in hardware or software) outputs of which are coupled to respective inputs of indicator means in the form of an appropriately programmed look-up table 31. The (twoline) output of table 31 is coupled to an output 32 of the apparatus 8.

The outputs 17, 20, 23 and 26 of the cross-correlator means 11, 12, 13 and 14 respectively are also coupled to inputs of magnitude-determining means 33, 34, 35 and 36 respectively. The outputs of the magnitude-determining means 33 and 34 are coupled to respective inputs of an adder 37 and the outputs of the magnitude-determining means 35 and 36 are coupled to respective inputs of an adder 38. The outputs of the adders 37 and 38 are coupled to respective inputs of a comparator 39 the output of which is coupled to a further output 40 of the apparatus 8. The components 33-39 (which may be implemented in hardware or software) together form a magnitude comparison means 41 having inputs 42, 43, 44 and 45 respectively and an output 46.

In operation, and assuming the operation of amplifier 1 is correct, cross-correlator means 11 cross-correlates the left stereophonic signal to be amplified by amplifier 1 with the corresponding signal after amplification. Similarly cross-correlator means 12 cross-correlates the right stereophonic signal to be amplified by amplifier 1 with the corresponding signal after amplification. Similarly cross-correlator means 13 cross-correlates the right stereophonic signal to be amplified with the left stereophonic signal after amplification. Similarly crosscorrelator means 14 cross-correlates the left stereophonic signal to be amplified with the right stereophonic signal after amplification. Thus, if amplifier 1 is operating correctly, in particular is not inverting either or both channel signals, none of the cross-correlation results will be negative.Thus the output signals of all the sign-determining means 27-30 will be binary "1" (say) giving a particular 2-bit combination at output 32. On the other hand, if amplifier 1 is inverting one or both channels (but is not interchanging the channels) the crosscorrelation result determined by at least cross-correlation means 11 and/or 12 will be negative.

Thus the output signal of at least sign-determining means 27 and/or 28 will be binary "O" (say), giving particular 2-bit combinations at output 32 indicating this fact. If in addition amplifier 1 is interchanging the channels then the cross-correlation result determined by at least cross-correlation means 13 and/or 14 (rather than 11 and/or 12) will be negative. Thus the output signal of at least sign-determining means 29 and/or 30 will be binary "0" (say) giving particular 2-bit combinations at output 32 indicating this fact.

If amplifier 1 is not interchanging the channels then the magnitudes of the crosscorrelation results determined by cross-correlation means 11 and 12 will be greater than those determined by cross-correlation means 13 and 14, whereas if amplifier 1 is interchanging the channels the converse will be the case. In the former case the output signal of adder 37 will be greater than the output signal of adder 38, giving a particular output signal, say binary "1", from comparator 39 and hence at output 40. In the latter case the output signal of adder 37 will be less than the output signal of adder 38, giving another output signal, say binary "0", from comparator 39 and hence at output 40.

Thus, overall, the 2-bit signal generated at output 32 indicates which (if any) of the left and right stereophonic signals is being inverted by amplifier 1 and the single-bit generated at output 40 indicates whether amplifier 1 is interchanging the two signals between its inputs 2 and 3 and its outputs 4 and 5.

Although as described the plural-channel audio signal processing system 1 takes the form of a stereophonic amplifier, it will be appreciated that this is not essential. As an alternative the system 1 may take the form, for example, of an audio magnetic tape duplication system. In such a case the signals applied to the inputs 9 and 10 of apparatus 8 may be left and right stereophonic signals respectively derived from a master tape and the signals applied to the inputs 6 and 7 may be corresponding left and right stereophonic signals respectively derived from another tape which has been duplicated from the master.In such a case it is difficult to maintain exact time-correspondence between the signal read from the master tape and the corresponding passage read from the other tape because, for example, of mechanical tolerances in the machines used to play back the master and other tapes, and this may result in unsatisfactory operation of the cross-correlation means 11-14. This problem may be alleviated to some extent by low-pass filtering the input signals to the crosscorrelation means 11-14, this filtering having a cut-off frequency of. for example, 500Hz.

Alternatively or in addition digital samples of corresponding passages from the master and other tapes may be stored prior to assessment by the apparatus 8, a method as described and claimed in co-pending patent applications GB 9203911.4 and PCT/GB93/00381, which are incorporated herein by reference, being used to ensure that the stored passages are accurately aligned one with the other. In such a case the inputs 9 and 10 may be omitted if desired, the digitally sampled passage from the master tape being stored in the apparatus 8 prior to the inputting of the corresponding passage from the tape duplicated therefrom.

It has been assumed in the description above with reference to the operation of the look-up table 31 that the input signals to the system 1 are correct, i.e. that one is not inverted while the other is not. If the possibility of such inversion is present then a further crosscorrelator means (not shown) may be provided in the apparatus 8, its two inputs being fed from respective ones of the inputs 9 and 10 and its output being coupled to a further input (not shown) of look-up table 31 via a further sign-determining means (not shown) to appropriately modify the output signal of table 31 in the event of there being phase inversion between the two input signals (which will tend to cause the cross-correlation result given by cross-correlation means 13 and/or 14 to be negative even if the system 1 is operating correctly in respect of not giving rise to channel inversion).

Claims (6)

1. Apparatus for testing a plural-channel audio signal processing system, said apparatus having first and second inputs for connection to outputs of the system for first and second channel signals processed by the system respectively, said apparatus comprising first crosscorrelator means coupled to said first input for cross-correlating a signal presented to said first input or a filtered version thereof with the first channel signal to be processed by the system or a filtered version thereof, and second cross-correlator means coupled to said second input for cross-correlating a signal presented to said second input or a filtered version thereof with the second channel signal to be processed by the system or a filtered version thereof.

2. Apparatus as claimed in Claim 1, including indicator means coupled to outputs of said first and second cross-correlator means for indicating whether any cross-correlation determined thereby is negative.

3. Apparatus as claimed in Claim 1 or Claim 2, including third cross-correlator means coupled to said first input for cross-correlating a signal presented to said first input or a filtered version thereof with the second channel signal to be processed by the system or a filtered version thereof, and fourth cross-correlator means coupled to said second input for cross-correlating a signal presented to said second input or a filtered version thereof with the first channel signal to be processed by the system.

4. Apparatus as claimed in Claim 3 when appended to Claim 2, wherein the indicator means is coupled to outputs of said third and fourth cross-correlator means for indicating whether any cross-correlation determined thereby is negative.

5. Apparatus as claimed in Claim 3 or Claim 4, including magnitude comparison means having respective inputs coupled to outputs of the first, second, third and fourth crosscorrelator means for comparing the sum of the magnitudes of the outputs of the first and second cross-correlator means with the sum of the magnitudes of the outputs of the third and fourth cross-correlator means.

6. Apparatus for testing a plural-channel audio signal processing system substantially as described herein with reference to the drawing.