Method and apparatus for suppressing interference in a signal, in particular an audio signal

For suppressing interference in an audio signal comprising a useful signal component and an interference signal component in the method according to the invention firstly the original audio signal is sampled and digitally recorded. At the same time the level of the interference signal component is separately detected and sampled, the sample values being stored as reference interference signal under the control of a computer retaining the phase relationship with respect to the original audio signal. The recorded digitized signal is then processed under the control of a computer at a predetermined speed and the interference signal component thereof is compensated in dependence upon the stored reference interference signal. An apparatus for carrying out this method comprises an analog reproduction unit (1) for an original master tape (3), an analog/digital converter (4), a digital recording unit (6), an interference signal separating means (7), a memory means (11) for storing the reference interference signal, a digital reproduction unit (24) and a processor (9) in which the compensation of the signal reproduced by the digital reproduction unit (24) is effected in dependence upon the interference signal data present in the memory means (11).

CROSS REFERENCE TO RELATED APPLICATION(s) 
This U.S. application stems from PCT International Application No. 
PCT/DE84/00285 filed Dec. 31. 1984. 
The invention relates to a method for suppressing interference in an 
original signal, in particular an audio signal, comprising a useful signal 
component and an interference signal component, and to an apparatus for 
carrying out said method. 
As is known, in an electrical signal only the component which lies above 
the level of the interference signal component present in the signal can 
be utilized. In particular, in the case of an electroacoustic or audio 
signal the interference signal component is no longer subjectively 
perceived in those regions of the signal in which the signal amplitude is 
above the interference level caused for example by noise. However, to make 
the interference level inaudible also in the regions of the signal in 
which the amplitude of the useful signal component is beneath the 
interference level, it is necessary to carry out a suppression operation 
to remove the signal regions dominated by the interference level. To avoid 
then causing any audio physiological interference point the curve ends of 
the remaining signal regions must then be connected together in suitable 
manner. However, at the moment there would appear to be no suitable known 
method for this which can be carried out with an economically acceptable 
expenditure. 
The invention is based on the problem of providing such a method and an 
apparatus for carrying out said method by which with relatively low 
expenditure a substantially complete interference suppression can be 
obtained. 
This problem is solved as regards the method in that the original signal is 
digitally recorded with a sampling frequency corresponding at least to the 
essential information content of its useful signal component, that the 
interference signal component is detected and its level stored as 
reference interference signal with a sampling frequency corresponding at 
least to a level change rate of the interference signal component under 
the control of a computer in phase synchronism with the recorded digitized 
signal, and that the recorded digitized signal is processed with a speed 
independent of its recording speed and its interference signal component 
compensated under computer control in dependence upon the stored reference 
interference signal. 
By the digital recording of the generally analog original signal, on the 
one hand a coding of the signal suitable for the computer-controlled 
processing is obtained. On the other hand, this recording operation 
permits the method to be carried out independent of the recording and 
time. Due to the related possible choice of the processing speed 
independently of the recording speed the difficulty of a data flow of the 
digitally recorded signal which is too high for the computer-controlled 
processing is also eliminated. Likewise, the data flow corresponding to 
the interference signal component can readily be reduced to a speed 
suitable for computer-controlled storing because for carrying out the 
method it suffices to store the level change of the interference signal 
component which can be detected with a substantially reduced data quantity 
per time unit. This is because it suffices for the compensation following 
the readout from the store to know the level of the interference signal 
component, the detailed variation of the interference signal component not 
being important. 
The main use of the method according to the invention will no doubt lie in 
commercial music production. For example, by compensating the interference 
signal component in historic recordings the latter could be made available 
in modern sound quality. 
Preferably the method according to the invention is carried out in that the 
interference signal component is split into a hum component and a noise 
component and the corresponding reference interference signals are 
separately stored. This measure takes account of the different nature of 
the hum component and the noise component. Whereas the hum component is 
formed by a low-frequency ripple which can easily be compensated by a 
phase-inverted superimposing of a corresponding ripple signal, because of 
the separate storing of the corresponding reference interference signals 
the noise component can be treated separately from the hum component. 
In particular, to compensate the hum component it suffices to store as 
reference interference signal for the hum component the amplitude thereof 
and the phase thereof with respect to the original signal. This completely 
defines the phase-inverted signal necessary for the compensation. 
In a convenient embodiment of the method according to the invention the hum 
component is determined as the frequency component of the original signal 
lying in a predetermined hum frequency interval. This can easily be 
effected by a suitable filter. Any small loss of the useful signal 
component proves to be negligible. 
It is also found convenient and simple to detect the noise component as the 
component of the original signal lying below a predetermined amplitude 
limit small with respect to the mean level of the original signal. Instead 
of this detection, very simple to carry out, or alternatively additionally 
thereto it is further possible to detect the noise component as the 
component of the original signal lying above a predetermined frequency 
limit disposed beyond the essential frequency components of the useful 
signal component. Admittedly, with the first detection method of the noise 
component by means of the amplitude limit in the noise component a 
correspondingly small useful signal component may also be contained. This 
is however of no consequence to the execution of the method according to 
the invention because the method according to the invention requires only 
a knowledge of the level of the noise component. 
However, a still more exact detection is achieved in a preferred embodiment 
of the method according to the invention in that the noise component is 
detected in the pauses of the useful signal. Such pauses frequently occur 
especially in speech and music. Alternatively or additionally it is 
further provided in an advantageous embodiment of the method according to 
the invention that the noise component is detected as the signal component 
remaining in the region of the zero passages of the original signal. These 
two embodiments achieve firstly an adequately accurate detection of the 
level of the noise component and secondly the number of sampling points 
per unit time is small enough to permit readily the computer-controlled 
storing as reference interference signal. 
A particularly advantageous procedure is distinguished in that in the 
compensation firstly a compensation signal corresponding to the stored hum 
interference signal in amplitude and frequency is superimposed 
phase-inverted on the recorded digitized signal, and that the signal from 
which the hum component has been removed in this manner is recorded as new 
digitized signal and thereafter the noise component is compensated. This 
sequence is particularly convenient because the compensation of the noise 
component can be carried out particularly accurately and reliably on the 
signal already free from hum. 
In a preferred embodiment of the method according to the invention the 
compensation of the noise component is carried out in that the amplitude 
of the digitized signal is compared in phase synchronism with the 
reference interference signal corresponding to the noise component, that 
the digitized signal is put equal to zero at the points at which its 
amplitude is smaller or equal to the reference interference signal, and 
that the remaining digitized signal in the regions set to zero is 
supplemented under computer control by interpolation. By this zeroing 
firstly all the signal components identified as noise interference are 
eliminated. Nevertheless, this does not introduce any appreciable 
perceptible interferences because by the subsequent interpolation the wave 
train of the useful signal component interrupted by the regions set to 
zero is supplemented in natural manner. 
According to a further idea of the invention a correction is also provided 
of dynamic compressions. Such dynamic compressions are carried out in the 
analog recording of sound recordings on changing from piano to forte 
sections to avoid exceeding the permissible maximum modulation of the 
recorders, for example of an analog magnetic tape. In detail, the piano 
section modulated beyond its natural level is modulated down shortly 
before the start of the forte section so that another rise to the full 
modulation does not occur until in the forte section. This change in the 
variation of the modulation results of course in a corresponding change in 
the level of the noise component. Therefore, in a further development of 
the method according to the invention the level variation of the original 
presentation on which the original signal was based is re-established in 
that the level of the signal from which the hum has been removed is 
modulated again in proportion to a change of the level of the noise 
component. 
In the invention account has also been taken of the fact that the original 
signal can be distorted by the influence of amplitude limiters, i.e. cut 
off in the amplitude. In a preferred embodiment of the method according to 
the invention re-establishment of the signal corresponding to the original 
presentation is made possible in that the compensated digitized signal is 
replaced in the regions which have amplitude limitation under the control 
of a computer by an interpolation curve. For carrying out the method 
according to the invention the phase relationship between the digitized 
signal and the reference interference signal must be retained. This may be 
done for example in particularly simple manner in that the phase 
synchronization is carried out by continuous consecutive counting of the 
bits of the digitized signal. 
An apparatus provided within the scope of the invention for carrying out 
the method comprises an analog reproduction unit by which the original 
signal recorded analog on a recording medium can be reproduced, and an 
analog/digital converter by which the reproduced original signal can be 
converted with the sampling frequency associated with the useful signal 
component to the digitized signal, a digital recording unit for recording 
the digitized signal, an interference signal separating means by which the 
interference signal component contained in the original signal can be 
detected and can be digitized with the sampling frequency associated with 
the interference signal component, a memory means which is controlled by a 
processor and into which the reference interference signal formed by the 
sampling of the interference signal component can be entered according to 
amplitude and phase position related to the digitized signal, a digital 
reproduction unit by which the recorded digitized signal can be reproduced 
with the predetermined processing speed and a compensation means which is 
controlled by the processor and in which the interference signal component 
in the reproduced digitized signal can be compensated in dependence upon 
the reference interference signal present in the memory means. 
In an advantageous embodiment of the apparatus the interference signal 
separating means comprises a band filter means which is tunable to the hum 
frequency and has adjustable band width, by which the hum component of the 
interference signal component can be detected and a corresponding 
separately storable reference interference signal can be produced. This 
band filter means permits in simple manner the separation of the hum 
component and by the adjustability is adaptable to various conditions of 
the hum-disturbed original signal. 
Similarly, for simple construction of the apparatus it is convenient for 
the interference signal separating means to comprise a combined amplitude 
filter and high-pass means by which the noise component of the 
interference signal component is detected and a corresponding separately 
storable reference interference signal can be generated. 
As already explained above in conjunction with an advantageous embodiment 
of the method, with a dynamic compression of the original signal the level 
of the noise component is correspondingly altered. It is therefore 
provided in an advantageous further development of the apparatus according 
to the invention that the compensation means comprises a level controlled 
by the processor by which the reproduced signal can be remodulated in 
dependence upon the level of the noise component. By this remodulation the 
dynamics of the original recording on which the original signal was based 
can be re-established. 
To enable the digitized signal corresponding to the original signal to be 
buffered between individual processing stages it is further provided in a 
particularly preferred embodiment that the memory means comprises a memory 
area into which the digitized signal may be entered. 
Finally, in another further development of the apparatus according to the 
invention the signal separating means comprises a measuring device by 
which the level variation of the noise component detected can be 
indicated. In this manner an additional monitoring of the suppression 
operation by the user is possible, similarly to the monitoring made by a 
sound engineer of the original signal during the recording.

According to FIG. 1a an apparatus for suppressing interference in an 
original signal comprising a useful signal component and an interference 
signal component includes an analog reproduction unit 1 from the output 2 
of which the original signal obtained by reproduction from an analog 
master tape 3 and formed for example by an audio signal can be taken. 
Connected to the output 2 is an analog/digital converter 4 whose output is 
connected via a signal path 5 to the input of a digital recording unit 6. 
The output 2 of the analog reproduction unit 1 is further followed by an 
interference signal separating means 7 whose output is coupled via a 
signal path 8 to the input of a processor 9. The processor 9 is connected 
via a signal path 10 to a memory means 11. A synchronization connection 
existing between the digital recording unit 6 and the processor 9 
controlling the memory 11 is indicated symbolically by a dashed line 12. 
According to FIG. 2a there is superimposed on the useful signal component 
13 of the original signal an interference signal component which lies 
between a lower envelope curve 14 and upper envelope curve 15 and the 
maximum size of which limited by the spacing between the lower and upper 
envelope curves 14, 15 is exaggerated in FIG. 2a. Apart from this 
interference signal component illustrated in FIG. 2a and formed by a noise 
component a hum component not shown in FIG. 2a having a low-frequency hum 
frequency is also present. This disturbed original signal appearing at the 
output 2 according to FIG. 1a is applied to the input of the interference 
signal separating means 7 by which firstly the hum component and secondly 
the noise component is detected. In particular, the interference signal 
separating means 7 comprises for this purpose a filter means 16 which may 
comprise on the one hand a band filter means tunable to the hum frequency 
and having an adjustable band width and on the other a combined amplitude 
filter and high-pass means. The former serves to separate the hum 
component and the latter to separate the noise component. Whereas the hum 
component is defined by the hum frequency the noise component may be 
detected by its level limited by the spacing of the lower and upper 
envelope curves 14, 15. 
In particular, provided in the interference signal separating means 7 is a 
measuring means 17 by which the level variation of the noise component is 
detected. This is done for example according to FIG. 2b in the region of 
the points at which the useful signal component 13 assumes the value zero. 
This is the case in FIG. 2b for the entire region of the original signal 
up to the instant t.sub.1 which for example corresponds to a pause in the 
piece of music on which the original signal was based. Likewise, the level 
of the noise component may also be detected in the region of the instant 
t.sub.2 which corresponds to a zero passage of the useful signal component 
13. The measuring means 17 may also include a measuring device by which 
the level variation of the noise component is visibly displayed for a 
user. 
The level variation of the noise component detected in this manner is 
sampled in an analog/digital converter provided in the signal separating 
means 7 with a sampling frequency corresponding at least to its rate of 
change and thereby a digital reference interference signal R is generated 
which with the aid of the processor 9, which due to the synchronization 
connection 12 also forms phase information on the reference interference 
signal with respect to the original signal, is entered into the memory 
means 11. Likewise, the amplitude and the phase of the hum component is 
entered into the memory means 11 as further reference interference signal 
B for the hum component by means of the processor 9. 
As exactly apparent from FIG. 3a the original signal 19 is greater between 
the instants t.sub.a and t.sub.b than the level 20 of the noise component, 
which is shown exaggerated in FIG. 3a. Although the original signal 19 has 
the interference component due to the noise in the hatched region 21 
included between the instants t.sub.a and t.sub.b, this interference 
component is not subjectively perceived as detrimental because of the 
dominance in magnitude of the useful signal component 13. In contrast, the 
region of the useful signal component outside the interval t.sub.a, 
t.sub.b is submerged in the interference signal component, as indicated by 
the dashed curve 22. Accordingly, the digitized signal 23 recorded on the 
digital recording unit 6 and illustrated in FIG. 3c represents a useful 
signal component only at the sampling points t.sub.c lying in FIG. 3b 
between the sampling points t.sub.a and t.sub.b whilst for example at the 
sampling points t.sub.d and t.sub.e outside this interval only a value 
concealed by the noise interference is obtained. 
The digitized signal 23 recorded on the digital recording unit 6 and 
represented in FIG. 3 is compensated in a first step with regard to the 
hum component contained in its interference signal component. For this 
purpose the recorded digitized signal 23 according to FIG. 1b is 
reproduced with a predetermined processing speed by a digital reproduction 
unit 24 which is synchronized via a synchronization connection 12' with 
the processor 9. There is then superimposed on the reproduced signal 
appearing at the output 25 of the digital reproduction unit 24 in an adder 
stage 26 a signal with inverted phase which corresponds to the hum 
component and which is generated in a quartz-stable oscillator 27 which is 
controlled by the processor 9 in accordance with the reference 
interference signal which is present in the memory means 11 and which 
contains information on the phase and amplitude of the hum component. 
Thus, at the output 28 of the adder stage 26 the digitized signal freed 
from its hum component is available. 
However, as apparent from the level diagrams illustrated in FIGS. 5a to d, 
the signal freed from its hum component may comprise a level variation 
changed with respect to the original dynamic and due to a dynamic 
compression. In FIG. 5a the level of the full modulation of the analog 
master tape 3 is represented by the straight line 29 parallel to the time 
axis whilst the curve 30 represents the actual level variation of the 
original signal freed from its hum component. A signal section illustrated 
for the time t&lt;t.sub.p corresponds to a piano section of the original 
presentation on which the original audio signal was based, said piano 
section having been recorded however with an increased level with respect 
to the original presentation. Before the start of a forte section 
represented for the time t&gt;t.sub.f, the level variation 30 in the time 
interval t.sub.p &lt;t&lt;t.sub.f is therefore reduced by a sound engineer 
monitoring the recording so that at the dynamic jump starting at the time 
t.sub.f no overmodulation of the master tape 3 can occur. As a result, 
even the forte section present at the time t&gt;t.sub.f remains below the 
level of full modulation 29. This reducing of the modulation results as 
illustrated in FIG. 5b to an enlarged amplitude scale in a corresponding 
lowering of the level 20 of the noise component. In particular, during the 
piano section at the time t&lt;t.sub.p the noise component thus has a 
relatively high level whilst due to the lower modulation at the time 
t&gt;t.sub.f of the forte portion the level 20 of the noise component is 
relatively low. These conditions obtaining in the dynamic compression are 
illustrated jointly in FIG. 5c in which apart from the level variation 30 
of the original signal freed from hum and recorded on the master tape 3 
and the level variation 20 of the noise component the actual level 
variation 31 of the original underlying presentation is shown. 
This property by which the level 20 of the noise component represents a 
measure of the dynamic compression is used according to FIG. 1b to correct 
this dynamic compression by remodulation. For this purpose, according to 
FIG. 1b the digitized signal freed from hum is converted back in a 
digital/analog converter 32 to an analog signal and in a following level 
controller 33, which is controlled by the processor 9, in dependence upon 
the interference signal stored in the memory means 11 for the noise 
component and representing the level 20 of the noise component is 
remodulated in such a manner that the noise level 20 of the 
dynamic-compressed signal is raised at the point of the dynamic jump at 
t=t.sub.f to its value obtaining prior to the dynamic jump. This means a 
remodulation of the level variation 30 to a level variation corresponding 
to the original presentation and indicated in FIG. 5d by 31'. 
The signal dynamic-corrected in this manner is redigitized in an 
analog/digital converter 34 following the level controller 33 and recorded 
on the digital recording unit 6. 
The digitized signal freed from hum and dynamic-corrected according to FIG. 
1b is transmitted in FIG. 1c by means of the digital reproduction unit 24 
under the control of the processor 9 to the memory means 11 in which for 
this purpose a memory area 35 is provided in which the digitized signal 23 
is entered as illustrated in FIG. 3c. 
Thus, the compensation of the noise component is under the control of the 
processor 9. Under the control of a control program stored in the program 
memory 36, the processor 9 compares each of the digitized sample values of 
the digitized signal represented in FIG. 3c with the reference 
interference signal for the noise component stored in the memory means 11. 
Whereas the values exceeding the level 20 of the noise component 
illustrated in FIG. 3b are taken over as useful signal component the 
digitized signal is set to zero as illustrated in FIG. 3d at all points at 
which its value lies beneath the level 20 of the noise component. With the 
aid of the control program in the program memory 36 this remaining 
digitized signal 38 is then supplemented in accordance with the 
illustration of FIG. 3e in the regions set to zero by suitable 
interpolation values. These interpolation values are chosen in such a 
manner that the analog curve corresponding to the interpolated digitized 
signal of FIG. 3e and illustrated in FIG. 3f has a smooth transition path 
also in the region of the signal set to zero. This avoids the noise 
suppression resulting in disturbing transitions at the start and end of 
the regions set to zero. 
Along with the compensation of the noise component explained with reference 
to FIGS. 3d to f, in accordance with the illustration of FIGS. 4a and 4b 
any limiter influences may be eliminated. These manifest themselves 
according to FIG. 4a in that the signal amplitude is clipped, i.e. 
constant with time, in the peak regions 39 of the wave. The control 
program can be designed so that on detection of equality of three or more 
successive digitized sample values only the first and last value of this 
sequence of identical sample values are accepted in each case and the 
intermediate region bridged by an interpolation curve 40 represented in 
FIG. 4b. As a result of this compensation in accordance with FIG. 1d a 
compensated digitized signal is stored in a memory area 35'. 
In accordance with FIG. 1e this compensated digitized signal is transmitted 
under the control of the processor 9 from the memory area 35' to the 
digital recording unit 6. In this manner a new master tape 37 is obtained 
which is freed from all interference signal components. 
Whereas in the embodiment described above the compensation of the hum 
component and remodulation of the dynamics according to FIG. 1b is carried 
out in analog form of the signal, in an embodiment illustrated in FIG. 6 
this compensation step proceeds from the digitized signal reproduced by 
the digital recording unit 6. Otherwise, the procedure cycles correspond 
to the operations explained with reference to FIGS. 1a to e and identical 
reference numerals are also used for identical logic blocks. Going beyond 
the embodiment described with reference to FIG. 1 the analog reproducing 
unit 1 may be followed by a decoding means 41 which is indicated as 
alternative in dashed line in FIG. 6. This decoding means 41 serves when 
required to provide a decoding in the case where the recording is made by 
the systems Dolby A, DBX I, Telcom and the like. Furthermore, the 
processor 9 is provided with an input keyboard 42 on which the user can 
input additional corrections for the signal to be compensated. 
An embodiment illustrated in FIG. 7 of the apparatus for suppressing 
interference in an original signal comprising a useful signal component 
and an interference signal component differs from the embodiment 
illustrated in FIG. 1a firstly in that the processor 9 and the memory 
means 11 connected thereto via the signal path 10 are replaced by a 
further digital recording unit 50, a synchronizing signal generator being 
connected via a synchronization connection 52 to said further digital 
recording unit 50 and via a synchronization connection 53 to the digital 
recording unit 6 also illustrated in FIG. 1a . Furthermore, the reference 
numerals 1 to 8 and 16 to 18 designate the same blocks as in the 
embodiment illustrated in FIG. 1a. Secondly, in addition to the embodiment 
described in FIG. 1a in the embodiment of FIG. 7 a dual-beam oscilloscope 
54 is connected with its one input to the analog reproducing unit 1. A dc 
voltage generator 54 having both an analog output 56 and a digital output 
57 is connected with its analog output 56 to the other input of the 
dual-beam oscilloscope 54. The digital output 57 carrying a digitized dc 
voltage corresponding to the dc voltage appearing at the analog output 56 
is coupled to the further digital recording unit 50. The dc voltage level 
furnished by the dc voltage generator 54 is adjustable in its magnitude by 
means of a manual regulator 58 connected to the dc voltage generator 55. 
Whereas in the embodiment illustrated in FIG. 1a the interference signal 
component is acquired by means of the CPU 9 and the memory 11, in the 
embodiment illustrated in FIG. 7 the digital recording unit 50 serves to 
acquire the interference signal component. In particular, for this purpose 
the noise component 59 contained in the interference signal component is 
made visible by a suitable adjustment of the screen image resolution of 
the dual-beam oscilloscope 54 at the zero passages of the original signal 
60, which is illustrated with the aid of FIG. 8. The dc voltage applied to 
the other input of the dual-beam oscilloscope 54 from the dc voltage 
generator 55 generates on both sides of the zero line of the screen image 
dc voltage lines 61 whose spacing is set in each case by means of the 
manual regulator 58 so that the two dc voltage lines 61 exactly border the 
noise component. At the same time the noise level determined in this 
manner is recorded on the further digital recording unit 50 in that the 
digitized dc voltage appearing at the digital output 57 and corresponding 
to the spacing of the dc voltage lines 61 is recorded under the control of 
the synchronizing signal generator 51. 
At the same time the original signal also passes through the interference 
signal separating means 7 whose filter means 16 comprises a switchable 
narrowband filter which allows to pass frequencies of either 47 to 54 Hz 
or 57 to 64 Hz. The hum component is thereby filtered out and also 
recorded synchronously with the noise component on the further digital 
recording unit 50. As already explained in conjunction with FIG. 1a, 
furthermore in synchronism therewith the original signal is recorded 
together with the synchronizing signal on the digital recording unit 6. 
Thus, on the further digital recording unit 50 altogether the hum 
component, the digital dc voltage signal corresponding to the noise 
component and the synchronizing signal are recorded whereas simultaneously 
on the digital recording unit 6 the original signal is recorded together 
with the synchronizing signal. 
The original signal from the digital recording unit 6 digitized in this 
manner and provided with the synchronizing signal and the interference 
signal component consisting of the hum component and noise component 
together with the synchronizing signal from the further digital recording 
unit 50 are now transferred simultaneously or in succession to a data 
processing system in which the interference signal compensation described 
with reference to FIGS. 3a to 5d is carried out by suitable programs. 
Preferably, the sampling frequency and the digital quantization of the two 
digital recording units 6, 50 are identical. In particular, all digital 
values of the original signal lying beneath the noise level are erased and 
replaced by the interpolation curves described with the aid of FIGS. 3a to 
3f. To eliminate the hum component the recorded hum component is negated 
and added to the recording of the original signal. Furthermore, the 
original dynamics as described in conjunction with FIGS. 5a to 5d are 
reconstructed and any limiter influences eliminated as described in 
conjunction with FIGS. 4a and 4b. 
The method described above is also suitable for suppressing noise in video 
signals and processing the latter. During the transmission of the 
numerical values of the digitized original signal on the data processing 
system any vertical blanking intervals can be removed from the numerical 
value and stored as required together with the typical record structure. 
If the numerical values are coded in matrix form the latter is decoded and 
the type of coding stored as required. In the processing of colour video 
signals subsequent colour corrections may be made.