Method of transmitting an audio signal

A method of digital transmission of an audio signal, in which in order to save transmission and/or memory capacity, the digital signal is converted, before transmission, into signals representing the short-term spectrum of time sections of the digital signal and portions of this signal are processed on the basis of psycho-acoustic laws, with portions of this signal lying below given thresholds left unconsidered, with at least one first threshold being defined below which only amplitude values outside of psycho-acoustically differentiable frequency ranges are left unconsidered.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of transmitting an audio signal 
wherein an analog signal is converted to a digital signal, is coded, is 
digitally transmitted and reconverted to an analog signal, with the signal 
being converted before transmission into a digital signal representing the 
short-term spectrum, and portions of this signal which lie below given 
thresholds are left unconsidered on the basis of psycho-acoustic laws 
during coding of the digital signal to be transmitted. 
Such a method is disclosed already in DE-OS No. 3,506,912. With the aid of 
this method, it is possible to keep the required transmission bandwidth 
small and manage with a narrowband channel, transmit as many audio signals 
as possible simultaneously over an existing channel or utilize an existing 
mass memory as economically as possible. This is based on the realization 
that, due to the psycho-acoustic characteristics of the human ear, a 
listener is unable to discern all components of a sound reproduction, 
i.e., some of these components are irrelevant in a data transmission 
sense. These components, which would take up a considerable portion of the 
otherwise required transmission capacity, are omitted. 
It has been found that the decision as to which components may be omitted 
requires the consideration of various criteria separately or in 
combination with other criteria. If such consideration is not made, the 
omission of certain components may lead to unsatisfactory sound 
experiences in certain unusual sound events. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a method of 
transmitting an audio signal in which a data reduction is made in such a 
manner that even for extraordinary sound events the sound experience is 
not worsened. 
This is accomplished in a method of transmitting an audio signal in which 
an analog signal is converted to a digital signal, is coded, is digitally 
transmitted and reconverted to an analog signal, with the signal being 
converted, before transmission, into a digital signal representing the 
short-term spectrum, and portions of this signal which lie below given 
thresholds are left unconsidered on the basis of psycho-acoustic laws 
during coding of the digital signal to be transmitted wherein only the 
amount values of the short-term spectrum outside of psycho-acoustically 
differentiable frequency ranges are left unconsidered. 
In the conversion of the signal into a signal representing the short-term 
spectrum, which is done by blockwise transformation, spectral components 
may occur which are not present in the long-term spectrum. The 
consideration of these spectral components during retransformation is 
important in order to again obtain a true image of the original signal. If 
the spectral components were suppressed in the course of data reduction, 
this would lead to a noticeable falsification of the sound offering. 
However, one of the data reduction measures resides in suppressing spectral 
components having a value which lies below a given threshold or to leave 
it unconsidered. The basic reliability of this measure in view of the 
contradictory requirement for high reproduction quality resides in the 
psycho-acoustic masking effect. This masking effect is an inherent 
property of the human ear and the process of hearing. The human ear is 
acting similar to a spectralanalyzer using about 26bandfilters distributed 
over the hearable frequency range from 20 Hz to 20 kHz. Within each of 
these frequency ranges any signal with magnitude of about 26 . . . 30 dB 
lower than the maximum signal in the same range cannot be detected by the 
ear and is therefore said to be "masked". As these signals do not 
contribute to the sensation of a certain sound they do not have to be 
transmitted. 
It has now been found that this masking effect cannot always be utilized if 
spectral components are affected by the suppression which were 
additionally produced in the short-term spectrum as a result of blockwise 
transformation. This is true even if the spectral components lie below the 
threshold at which the suppression of other spectral components would not 
result in an audible loss of quality. 
It would be desirable if the additional spectral components produced by 
blockwise transformation could be distinguished from the others so that it 
would be possible to group them into "relevant" and "irrelevant" spectral 
components. But this is not the case. 
Therefore, suppression of spectral components must be limited in those 
frequency ranges in which a masking effect is no longer reliably ensured. 
To nevertheless be able to maintain data reduction by suppression of 
certain low value spectral components, this measure is implemented below a 
first defined threshold only in frequency ranges which lie outside of 
psycho-acoustically differentiable frequency ranges. 
A further feature of the invention provides for concrete frequency ranges 
in which the masking effect does not reliably become effective and 
therefore the suppression of spectral components could be disadvantageous. 
This refers, on the one hand, to the low frequency range below about 1 kHz 
and, on the other hand, to the immediate vicinity of the greatest spectral 
component of the audio signal. The mention of the latter frequency ranges 
is based on the realization that this is where the "relevant" spectral 
components occur with particularly high probability and their suppression 
has a quality reducing influence even if their amounts lie far below the 
given threshold.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The signal shown in FIG. 1 results from a circuit arrangement as shown 
schematically in FIG. 3 in which a lowpass audio signal filtered by 
lowpass filter 30 is converted to a digital signal by an analog/digital 
converter 32, a time window 34 outs certain, preferably 20 ms, sections of 
the digital audio signal out of the long-term signal and the resulting 
blocks are transformed by a fourier transform circuit 36 from the time 
domain into the frequency domain, similarly to the procedures followed in 
the method disclosed in DE OS No. 3,506,912. 
Following the transformation, certain frequency components are suppressed 
by suppressor encoder, e.g., computer, 38 in a manner to be described 
below. The remaining signal is transmitted by a transmitter 40 over 
channel 41 to a receiver 42 and is then transformed back to the time 
domain by transformer 44 and respective time sections recombined to a 
continuous digital signal in recombiner 46 before being converted back to 
an analog signal by D/A connector 48. 
In the illustrated embodiment, the frequency axis is divided into 513 loci 
having the indexes 0 . . . 512, which can be associated with spectral 
components. A plurality of these loci are combined preferably in the 
manner as described in DE-OS No. 3,506,912into frequency groups of which 
there are a total of 26. The number of loci for spectral components in the 
individual frequency groups g differs and increases from low frequency 
groups to higher frequency groups. 
Due to the large number of loci, the graph plotted over the frequency axis 
is not to scale. Only those regions are shown in an expanded manner which 
are of significance for an explanation of the solution according to the 
invention. Shown is the frequency range below somewhat more than 1 kHz, 
here represented by groups 0 to 12, and the remaining frequency range from 
which, as an example, groups 16, 17 and 18 have been selected. 
The amounts of the spectral components are logarithmed on the ordinate and 
are related to the value of the absolute maximum (which is given a dB 
value of zero). The spectrum shown in the drawing is a short-term spectrum 
whose spectral-lines are produced, on the one hand, by the original signal 
and, on the other hand, also by blockwise transformation, i.e. by folding 
the signal function with the transform of the time window function. The 
spectral lines additionally produced by the blockwise transformation are 
also of significance for obtaining a true image of the original signal 
after transmission and retransformation of the signal from the frequency 
domain into the time domain. 
To save transmission or memory capacity during transmission or storage, 
respectively, the procedure in the past has been to leave unconsidered or 
suppress, e.g. set back to a very low amount or to zero, all those 
spectral components below a threshold 1, According to the psycho-acoustic 
effect of masking, this threshold could be in the range of 26 to 30 dB. It 
has been realized that this leads to unsatisfactory results. Therefore, in 
the method disclosed in DE-OS No. 21 513 the thresholds are calculated by 
determining in each group g separately the logarithm ratio of the 
effective value of the maximum amount value to the square root of the mean 
energy of all spectral components of this group, multiplied by a factor of 
approximately 3. This factor has been determined by experiments and can 
lie in a range between 2 to 5. The calculated range of the threshold is 
then limited to a minimum of 30 dB in order not to be below the range 
given by the psycho-acoustic effect of masking. This leads to a variation 
range of the first threshold over all groups g, of 30 to 70 dB. However, 
this may lead to unsatisfactory results if the unconsidered spectral 
components were additionally produced by blockwise transformation. Since 
these cannot be distinguished from other spectral components. the method 
is performed by suppressor 38, modified in the following manner, in 
deviation from the general nonconsideration or suppression of spectral 
components below a threshold 1. 
In the range below about 1 kHz, i.e. herein groups 1 to 10, an additional 
threshold 2 is defined which lies below threshold 1, preferably 10 dB 
below it. If spectral components, e.g. the spectral components marked 3, 
lie above threshold 1, they are coded and transmitted. If they lie below 
threshold 1 but still above threshold 2, as is the case for spectral 
component 4, they are also transmitted. Only the spectral components 
marked 5, which also lie below threshold 2, are suppressed. 
A further modification takes place in the immediate vicinity of the 
absolute maximum of the amount values of the spectrum. In the spectrum 
here serving as an example, the spectral component having the highest 
amount value 6 belongs to group 17. Independently of the fact whether the 
adjacent spectral components 7 and 8 lie above threshold 1, as is the case 
for spectral components 7, or below threshold 1, as is the case for 
spectral components 8, they are not suppressed. This applies regardless to 
which group g spectral component 6 belongs which has the absolute maximum 
of the amplitude value. Even if this is the case in the frequency range of 
groups 0 to 10, the spectral components are not suppressed. 
In the subsequent lower or upper groups 16 and 18,however, the spectral 
components lying above threshold 1, here marked 9, are transmitted and the 
spectral components 10 below the threshold are suppressed. 
The method is advisably executed with the same computer with which 
transformation and coding are effected. To describe the method steps, 
reference is made to FIG. 2 which is a flow diagram. After the start in 
step 11, the group having the group number g=1 is addressed. Group g=0 
takes on a special position since it represents the dc - value of the 
short-term spectrum and is of no further significance for the execution of 
the method. In step 13, a decision is now made whether the examined group 
9 contains the absolute maximum of the amount value. If that is the case, 
the spectral components disposed in this group are not suppressed but are 
instead branched off to step 14, the group number to be examined is 
changed to the next following number and a decision is made at step 15 
whether all groups have been examined. If this is the case, the 
examination for the block in question is terminated and routine step 16 is 
completed. For the recovery of the next block, the system then jumps back 
to start 11 step and the routine begins anew. 
If this is not the case, i.e., the group being examined does not contain 
the absolute maximum, a jump back to step 13 is made and it is determined 
whether the group g now being examined contains the absolute maximum. If 
the group g does not contain the absolute maximum, a check is made in step 
17 whether the group being examined lies in the range of ordinals from 1 
to 10. If this is the case, the threshold is established to be threshold 2 
in step 18 for nonconsideration or suppression of the spectral components, 
this threshold lying 10 dB lower than threshold 1. If the group lies 
outside of the range of groups 1 to 10, threshold 1 in step 19 is 
established for nonconsideration or suppression of spectral components. 
In step 20, the group maximum, i.e. the spectral component of the maximum 
amount value in the same group, is now determined for the group being 
examined. Then a check is made of the further spectral components of this 
group. For this purpose, the spectral components having the lowest index 
within the group g presently being examined is examined in step 21. A 
check is made in step 22 whether the spectral component lies below the 
appropriate threshold 1 or 2. This check is made under consideration of 
the group maximum. For this purpose, the difference is formed of the group 
maximum and the amount of the respective spectral component. If the 
difference is greater than the threshold, i.e. if the spectral component 
lies below threshold 1 or 2, the spectral-component is marked for 
suppression in step 23. In the other case, no marking takes place. 
In step 24, the index of the spectral component presently being examined is 
moved on by 1 and in step 25 it is determined whether the index i is 
greater than the maximum index of the group. If this is not the case, the 
next group 9 is addressed. If it is the case, the spectral component with 
the next index is examined in the same manner. 
After examination of the spectral components with index 512 in group 25 the 
examination is completed, as already mentioned, and during the now 
following coding of the transformed signal blocks the marked spectral 
components are suppressed correspondingly. If the next transformed signal 
block is present, the process begins anew at start step 11. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.