Patent Application
20090276226
The invention relates to a method for encoding an analog signal by means of an analysis based on synthesis methods.
A topic much discussed at the present time is the idea of expanding the bandwidth for acoustic signals, e.g. expanding from 4 kHz telephony bandwidth to 8 kHz broadband telephony, since this will be accompanied by a significant improvement in the quality of the voice signal.
However, bandwidth is a limited resource, in particular in mobile cellular communications, in which at least a part of the transmission takes place over a radio link. That is to say that the predefined, limited bandwidth has to be distributed among a plurality of users. If the bandwidth is then increased for one user, it necessarily follows, assuming the number of users remains the same, that the bandwidth available to the remaining users will be reduced.
Various methods are therefore applied in order to construct from the excitation signal in the narrowband, i.e. for example with a 4 kHz bandwidth in the range from 0 to 4 kHz, a signal of higher bandwidth, for example 8 kHz bandwidth from 0 to 8 kHz.
This is accomplished for example by squaring the narrowband signal in the time domain and generating the missing band by mirroring or shifting the narrowband in the frequency domain. For the example of the 4 kHz bandwidth and a desired bandwidth of 8 kHz, this means that the spectrum from 0 to 4 kHz is mirrored at, for example, 4 kHz, thereby generating the spectrum from 4 to 8 kHz. Alternatively a shifting by 4 kHz is possible. By means of these methods a broadband signal can thus be constructed from a narrowband signal, albeit with the resulting disadvantage that these methods either distort the spectrum of the narrowband excitation signal or else cause data errors in the spectrum.
Proceeding from the basis of this prior art, the object of the present invention is to provide a means of creating a signal that is of high quality compared to the prior art while at the same time requiring only a small amount of transmission bandwidth.
This object is achieved by the independent claims. Advantageous developments are the subject matter of the dependent claims.
An analog signal is broken down into time frames for encoding purposes and a synthetically produced signal is matched to the analog signal time frame by time frame. The synthetic signal is generated as the output signal of a synthesis filter which is excited by means of an excitation signal as input signal.
In order to form the excitation signal use is made of at least one adaptive codebook which contains the excitation signal for earlier time frames. The earlier excitation signal is represented in this case as a plurality of sampled values.
In order to represent the current excitation signal, a segment corresponding to the length of the current time frame is selected from the plurality of sampled values contained in the adaptive codebook. The selection is made using a reference parameter which is dependent on a basic voice frequency and which can also assume non-integer values, i.e. points to locations for intermediate values lying between the actually present sampled values.
If the basic voice frequency parameter now assumes a non-integer value, intermediate values corresponding to the sampled values are chosen in the selected segment. As already described, the segment corresponds in its length to the current time frame and its position in the adaptive codebook is specified by the basic voice frequency parameter.
This forming of intermediate values is accomplished for example by means of interpolation. An interpolation can be performed in particular by means of a (sin x)/x function.
The core of the invention is thus to use the totality of sampled values and interpolation values for forming the excitation signal.
This has the advantage that an effective higher bandwidth is achieved which is produced from the effectively higher sampling rate for the sampled values and intermediate values. This enables the quality of a synthetic signal reproduced on the receiver side and corresponding as closely as possible to the actual analog signal to be considerably improved. This improvement happens without an increase in the demand for transmission bandwidth, since the same encoding parameters are transmitted as in the case of a narrowband solution.
The improvement is achieved in that already generated intermediate values in the codebook—in particular on the transmitter and receiver side—are retained and used to generate the excitation signal.
This is in contrast to prior art solutions in which, despite the fact that a non-integer basic voice frequency parameter was provided which specified the position of the segment in the adaptive codebook, the interval between the intermediate values used for generating the excitation signal was not reduced.
To express it in different words, if, for example, the basic voice frequency parameter specifies the start of the selected segment and points to the value 5⅓, the corresponding intermediate values 5⅓, 6⅓, 7⅓ etc. are formed and only these are used for generating the excitation signal and retained in the adaptive codebook. According to the invention, however, the values 5⅓, 5⅔, 6, 6⅓, 6⅔ etc. would be used, which can be accomplished without additional transmission of information. In this way an improvement in quality is produced while at the same time achieving an efficient utilization of transmission capacity.
In particular the basic voice frequency parameter can be represented as a fraction of a whole number N. This then results in a reduction in the time interval by 1/N. If, for example, N=2 or 3 is chosen, which corresponds to a doubling or tripling of the bandwidth of the excitation signal to be represented, the interval reduces between a sampled value and an intermediate value to ½ or ⅓. Similarly, in the case where N is greater than or equal to 3, the interval between two intermediate values is reduced to the same value.
The excitation signal can also be generated in particular by means of a fixed codebook. Fixed excitation signals, for example, are contained in a fixed codebook.
According to an advantageous embodiment it is provided to retain the fixed codebook in its originally specified bandwidth or, as the case may be, the original sampled values and to achieve a higher bandwidth only by means of the adaptive codebook. This has the advantage of a particularly simple implementation.
In order to create intermediate values between the originally present fixed excitation signals also in the case of the fixed codebook, a fixed codebook entry can be shifted while retaining the time intervals between the signal components. If, for example, a fixed codebook entry of length 4 has a signal component at times 1 and 3, and no signal component or a zero value of the signal component at times 0, 2 and 4, then a shift would take place to the times ⅓ to 4⅓.
Alternatively it can be provided to determine intermediate values by interpolation in the case of the fixed codebook also.
In addition or alternatively to the fixed codebook, a white, i.e. essentially frequency-independent, noise signal can be used for generating the excitation signal. This can save on the need for the fixed codebook, for example. Experience has shown that in this way, in particular with voice signals, a very satisfactory quality of the signal generated on the receiver side can be guaranteed.
The noise signal is recorded from the environment or generated by means of a noise generator.
In order, for example, to avoid an overemphasizing of the harmonic structure in the thus expanded frequency range, that is to say, for example, the frequency range between 4 and 8 KHz in the case of a narrowband signal with a 4 kHz bandwidth, a filtering of the formed excitation signal can be provided, in particular before it is used as an input signal for the synthesis filter. Wiener FIR (Finite Impulse Response) filtering, for example, can be performed in this case.
The proposed methods can be performed in a communication terminal device having an encoding unit, such as, for example, a mobile phone, a PDA (Personal Digital Assistant), a computer or a fixed-network telephone, etc.
A corresponding receiver, for example interworking elements between different communication systems, a TRAU (Transmission and Rate Adaption Unit) has a corresponding decoding unit.
A suitable communication system has at least one communication terminal and one receiver.
Further advantages are presented with reference to exemplary embodiments, some of which are also depicted in the figures, in which:
a: shows the generation of a synthesized signal;
b: shows the generation of an excitation signal for a broadband solution;
FIG 1a shows the use of an excitation signal exc for exciting a synthesis filter A(z) . The synthesis filter A(z) simulates in the case of voice signals in the human vocal tract, with the result that in this case a synthetic acoustic signal AS_syn is generated by means of a suitable excitation signal exc. Said synthetic acoustic signal AS_syn is compared with the actual acoustic signal as by means of a comparator C. The excitation signal exc is successively matched in such a way that the synthetic acoustic signal AS_syn simulates the actual acoustic signal as as closely as possible.
b then shows the generation of the excitation signal exc. Several parameters are used for this purpose, which parameters are finally transmitted for effective use of the bandwidth, since the transmission of said parameters requires less transmission capacity than the transmission of the excitation signal exc itself.
b shows the generation of an excitation signal exc in the case of a broadband solution.
What is understood by broadband solution in this case is that the bandwidth of the signal reconstructed on the receiver side is greater than originally provided e.g. by the embodiment of codebooks. In the case of an extension of the G.729, a signal with 4 kHz bandwidth is referred to as a narrowband signal, and a signal expanded to 8 kHz bandwidth is referred to as a broadband signal.
In order to generate the excitation signal, an adaptive codebook ACB is provided by means of which harmonic components of the acoustic signal are represented. For that purpose the adaptive codebook includes earlier excitation signals old_exc, i.e. signals from preceding time frames or time slots. An entry is chosen from the adaptive codebook ACB by way of a non-integer basic voice frequency parameter p which is represented by its integer component N*(int p), where N represents an integral number., and the fraction p_frac.
The basic voice frequency parameter in
In this case
In the following reference is once again made to
In order to obtain a bandwidth-expanded fixed excitation signal exc_s, values are optionally inserted between the existing values in the fixed codebook. The number of values inserted therebetween depends on the desired bandwidth expansion. Said insertion is intended to be made clear by means of the entry int N.
Sampled value is the term used to denote the values sampled in an original first sampling frequency. The values initially synthetically inserted therebetween are referred to as intermediate values, which initially assume the value 0 and then values≠0 as a function of the respective new time frames of the signal. In line a), positions at which sampled values are provided in the original smaller bandwidth are circled, while the values lying between are intermediate values.
For the first frame (frame 1), the adaptive codebook ACB is empty, i.e. only zero values are present at the times which correspond to a desired sampling rate. At the same time zeros are already inserted as intermediate values, with the result that in line a) in the adaptive codebook zero values are present at the times which already correspond to a higher sampling rate.
If the first frame is present for example only in a first sampling rate, for example 4 kHz, as for instance by means of the non-zero values of the current frame in line a, and if, however, a subsequent encoding for a tripled sampling rate, for example 12 kHz, is to be performed, a corresponding number of zero values is inserted between the existing sampled values. This is also shown in line a for the current frame.
If, for example, the rate is expanded to the tripled sampling rate, which then corresponds to a tripled bandwidth of the signal achievable thereby, then 3 minus 1 intermediate values are inserted between existing sampled values. For the second frame (frame 2), the first frame is already contained in the adaptive codebook. Using an index by means of which each of the sampling points and intermediate values can be selected, a suitable segment is selected from the adaptive codebook. The adaptive codebook ACB contains a number of M1 values, where M1=M0*M3 if M0 represents the number of values present for the first sampling rate, i.e., for example, at 4 kHz. With regard to the lower first sampling rate (of, for example, 4 kHz) against the intermediate values lying between the original sampled values in the case of non-integer basic voice frequency parameters p.
The second frame is represented for example by the elliptically circled segment from the adaptive codebook ACB.
For the third time frame (line D), which is represented by the elliptically circled segment from the adaptive codebook ACB, intermediate values≠0 are present in the adaptive codebook ACB. An adaptive codebook is built up successively in the manner shown.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 000 828.3 | Jan 2005 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/56479 | 12/5/2005 | WO | 00 | 7/3/2007 |
