CN1485849A - Digital audio encoder and its decoding method - Google Patents
Digital audio encoder and its decoding method Download PDFInfo
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Abstract
The invention relates to a digital audio code device and decoding method, wherein the device comprises a FFT device for transforming the audio frequency into frequency domain spectrum, an acoustic device for obtaining concealed critical value from the frequency spectrum, a MDCT device for selecting long transformation or short transformation according to the concealed critical value, a quantizer for completing quantification calculation to the frequency band sample through the internal and external loop. By making use of linear prediction coefficient for frequency spectrum masking signal transmission on the basis of the MP3 bit stream additional information, the invention can rendition high pitch portion on a decoder, thus improving the effect of audio compression deacidizing tone quality for MP3.
Description
(1) technical field
The present invention relates to a kind of regeneration techniques of DAB, is digital audio encoder and the coding/decoding method that the high pitch of DAB effectively can be regenerated.
(2) background technology
General, the DAB that is commonly referred to as digital MPEG (Moving picture Experts Group) is the standard mode of the International Organization for Stand (ISO/IEC) of high-quality high-performance stereo coding.
Above-mentioned MPEG, warp and closed circuit combination can be realized high efficiency multimedia information compression, have constantly occurred digital TV (DTV) recently, DVD, various application products such as digital music broadcasting (DAB:Digital AudioBroadcasting) and MP-3 player.
The MP3 audio frequency is recently in the mode of widely used MP3 expansion, means the sign indicating number that the mode of weaving into MPEG-1 audio frequency layering 3 is compiled.
The contraction principle of mpeg audio is a sense quality of utilizing the people, omits the low tiny information of sensitivity, employing be " perceptual coding " (Perceptual Coding) method of reducing encoding amount.
Here the perceptual coding that refers to is the coding method that utilizes the minimum audible limit of sound and especial sound phenomenon (because of the influence of especial sound, and can't hear the hidden phenomenon of other sound).
At this moment, the above-mentioned limit of listening is different with frequency (height of sound), and hidden phenomenon is with hidden sound (Masker) with by the frequency of hidden and unheard sound (Maskee) and different.
Especially, the critical band that will cause that the frequency bandwidth of hidden effect is called (Critical Band), but the perceptual signal band noise ratio (S/N) in this critical band is very low.
On MPEG, then utilize above-mentioned character,, make it not hear the quantification noise along with the quantification noise that digitizing produced is mixed in the critical band.
Accompanying drawing below with reference to the play-back technology of the DAB that replenishes explains.
Fig. 1 is the block diagram of the general digital audio encoder structure of standard, on the positive print shown in, by the bank of filters 1 that sound signal is subdivided into 32 sub-frequency bands; With the discontinuous cosine transform device of the mitigation MDCT that above-mentioned bank of filters is divided into finer frequency band (Modified Discrete Cosine Transform2 and above-mentioned sound signal is transformed into the fast fourier transformer FFT (Fastfourier Transform) 3 of the frequency spectrum of frequency field; With utilize the psychological sounder (psycho Acoustic) 4 that produces hidden curve from the frequency spectrum of above-mentioned FFT device 3 outputs; With the hidden curve of output on DCT coefficient of exporting on the above-mentioned MDCT device 2 of input and the psychological sounder 4, carry out the quantizer 5 of the quantization unit 5c of quantization operations by internal loop 5b and external loop-around 5a; Additional information follower 6 with used output additional information when the above-mentioned quantization operations; Huffman encoder 7 with output that the signal that quantizes on above-mentioned each sub-frequency bands is encoded; With in said additional information with carry out the coded audio follower 8 that the mistake stream check digit output mpeg audio of additional secondary data on the signal of huffman coding performs in a radio or TV programme and constitute.Below just describe at the operation of this constructional device.
Bank of filters 1, the input of pressing digital audio and video signals is subdivided into 32 sub-frequency bands output with it, FFT its 3, the frequency spectrum that then above-mentioned sound signal is transformed into frequency field is exported.
At this moment, 4 of psychological sounders are collected actual former sound on the curve of former sound and minimum audition limit, but obtain the hidden curve in the limit of perception, and above-mentioned MDCT device 2 just is subdivided into finer frequency band with the sub-band of output in the above-mentioned bank of filters 1.
After this, 5 of quantizers are adjusted each sub-frequency bands, promptly regulate each frequency band constant multiplier and quantized level size, will quantize noise and be encoded to below the hidden curve.
That is, constant multiplier is that the quantification noise degree by each sub-frequency bands decides, and adjust the size of the quantized level of each sub-band, makes the energy level of the quantification noise ratio hidden curve of each sub-band through quantizing to take place little.
At this, though above-mentioned quantized level size transfers big compressibility high more more, quantize the noise level and become big, the enabled stage size decreases, compressibility also diminishes though quantification noise level has diminished simultaneously.
At the loudness height, the hidden part that does not play much effects makes quantized level diminish a bit to reduce and quantizes noise, then allows quantized level more greatly to the big part of hidden influence.
Especially,, become minimum, use two circulation roads that form by external loop-around 5a and internal loop 5b in order in fixing bit stream, to make the quantification noise at above-mentioned quantizer 5.
At this, said external loop 5a, to the noise energy level frequency band bigger that generates at frequency field than hidden curve, to allow constant multiplier become big, it is low that the noise energy level is adjusted, and internal loop 5b is because bit (bit rate) instability of huffman code data, so, if then play the effect that strengthens quantization step and play adjustment when bigger than available bits figure place.
At this moment if quantized level step-length increasing, because of noise also increases together, thus repeat external loop-around 5a round-robin process once more, till the value that satisfies it.
The result that will obtain through above-mentioned repetitive process, after huffman encoder 7 carries out huffman coding, with its on coded audio follower 8 with the additional information stack of additional information follower 6, obtain the MP3 bit stream.
But, the psychological volume model that uses on above-mentioned MP3 algorithm is as benchmark with the mean value of everyone sense of hearing, can not provide and to satisfy proprietary reduction sound, especially, the absolute hearing limit that the above high pitch of 15khZ uses in psychological sound equipment model approximately is more than the 60dB, so be difficult to reduction in most sound signal, even also have the problem that is difficult to abundant reproduction at the frequency band of 10～15Knz.
(3) summary of the invention
The present invention considers the problems referred to above and invents, and the high pitch with DAB is provided, and through being reproduced as little bits of encoded, improves the audio coder and the coding/decoding method of the tonequality of decoded audio.
In order to achieve the above object, the present invention includes:
Sound signal is transformed into the FFT device of the fast fourier transform of frequency field frequency spectrum;
With the frequency spectrum that utilizes by described FFT device output, obtain the psychological sounder of hidden critical value;
According to the hidden critical value of described psychological sounder, select to have the MDCT device of the discontinuous cosine transform of mitigation of the frequency band sample of long conversion or short conversion;
According to the hidden critical value of described psychological sounder, described frequency band sample is finished the quantizer of quantization operations by internal loop and external loop-around;
The frequency spectrum of described FFT device output will be inserted, the linear predictor coefficient follower of output linear predictor coefficient on its frequency spectrum;
The additional information follower of used additional information during with the described quantization operations of output;
The signal that quantizes on described each sub-band is carried out the huffman encoder of huffman coding output;
With the additional coded audio follower that utilizes linear predictor coefficient shielding output mpeg audio on the signal of described additional information device and huffman encoder.
The coding/decoding method of a kind of aforesaid digital audio encoder of the present invention comprises:
Input MP3 bit stream is with the step of source information and additional information decoding;
If on described MP3 bit stream, have linear predictor coefficient, its linear predictor coefficient be reduced into the step of high band audio signal;
After constant multiplier and huffman code data decoded, its decoded data is carried out the step of re-quantization;
Comprehensively after sub-band, the sub-band that this is comprehensive is obtained the step of MP3 reduction sound by bank of filters with the subregion frequency band;
Superpose with sound signal and to obtain the step of the recovering signal that high pitch reproduced described MP3 reduction sound and described high frequency band.
Effect of the present invention: the present invention utilizes linear predictor coefficient to transmit the spectrum mask signal on the additional bit information of MP3 bit stream, through reproducing high line, will improve the effect of MP3 audio compression reduction tonequality on demoder.
For further specifying above-mentioned purpose of the present invention, design feature and effect, the present invention is described in detail below with reference to accompanying drawing.
(4) description of drawings
Fig. 1 is the block diagram of the existing digital audio encoder structure of expression;
Fig. 2 is the block diagram according to digital audio encoder structure of the present invention;
Fig. 3 is the operational flowchart to digital audio decoding code method of the present invention;
Fig. 4 is huffman coding frequency field block plan among the present invention.
(5) embodiment
Below just with reference to the effect and the effect of digital audio encoder of the present invention and coding/decoding method, and elaborate with reference to the accompanying drawings:
Fig. 2 is the block diagram of expression digital audio encoder structure of the present invention, just as shown in FIG., and by the bank of filters 1 that sound signal is subdivided into 32 sub-frequency bands; With FFT device 3 with sound signal transform band frequency spectrum; Obtain the psychological sounder 4 of hidden critical value with the frequency spectrum that utilizes 3 outputs of above-mentioned FFT device; With hidden critical value, has the MDCT device 2 of the selection frequency band sample of long conversion (Long Transform) or short conversion (Short Transform) according to above-mentioned psychological sounder 4; With hidden critical value, pass through internal loop 5b and the external loop-around 5a of above-mentioned frequency band sample finished the quantizer 5 that quantizes the operand 5c of portion according to above-mentioned psychological sounder 4; With the frequency spectrum that inserts above-mentioned FFT device 3 outputs, the linear predictor coefficient follower 100 of output linear predictor coefficient in its frequency spectrum; Additional information follower 6 with output used additional information when the above-mentioned quantization operations; Carry out the huffman encoder 7 that huffman coding is exported with the signal that will on each sub-frequency bands, quantize; With the shielding that utilizes linear predictor coefficient on the signal that is attached to said additional information and huffman coding, the coded audio follower 8 of output mpeg audio constitutes.
Figure is the operational flowchart of digital audio decoding code method of the present invention, on the figure shown in:
S1, S2: in case output MP3 bit stream, the step of decoding source information and additional information;
S9～S11:, its linear predictor coefficient is reduced into the step of high band audio signal if in the MP3 bit stream, have linear predictor coefficient;
S3～S5: after the decoding of constant multiplier and huffman data, the data of decoding are carried out the step of re-quantization, huffman data is to comprise sampling frequency, bit stream, level;
S6, S7: comprehensively after sub-band, the sub-band that it is comprehensive is obtained the step of MP3 reduction sound by bank of filters with the subregion frequency band;
S8: the sound signal of above-mentioned MP3 reduction sound and above-mentioned high frequency band stacked up obtain the step of the recovering signal that is regenerated as high pitch.
Operation of the present invention (simultaneously in conjunction with Fig. 2) will be described below.
At first, bank of filters 1 is segmented 32 sub-frequency bands to sound signal, and FFT device 3, the frequency spectrum that sound signal is transformed to frequency band is exported.
At this moment, psychology sounder 4 is to utilize the frequency spectrum of above-mentioned FFT device 3 outputs to obtain hidden critical value, promptly, will be from the frequency spectrum of above-mentioned FFT device 3 outputs, by each critical band and covert property modelling the long-pending Convolution of spread function Zhe after, its Zhe amassed add offset offset on the value and obtain hidden critical value.
Also have, MDCT device 2 is the hidden critical values according to above-mentioned psychological sounder 4, just selects the frequency band sample with long conversion or with short conversion in order to improve the frequency discrimination degree, and it is attached on the quantizer 5.
Thus, above-mentioned quantizer 5, input is from 576 frequency field samples of above-mentioned MDCT device 2 outputs, after its quantification, with the signal that quantizes on each sub-band, output after huffman encoder 7 carries out huffman coding, additional information follower 6 is output used additional information when above-mentioned quantization operations then.
At this moment, 5 of above-mentioned quantizers are finished internal loop 5b or external loop-around 5a according to two kinds of restrictive conditions.
Above-mentioned internal loop 5b then (surpasses) the adjustment constant multiplier and adjusts bit stream for not making available bits stream in frame, but after the Hofmann decoding, set bit stream if surpass, and adjusts constant multiplier and satisfies bit stream.
Said external loop 5a will quantize noise and be transformed into below the hidden critical value then at each critical band, allow to eliminate noise.
At this moment, when above-mentioned hidden critical value is high frequency band, approximately be the above value of 60dB, thus can not in most of sound signal, encode, even also be difficult to reproduce fully reduction at 10～15KHz band signal.
Thus, the present invention encodes in order only to carry out high frequency band with few bit number, use be linear predictor coefficient, below this point is illustrated.
The frequency spectrum of output on the at first above-mentioned FFT device 3, be input to linear predictor coefficient follower 100 and in its frequency spectrum, export linear predictor coefficient, but, in addition, also can generate its linear predictor coefficient in the sound signal in the time field by high band filter is less than 4 times and also can obtains effect of sufficient.
Then, coded audio follower 8 is attached to the shielding that utilizes linear predictor coefficient on the signal of said additional information and huffman coding, the output mpeg audio.
At this, Fig. 4 is the block plan of expression huffman coding frequency field, and frequency field is divided into 3 parts, for being replaced as " 0 " value, is the number of only knowing " 0 " value on demoder when the high frequency field, so can't reduce the sound signal of high frequency band.
For the sound signal of the high frequency band of decoding, utilize above-mentioned linear predictor coefficient, after the spectrum mask modelling, it is long-pending that its value and noise are carried out Zhe, just can reduce the frequency band sound signal of having removed.
In more detail, with reference to Fig. 3 the digital audio decoding code method is described.
At first, in case input MP3 bit stream, will be to having sampling frequency, bit stream, the Huffman information of information such as level and add additional information decode S1, S2.
Secondly, if in above-mentioned MP3 bit stream, have linear predictor coefficient, just its linear predictor coefficient is reduced into the sound signal S9～S11 of high frequency band.
That is, the S9 that will the property selected predictive coefficient decodes, after the spectrum mask signal modeling, it is long-pending that above-mentioned spectrum mask signal and noise are carried out Zhe, sound signal S10, the S11 of reduction high frequency band.
At this, the sound signal of above-mentioned high frequency band just in order to prevent the rapid energy level variations between the frame, be passed through intermediary's wave filter.
Said additional information in decoded sound signal after, inner constant multiplier and huffman data decode S3, S4 will be included in, its data of having separated sign indicating number are carried out re-quantization S5, the subregion frequency band is comprehensively become after the sub-band S6, the sub-band that it is comprehensive on bank of filters by after obtain MP3 reduction sound S7.
After this, above-mentioned MP3 reduction sound and above-mentioned high band audio signal are stacked up just can obtain the recovering signal S8 of the high pitch of regenerating.
Certainly, those of ordinary skill in the art will be appreciated that, above embodiment is used for illustrating the present invention, and be not to be used as limitation of the invention, as long as in connotation scope of the present invention, all will drop in the scope of claims of the present invention variation, the modification of the above embodiment.
Claims (9)
1. digital audio encoder is characterized in that described scrambler comprises:
Sound signal is transformed into the FFT device of the fast fourier transform of frequency field frequency spectrum;
With the frequency spectrum that utilizes by described FFT device output, obtain the psychological sounder of hidden critical value;
According to the hidden critical value of described psychological sounder, select to have the MDCT device of the discontinuous cosine transform of mitigation of the frequency band sample of long conversion or short conversion;
According to the hidden critical value of described psychological sounder, described frequency band sample is finished the quantizer of quantization operations by internal loop and external loop-around;
The frequency spectrum of described FFT device output will be inserted, the linear predictor coefficient follower of output linear predictor coefficient on its frequency spectrum;
The additional information follower of used additional information during with the described quantization operations of output;
The signal that quantizes on described each sub-band is carried out the huffman encoder of huffman coding output;
With the additional coded audio follower that utilizes linear predictor coefficient shielding output mpeg audio on the signal of described additional information device and huffman encoder.
2. digital audio encoder as claimed in claim 1, it is characterized in that described psychological sounder then will be at the frequency spectrum of FFT device output, will with by each critical band covert property modelling spread function Zhe long-pending after, will on the long-pending value of its Zhe, add offset and obtain hidden critical value.
3. digital audio encoder as claimed in claim 1 is characterized in that described internal loop then adjusts constant multiplier, makes to be no more than available bits stream in studio equipments inside and to adjust bit stream.
4. digital audio encoder as claimed in claim 1 is characterized in that described external loop-around on each critical band, will quantize noise and be deformed into below the hidden critical value.
5. digital audio encoder as claimed in claim 1 is characterized in that described linear predictor coefficient follower then utilizes the time field sound signal by high band filter, the output linear predictor coefficient.
6. the coding/decoding method of a digital audio encoder is characterized in that described coding/decoding method comprises:
Input MP3 bit stream is with the step of source information and additional information decoding;
If on described MP3 bit stream, have linear predictor coefficient, its linear predictor coefficient be reduced into the step of high band audio signal;
After constant multiplier and huffman code data decoded, its decoded data is carried out the step of re-quantization;
Comprehensively after sub-band, the sub-band that this is comprehensive is obtained the step of MP3 reduction sound by bank of filters with the subregion frequency band;
Superpose with sound signal and to obtain the step of the recovering signal that high pitch reproduced described MP3 reduction sound and described high frequency band.
7. digital audio encoder as claimed in claim 6, describedly on described MP3 bit stream, there is linear predictor coefficient if it is characterized in that, its linear predictor coefficient is reduced into finishes in the step of high band audio signal with the linear predictor coefficient decoding, with the step of spectrum mask signal modeling; With described spectrum mask signal and noise to be carried out Zhe long-pending, be reduced into the step of high band audio signal.
8. as claim 6,7 described digital audio encoders, it is characterized in that described high band audio signal,, pass through intermediary's high frequency filter for preventing the variation of energy level rapid between the frame.
9. digital audio encoder as claimed in claim 6 is characterized in that described huffman code data is to comprise sampling frequency, bit stream, level.
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