Method and apparatus for selecting an encoding rate in a variable rate vocoder

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IMPROVED METHOD AND APPARATUS FOR SELECTING AN ENCODING RATE IN A VARIABLE RATE VOCODER

[0<ΓΑ155| 114874/2

BACKGROUND OF THE INVENTION I. Field of the Invention

The present invention relates to vocoders. More particularly, the present invention relates to a novel and improved method for determining speech encoding rate in a variable rate vocoder. II. Description of the Related Art

Variable rate speech compression systems typically use some form of rate determination algorithm before encoding begins. The rate determination algorithm assigns a higher bit rate encoding scheme to segments of the audio signal in which speech is present and a lower rate encoding scheme for silent segments. In this way a lower average bit rate will be achieved while the voice quality of the reconstructed speech will remain high. Thus to operate efficiently a variable rate speech coder requires a robust rate determination algorithm that can distinguish speech from silence in a variety of background noise environments.

One such variable rate speech compression system or variable rate vocoder is disclosed in copending U.S. Patent No. 5,414,796 filed June 11,1991, entitled "Variable Rate Vocoder" and assigned to the assignee of the present invention, the disclosure of which is incorporated by reference. In this particular implementation of a variable rate vocoder, input speech is encoded using Code Excited Linear Predictive Coding (CELP) techniques at one of several rates as determined by the level of speech activity. The level of speech activity is determined from the energy in the input audio samples which may contain background noise in addition to voiced speech. In order for the vocoder to provide high quality voice encoding over varying levels of background noise, an adaptively adjusting threshold technique is required to compensate for the effect: of background noise on the rate decision algorithm.

Vocoders are typically used in communication devices such as cellular telephones or personal communication devices to provide digital signal compression of an analog audio signal that is converted to digital form for transmission. In a mobile environment in which a cellular telephone or personal communication device may be used, high levels of

[QCPA155I 114874/2 2 background noise energy make it difficult for the rate determination algorithm to distinguish low energy unvoiced sounds from background noise silence using a signal energy based rate determination algorithm. Thus unvoiced sounds frequently get encoded at lower bit rates and the voice quality becomes degraded as consonants such as "s","x","ch","sh","t", etc. are lost in the reconstructed speech.

Vocoders that base rate decisions solely on the energy of background noise fail to take into account the signal strength relative to the background noise in setting threshold values. A vocoder that bases its threshold levels solely on background noise tends to compress the threshold levels together when the background noise rises. If the signal level were to remain fixed this is the correct approach to setting the threshold levels, however, were the signal level to rise with the background noise level, then compressing the threshold levels is not an optimal solution. An alternative method for setting threshold levels that takes into account signal strength is needed in variable rate vocoders. A final problem that remains arises during the playing of music through background noise energy based rate decision vocoders. When people speak, they must pause to breathe which allows the threshold levels to reset to the proper background noise level. However, in transmission of music through a vocoder, such as arises in music-on-hold conditions, no pauses occur and the threshold levels will continue rising until the music starts to be coded at a rate less than full rate. In such a condition the variable rate coder has confused music with background noise.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and apparatus for determining an encoding rate in a variable rate vocoder. It is a first objective of the present invention to provide a method by which to reduce the probability of coding low energy unvoiced speech as background noise. In the present invention, the input signal is filtered into a high frequency component and a low frequency component. The filtered components of the input signal are then individually analyzed to detect the presence of speech. Because unvoiced speech has a high frequency component its strength relative to a high frequency band is more distinct from the background noise in that band than it is compared to the background noise over the entire frequency band. (QCPA1551 114874/2 3 A second objective of the present invention is to provide a means by which to set the threshold levels that takes into account signal energy as well as background noise energy. In the present invention, the setting of voice detection thresholds is based upon an estimate of the signal to noise ratio (SNR) of the input signal. In the exemplary embodiment, the signal energy is estimated as the maximum signal energy during times of active speech and the background noise energy is estimated as the minimum signal energy during times of silence. A third objective of the present invention is to provide a method for coding music passing through a variable rate vocoder. In the exemplary embodiment, the rate selection apparatus detects a number of consecutive frames over which the threshold levels have risen and checks for periodicity over that number of frames. If the input signal is periodic this would indicate the presence of music. If the presence of music is detected then the thresholds are set at levels such that the signal is coded at full rate.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawing in which like reference characters identify correspondingly throughout and wherein:

Figure 1 is a block diagram of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Figure 1 the input signal, S(n), is provided to subband energy computation element 4 and subband energy computation element 6. The input signal S(n) is comprised of an audio signal and background noise. The audio signal is typically speech, but it may also be music. In the exemplary embodiment, S(n) is provided in twenty millisecond frames of 160 samples each. In the exemplary embodiment, input signal S(n) has frequency components from 0 kHz to 4 kHz, which is approximately the bandwidth of a human speech signal.

In the exemplary embodiment, the 4 kHz input signal, S(n), is filtered into two separate subbands. The two separate subbands lie between 0 and 2 kHz and 2 kHz and 4 kHz respectively. In an exemplary embodiment, the input signal may be divided into subbands by subband filters, the design of

IQCPA155I 4

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10 which are well known in the art and detailed in U.S. Patent Application Serial No. 08/189,819 filed February 1, 1994, entitled "Frequency Selective Adaptive Filtering", and assigned to the assignee of the present invention, incorporated by reference herein.

The impulse responses of the subband filters are denoted ht(n), for the lowpass filter, and hn(n), for the highpass filter. The energy of the resulting subband components of the signal can be computed to give the values Rl(0) and Rh(0), simply by summing the squares of the subband filter output samples, as is well known in the art.

In a preferred embodiment, when input signal S(n) is provided to subband energy computation element 4, the energy value of the low frequency component of the input frame, Rl(0), is computed as: L-l RL(0) = Rs(0) RhL(0) + 2. £RS(i)RhL(i), (1) i=l where L is the number taps in the lowpass filter with impulse response hL(n), where Rs(i) is the autocorrelation function of the input signal, S(n), given by the equation:

N

Rs(i) = Σδ&lt;η) · S(n - *)' for ie ί2) n=l where N is the number of samples in the frame, and where RhL is the autocorrelation function of the lowpass filter hL(n) 25 given by: L-l

RhL(i)= ZhL(n)hL(n_i)· forie(0,L-l] n=0 = 0 else (3) 30 The high frequency energy, Rh(0)/ is computed in a similar fashion in subband energy computation element 6.

The values of the autocorrelation function of the subband filters can be computed ahead of time to reduce the computational load. In addition, some of the computed values of Rs(i) are used in other computations in the

[QCPA155I 114874/2 5 coding of the input signal, S(n), which further reduces the net computational burden of the encoding rate selection method of the present invention. For example, the derivation of LPC filter tap values requires the computation of a set of input signal autocorrelation coefficients.

The computation of LPC filter tap values is well known in the art and is detailed in the abovementioned U.S. Patent No. 5,414,796. If one were to code the speech with a method requiring a ten tap LPC filter only the values of Rs(i) for i values from 11 to L-l need to be computed, in addition to those that are used in the coding of the signal, because Rs(i) for i values from 0 to 10 are used in computing the LPC filter tap values. In the exemplary embodiment, the subband filters have 17 taps, L=17.

Subband energy computation element 4 provides the computed value of Rl(0) to subband rate decision element 12, and subband energy computation element 6 provides the computed value of Rh(0) to subband rate decision element 14. Rate decision element 12 compares the value of RL(0) against two predetermined threshold values Tli/2 and Tt.MI and assigns a suggested encoding rate, RATEl, in accordance with the comparison. The rate assignment is conducted as follows: RATEl = eighth rate Rl(0) - Tli/2 (4) RATEl= half rate Tli/2 < Rl(0) - TLfull (5) RATEL= full rate Rl(0) > TLfull (6)

Subband rate decision element 14 operates in a similar fashion and selects a suggest encoding rate, RATEh, in accordance with the high frequency energy value Rh(0) and based upon a different set of threshold values THl/2 and THfull· Subband rate decision element 12 provides its suggested encoding rate, RATEl, to encoding rate selection element 16, and subband rate decision element 14 provides its suggested encoding rate, RATEh, to encoding rate selection element 16. In the exemplary embodiment, encoding rate selection element 16 selects the higher of the two suggest rates and provides the higher rate as the selected ENCODING RATE.

Subband energy computation element 4 also provides the low frequency energy value, Rl(0), to threshold adaptation element 8, where the threshold values Tli/2 and TLfull for the next input frame are computed. Similarly, subband energy computation element 6 provides the high frequency energy value, Rh(0), to threshold adaptation element 10, where the threshold values Thi/2 and THfull for the next input frame are computed.

[QCrA155| 114874/2 6

Threshold adaptation element 8 receives the low frequency energy value, Rl(0)z and determines whether S(n) contains background noise or audio signal. In an exemplary implementation, the method by which threshold adaptation element 8 determines if an audio signal is present is by 5 examining the normalized autocorrelation function NACF, which is given by the equation:

NACF(n) = max T N-1 £e(n)e(n-T) n=0_ 'N-1 N-1 Σ e2(n)+ £ e2(n-T) ,n=0 n=0 (7) where T > 0 and e(n) is the formant residual signal that results from filtering the input signal, S{n), by an LPC filter.

The design of and filtering of a signal by an LPC filter is well known in the art and is detailed in aforementioned U.S. Patent No. 5,414,796. 15 The input signal, S(n), is filtered by the LPC filter to remove interaction of the formants. NACF is compared against a threshold value to determine if an audio signal is present. If NACF is greater than a predetermined threshold value, it indicates that the input frame has a periodic characteristic indicative of the presence of an audio signal such as speech or 20 music. Note that while parts of speech and music are not periodic and will exhibit low values of NACF, background noise typically never displays any periodicity and nearly always exhibits low values of NACF.

If it is determined that S(n) contains background noise, the value of NACF is less than a threshold value TH1, then the value Rl(0) is used to 25 update the value of the current background noise estimate BGNl. In the exemplary embodiment, TH1 is 0.35. Rl(0) is compared against the current value of background noise estimate BGNl. If Rl(0) is less than BGNl, then the background noise estimate BGNl is set equal to Rl(0) regardless of the value of NACF.

30 The background noise estimate BGNl is only increased when NACF

is less than threshold value TH1. If Rl(0) is greater than BGNl and NACF is less than TH1, then the background noise energy BGNl is set apBGNL/ where ai is a number greater than 1. In the exemplary embodiment, αι is equal to 1.03. BGNl will continue to increase as long as NACF is less than 35 threshold value TH1 and Rl(0) is greater than the current value of BGNl, |QCPA155| 7 until BGNl reaches a predetermined maximum value BGNmax at which point the background noise estimate BGNl is set to BGNmax·

If an audio signal is detected, signified by the value of NACF exceeding a second threshold value TH2, then the signal energy estimate, SL/ is updated. In the exemplary embodiment, ΊΓΗ2 is set to 0.5. The value of Rl(0) is compared against a current lowpass signal energy estimate, SL· If RL(0) is greater than the current value of SLz then Sl is set equal to Rl(0). If RL(0) is less than the current value of SLz then Sl is set equal to a2*SLz again only if NACF is greater than TH2. In the exemplary embodiment, a2 is set to 0.96.

Threshold adaptation element 8 then computes a signal to noise ratio estimate in accordance with equation 8 below: SNRl = 10 · log

SL

BGNL (8)

Threshold adaptation element 8 then determines an index of the quantized signal to noise ratio ISNRL in accordance with equation 9-12 below: ISNRL = Hint SNR^ 2°], for 20 &lt; SNRL &lt; 55, (9) = 0, for SNRL £ 20, (10) =7 for SNRl 2: 55. where nint is a function that rounds the fractional value to the nearest integer.

Threshold adaptation element 8, then selects or computes two scaling factors, kLl/2 and kLfulb in accordance with the signal to noise ratio index, ISNRL· An exemplary scaling value lookup table is provided in table 1 below: |QCPA155| 8 ISNRL TABLE 1 KLi/2 KLfull 0 7.0 9.0 1 7.0 12.6 2 8.0 17.0 3 8.6 18.5 4 8.9 19.4 5 9.4 20.9 6 11.0 25.5 7 15.8 39.8 These two values are used to compute the threshold values selection in accordance with the equations below: 5 TLl/2= Kli/2‘BGNl, and (11) TLfull= KLfull’BGNL, (12) where Tli/2 is low frequency half rate threshold value and 10 TLfull is the low frequency full rate threshold value.

Threshold adaptation element 8 provides the adapted threshold values TLI/2 and TLfullto rate decision element 12. Threshold adaptation element 10 operates in a similar fashion and provides the threshold values Thi/2 15 and THfull to subband rate decision element 14.

The initial value of the audio signal energy estimate S, where S can be

Sl or Sh, is set as follows. The initial signal energy estimate, SiNIT/ is set to -18.0 dBmO, where 3.17 dBmO denotes the signal strength of a full sine wave, which in the exemplary embodiment is a digital sine wave with an 20 amplitude range from -8031 to 8031. SiNIT is used until it is determined that an acoustic signal is present.

The method by which an acoustic signal is initially detected is to compare the NACF value against a threshold, when the NACF exceeds the threshold for a predetermined number consecutive frames, then an acoustic 25 signal is determined to be present. In the exemplary embodiment, NACF must exceed the threshold for ten consecutive frames. After this condition is met the signal energy estimate, S, is set to the maximum signal energy in the preceding ten frames.

The initial value of the background noise estimate BGN’l is initially 30 set to BGNmax· As soon as a subband frame energy is received that is less [QCPA155) 114874/2 9 than BGNmax, the background noise estimate is reset to the value of the received subband energy level, and generation of the background noise BGNl estimate proceeds as described earlier.

In a preferred embodiment a hangover condition is actuated when 5 following a series of full rate speech frames, a frame of a lower rate is detected. In the exemplary embodiment, when four consecutive speech frames are encoded at full rate followed by a frame where ENCODING RATE is set to a rate less than full rate and the computed signal to noise ratios are less than a predetermined minimum SNR, the 10 ENCODING RATE for that frame is set to full rate. In the exemplary embodiment the predetermined minimum SNR is 27.5 dBas defined in equation 8.

In the preferred embodiment, the number of hangover frames is a function of the signal to noise ratio. In the exemplary embodiment, the 15 number of hangover frames is determined as follows: #hangover frames = 1 22.5 < SNR < 27.5, (13) #hangover frames = 2 SNR £ 22.5, (14) #hangover frames = 0 SNR > 27.5. (15) 20

The present invention also provides a method with which to detect the presence of music, which as described before lacks the pauses which allow the background noise measures to reset. The method for detecting the presence of music assumes that music is not present at the start of the call. 25 This allows the encoding rate selection apparatus of the present invention to properly estimate an initial background noise energy, BGNinit· Because music unlike background noise has a periodic characteristic, the present invention examines the value of NACF to distinguish music from background noise. The music detection method of the present invention 30 computes an average NACF in accordance with the equation below:

! T NACFAVE=l£NACF(i), (16) i=l where NACF is defined in equation 7, and 35 where T is the number of consecutive frames in which the estimated value of the background noise has been increasing from an initial background noise estimate BGNixjit· IQCPA155| 10

If the background noise BGN has been increasing for the predetermined number of frames T and NACFaVE exceeds a predetermined threshold, then music is detected and the background noise 5 BGN is reset to BGNinit· It should be noted that to be effective the value T must be set low enough that the encoding rate doesn't drop below full rate. Therefore the value of T should be set as a function of the acoustic signal and BGNipjfc.

The previous description of the preferred embodiments is provided 10 to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the 15 embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. WE CLAIM: