JP4627548B2 - Bandwidth expansion device, bandwidth expansion method, and bandwidth expansion program - Google Patents

Description

  The present invention relates to a band extension technique for extending a frequency band of a digital signal such as an audio signal.

Many band expansion techniques for generating a high-quality wideband signal by expanding the frequency band of a digital signal such as a band-limited audio signal or compression-encoded signal have been proposed. Such a band expansion technique, for example, reproduces a high-quality audio signal by extending the frequency band of an audio signal in a narrow telephone band, or compresses a compressed code using a compression method such as MP3 (MPEG-1 Audio Layer 3). The present invention can be applied to the case where a signal having a high sound range is reproduced by expanding the band of the low quality audio signal.
A technique using nonlinear processing is known as an audio signal band extending technique. For example, in a band expansion technique using nonlinear processing disclosed in a patent document (International Publication No. WO00 / 70769), an oversampling low-pass filter performs filtering on an audio signal that is an original signal to obtain a sampling frequency fs. A component having a frequency of approximately ½ or more is attenuated. Next, the nonlinear processing circuit generates a harmonic component by performing nonlinear processing on the output of the oversampling low-pass filter, and outputs a digital audio signal including the harmonic component. Further, a high-pass filter filters the digital audio signal to generate a high frequency component, that is, an extended band component. This high frequency component is added to the output of the oversampling type low pass filter, thereby generating a signal with an expanded frequency band. However, since the non-linear processing circuit performs non-linear processing, it generates not only harmonic components but also multi-modulation components such as chord components and difference sound components of a plurality of frequency components. This is a component unrelated to the frequency component of the signal, and there is a problem that the signal quality is degraded.
The bandwidth extension technique that does not use non-linear processing, for example, non-patent document (H.Yasukawa, "Spectrum Broadening of Telephone Band Signals Using Multirate Processing for Speech Quality Enhancement," IEICE Transactions on Fundamentals Electronics, Communications and Computer Sciences, Vol. E78-A, No. 8, pp. 996-998, 1995.) is known. FIG. 1 is a diagram schematically showing a configuration of a band extending apparatus 100 disclosed in this non-patent document. The band extending apparatus 100 includes an upsampler 101, a high-pass filter (HPF) 102A, a shaping filter 103A, a level adjuster 104A, a low-pass filter (LPF) 102B, a delay unit 103B, a level adjuster 104B, and an adder 105. It consists of
The upsampler 101 upsamples the input signal at a double rate. For example, when upsampling is performed on an input signal having the frequency spectrum (ω: angular frequency) shown in FIG. 2A at a double rate, an upsampling signal having the frequency spectrum shown in FIG. 2B is generated. The low-pass filter 102B performs filtering on the upsampling signal supplied from the upsampler 101 to generate a baseband component. The baseband component is delayed by the delay unit 103B, level-adjusted by the level adjuster 104B, and then supplied to the adder 105. For example, when the low-pass filter 102B having a pass band of | ω | <π / 2 filters the upsampling signal shown in FIG. 2B, a baseband component having the spectrum shown in FIG. 2C is obtained. .
On the other hand, the high-pass filter 102A filters the upsampling signal supplied from the upsampler 101 to generate an extended band component. The extended band component is shaped by the shaping filter 103A, level-adjusted by the level adjuster 104A, and then supplied to the adder 105. The adder 105 adds the extension band component supplied from the level adjuster 104A to the baseband component supplied from the level adjuster 104B to generate a band extension signal. For example, when the high-pass filter 102A having a pass band of | ω |> π / 2 filters the upsampling signal shown in FIG. 2B, an extended band component having the spectrum shown in FIG. 2D is obtained. . When the extension band component and the baseband component of FIG. 2C are added, a band extension signal having the spectrum shown in FIG. 2E is generated.
However, as shown in FIG. 2E, the band extension signal includes an extension band component whose spectrum is inverted with respect to the spectrum of the baseband component, so that the baseband component and the extension band component have a frequency of ω = ± π / 2. There is a problem of not connecting smoothly. Further, since the spectrum of the extended band component is inverted with respect to the spectrum of the baseband component, the level adjusters 104A and 104B may adjust the levels of the baseband component and the extended band component to appropriate levels. There is also a problem that it is difficult. Therefore, it is difficult to generate a high-quality and natural sound quality band extension signal with the technology of the above-mentioned non-patent document.

In view of the above, an object of the present invention is to provide a band expansion apparatus, a band expansion method, and a band expansion program that can generate a high-quality band expansion signal without using nonlinear processing.
The band extending apparatus according to the first aspect of the present invention extends the frequency band of an input signal. The band extending apparatus performs baseband processing on the input signal, generates a baseband signal limited to a predetermined frequency band by filtering the upsampled signal, and the input signal. An extension band processing unit that generates an extension band component from the baseband signal, and a mixing unit that mixes the extension band component with the baseband signal. The extension band processing unit is a band obtained from the input signal or the input signal. A spectrum inversion unit that inverts the spectrum of the signal to generate an inverted signal, an upsampler that upsamples the inverted signal, and a high frequency band of the baseband signal by filtering the output of the upsampler The frequency band component having a frequency that is substantially the same as the end frequency at the low end is the extension band. Including a band-pass filter to generate a minute, the.
The band extending method according to the second aspect of the present invention extends the frequency band of the input signal. The band extension method includes (a) generating a baseband signal limited to a predetermined frequency band by upsampling the input signal and filtering the upsampled signal; and (b) Generating an extension band component from the input signal; and (c) mixing the extension band component with the baseband signal, wherein the step (b) includes (b-1) the input signal or Inverting the spectrum of the band signal obtained from the input signal to generate an inverted signal; (b-2) performing upsampling on the inverted signal; and (b-3) the step (b-2). By filtering the inverted signal upsampled in step 1, the frequency is substantially the same as the frequency at the high end of the frequency band of the baseband signal. And generating a component of a frequency band with the frequency as low-frequency end as the extended band component, a.
The bandwidth extension program according to the third aspect of the present invention causes a processor to execute bandwidth extension processing for extending the frequency bandwidth of an input signal. The band extension processing includes: baseband processing for performing upsampling on the input signal and generating a baseband signal limited to a predetermined frequency band by filtering the upsampled signal; and from the input signal An extension band process for generating an extension band component; and a mixing process for mixing the extension band component with the baseband signal, wherein the extension band process is a spectrum of the band signal obtained from the input signal or the input signal. The spectrum inversion processing for generating an inverted signal by inverting the up-sampling processing for up-sampling the inverted signal, and filtering the output of the up-sampler to the high-frequency end of the frequency band of the baseband signal The frequency band component having a low frequency end that is substantially the same frequency as the frequency is expanded. Comprising a filter to generate a band component, a.

FIG. 1 is a diagram schematically showing a configuration of a conventional bandwidth extension apparatus,
2A to 2E are diagrams schematically illustrating a frequency spectrum for explaining a conventional band extension method.
FIG. 3 is a functional block diagram schematically showing the configuration of the bandwidth extension apparatus according to the first embodiment of the present invention.
4A to 4C are diagrams schematically showing frequency spectra for explaining the band extending method of the first embodiment.
5A to 5E are diagrams schematically showing frequency spectra for explaining the band extending method of the first embodiment.
6A and 6B are diagrams schematically showing a frequency spectrum for explaining the band extension method of the first embodiment,
7A and 7B are diagrams showing the frequency spectrum of the band extension signal generated according to the band extension method of the first embodiment and the frequency spectrum of the original signal,
FIG. 8 is a diagram illustrating a frequency spectrum for explaining another example of the calculation method of the mixing coefficient,
FIG. 9 is a diagram for explaining still another example of the calculation method of the mixing coefficient,
FIG. 10 is a functional block diagram schematically showing the configuration of the bandwidth extension apparatus according to the second embodiment of the present invention.
FIG. 11A to FIG. 11D are diagrams schematically showing a frequency spectrum for explaining the band extending method of the second embodiment,
12A and 12B are diagrams schematically showing a frequency spectrum for explaining the band extending method of the second embodiment,
FIG. 13 is a functional block diagram schematically showing the configuration of the bandwidth extension apparatus according to the third embodiment of the present invention.
FIG. 14A to FIG. 14F are diagrams schematically showing frequency spectra for explaining the band extending method of the third embodiment,
FIG. 15 is a functional block diagram schematically showing the configuration of the bandwidth extension apparatus according to the fourth embodiment of the present invention.
FIG. 16 is a diagram schematically showing a configuration of a mixing unit used in the band extending apparatus of the fourth embodiment.
FIG. 17A to FIG. 17D are diagrams schematically showing frequency spectra for explaining the band extending method of the fourth embodiment,
FIG. 18A to FIG. 18G are diagrams schematically showing a frequency spectrum for explaining the band extending method of the fourth embodiment,
19A to 19C are diagrams showing various application examples.

This application is based on Japanese Patent Application No. 2005-260541 and is incorporated herein by reference.
Hereinafter, various embodiments according to the present invention will be described.

したがって、Ｘ_Ｄ（ｋ）をダウンサンプリング信号ｘ_Ｄ（ｎ）の離散フーリエ変換としたとき、次式（２）が成立する。

式（２）によれば、周波数領域（ｆｒｅｑｕｅｎｃｙ ｄｏｍａｉｎ）において、Ｚ_Ｄ（ｋ）は、Ｘ_Ｄ（ｋ）をＬ／２点すなわち角周波数πだけシフトさせたものである。したがって、「スペクトル反転」とは、信号の周波数スペクトルを角周波数πだけシフトさせることを意味する。
次に、図３を参照すると、アップサンプラ３４は、２×Ｎ倍のレートすなわち２×Ｎのファクタで反転信号ｚ_Ｄ（ｎ）の系列にアップサンプリングを施して、アップサンプリング信号ｚ_Ｕ（ｎ）の系列を生成する。さらに、帯域通過フィルタ３５は、２π／（２×Ｎ）＜｜ω｜＜３π／（２×Ｎ）の通過帯域を有し、アップサンプリング信号ｚ_Ｕ（ｎ）の系列にフィルタリングを施すことで拡張帯域成分ｚ_Ｂ（ｎ）の系列を生成する。この拡張帯域成分ｚ_Ｂ（ｎ）の帯域の低域端（低域側端部）の周波数がベースバンド信号ｘ_Ｌ（ｎ）の帯域の高域端（高域側端部）の周波数と略同一になるように、帯域通過フィルタ３５のフィルタ特性（通過帯域、阻止域および遮断周波数など）が構成されている。本実施例の場合、拡張帯域成分ｚ_Ｂ（ｎ）の低域端の周波数は略±２π／（２×Ｎ）であり、ベースバンド信号ｘ_Ｌ（ｎ）の高域端の周波数も±π／Ｎであるので、両周波数は略一致することとなる。
図５Ｃ〜図５ＥにＮが「２」である場合の周波数スペクトルを例示する。このとき、スペクトル反転部３３は、図５Ｂのダウンサンプリング信号ｘ_Ｄ（ｎ）のスペクトルを反転して、図５Ｃに示されるスペクトルを持つ反転信号ｚ_Ｄ（ｎ）を生成する。また、アップサンプラ３４は、４倍のレートすなわち４のファクタで反転信号ｚ_Ｄ（ｎ）にアップサンプリングを施して、図５Ｄに示されるスペクトルを持つアップサンプリング信号ｚ_Ｕ（ｎ）を生成する。そして、２π／４＜｜ω｜＜３π／４の通過帯域を有する帯域通過フィルタ３５は、アップサンプリング信号ｚ_Ｕ（ｎ）にフィルタリングを施して、図５Ｅに示されるスペクトルを持つ拡張帯域成分からなるフィルタ信号ｚ_Ｂ（ｎ）を生成することとなる。かかる場合、図４Ｃに示したベースバンド信号ｘ_Ｌ（ｎ）の帯域の高域端の周波数±π／２は、図５Ｅの拡張帯域成分ｚ_Ｂ（ｎ）の低域端の周波数±π／２と一致する。
上記のベースバンド信号ｘ_Ｌ（ｎ）と拡張帯域成分ｚ_Ｂ（ｎ）とは、混合部４０で混合される。係数決定部２０は、入力信号ｘ（ｎ）の周波数スペクトルを算出しこれに基づいて拡張帯域成分ｚ_Ｂ（ｎ）に乗ずるべき混合係数を決定する処理ブロックである。係数決定部２０は、ＦＦＴ部（高速フーリエ変換部）２１、スペクトル算出部２２および係数算出部２３を有している。ＦＦＴ部２１は、入力信号ｘ（ｎ）の系列を高速フーリエ変換して周波数領域の信号系列を算出する。また、スペクトル算出部２２は、ＦＦＴ部２１から供給された周波数領域の信号系列に基づいて入力信号ｘ（ｎ）の周波数スペクトルを算出する。係数算出部２３は、スペクトル算出部２２で算出された周波数スペクトルに基づいて、π／２〜πの周波数範囲で線形または非線形の回帰曲線を算出し、この回帰曲線を利用して混合係数を算出することができる。具体的には、回帰曲線として、１次関数で表現された線形の回帰直線を使用すればよい。
第１実施例の場合、入力信号ｘ（ｎ）から、図５Ａに示されるような周波数帯域（第２周波数帯域π／２〜π）の成分が抽出され、かかる成分が、ダウンサンプリング、スペクトル反転、アップサンプリングおよびフィルタリングを施されることで、図５Ｅに示されるような拡張帯域成分ｚ_Ｂ（ｎ）が生成される。よって、混合係数Ｃ（ｎ）は、低域端の周波数ω＝π／２またはその近傍の点での回帰曲線のスペクトル値Ｌ_Ａに対する、高域端の周波数ω＝πまたはその近傍の点での回帰曲線のスペクトル値Ｌ_Ｂの比率Ｌ_Ｂ／Ｌ_Ａで与えられる。
図６Ａは、入力信号ｘ（ｎ）の周波数スペクトルと回帰曲線Ｌ_Ｒとを例示する図である。スペクトル曲線Ｃ_Ｓに基づいてπ／２〜πの周波数範囲の回帰曲線Ｌ_Ｒが算出されている。この周波数スペクトルはデシベル（ｄＢ）で示されているので、ω＝π／２の点での振幅をＶＡ、ω＝πの点での振幅をＶＢで表すとき、混合係数Ｃ（ｎ）は、Ｃ（ｎ）＝Ｌ_Ｂ／Ｌ_Ａ、Ｌ_Ｂ＝１０^{（ＶＢ／２０）}、Ｌ_Ａ＝１０^{（ＶＡ／２０）}、で与えられる。
混合部４０は、乗算器４１と加算器４２とで構成されている。乗算器４１は、係数決定部２０から供給された混合係数Ｃ（ｎ）を拡張帯域成分ｚ_Ｂ（ｎ）に乗算し、加算器４２は、その乗算結果を、ベースバンド処理部１０から供給されたベースバンド信号に加算する。その加算結果として帯域拡張信号ｙ（ｎ）が生成される。図６Ｂに、Ｎが「２」である場合の帯域拡張信号ｙ（ｎ）の周波数スペクトルを示す。この帯域拡張信号ｙ（ｎ）は、図４Ｃのベースバンド信号ｘ_Ｌ（ｎ）に図５Ｅの拡張帯域成分ｚ_Ｂ（ｎ）を混合した信号である。
図７Ａは、サンプリング周波数ｆｓが１１０２５Ｈｚである場合の原信号の周波数スペクトル（横軸：Ｈｚ）の測定値を例示するグラフであり、図７Ｂは、第１実施例の帯域拡張装置１を用いてこの原信号の周波数帯域を拡張し、その結果得た帯域拡張信号の周波数スペクトルの測定値を例示するグラフである。図７Ｂによれば、ベースバンド信号と拡張帯域成分とが滑らかに接続していることが分かる。
上記の通り、第１実施例の帯域拡張装置１は、周波数帯域が制限された帯域信号ｘ_Ｈ（ｎ）をダウンサンプリングして得られる帯域信号ｘ_Ｄ（ｎ）のスペクトルを反転している。帯域拡張装置１は、さらに、その反転信号ｚ_Ｄ（ｎ）をアップサンプリングし、当該アップサンプリングされた信号ｚ_Ｕ（ｎ）にフィルタリングを施すことで、ベースバンド信号ｘ_Ｌ（ｎ）の周波数帯域の高域端の周波数π／Ｎと略同一の周波数を低域端とする拡張帯域の成分ｚ_Ｂ（ｎ）を生成しており、かかる拡張帯域成分ｚ_Ｂ（ｎ）がベースバンド信号ｘ_Ｌ（ｎ）に混合される。このような帯域拡張装置１は、帯域信号ｘ_Ｄ（ｎ）のスペクトルを反転しているので、スペクトル反転を行わない従来技術と比べると、拡張帯域成分ｚ_Ｂ（ｎ）とベースバンド信号ｘ_Ｌ（ｎ）との接続性が良好になり、出力信号ｙ（ｎ）の品質向上が可能になる。
さらに、帯域拡張装置１は、入力信号ｘ（ｎ）の周波数スペクトルに基づいて、拡張帯域成分ｚ_Ｂ（ｎ）に乗ずるべき混合係数Ｃ（ｎ）を算出しているので、拡張帯域成分をベースバンド信号に滑らかに接続することができ、高品質で自然な周波数スペクトルを持つ帯域拡張信号ｙ（ｎ）を生成することが可能になる。
次に、混合係数Ｃ（ｎ）の算出方法の他の例について説明する。この算出法では、低域端の周波数ω＝π／２を含む狭帯域Δω_Ａにおける複数点のスペクトル値の第１平均＜Ｌ_Ａ＞と、高域端の周波数ω＝πを含む狭帯域Δω_Ｂにおける複数点のスペクトル値の第２平均＜Ｌ_Ｂ＞とが算出され、次いで、これら第１平均＜Ｌ_Ａ＞と第２平均＜Ｌ_Ｂ＞との比率が混合係数Ｃ（ｎ）として算出される。図８は、算出方法を説明するための周波数スペクトルを例示するグラフである。かかる場合、狭帯域Δω_Ａにおけるスペクトル値の第１平均＜ＶＡ＞と、狭帯域Δω_Ｂにおけるスペクトル値の第２平均＜ＶＢ＞とが算出される。周波数スペクトルはデシベル（ｄＢ）で示されているので、混合係数Ｃ（ｎ）は、Ｃ（ｎ）＝＜Ｌ_Ｂ＞／＜Ｌ_Ａ＞、＜Ｌ_Ｂ＞＝１０^{（＜ＶＢ＞／２０）}、＜Ｌ_Ａ＞＝１０^{（＜ＶＡ＞／２０）}、で与えられる。
上記算出法を採用すれば、係数算出部２３は、周波数スペクトルの回帰曲線を必要としないので、少ない演算量で混合係数Ｃ（ｎ）を算出することができ、低消費電力化も実現できる。
また、混合係数Ｃ（ｎ）のさらに他の算出法を図９を参照しつつ説明する。図９は、図３に示した係数決定部２０に代わる係数決定部２０Ｄの構成を概略的に示す機能ブロック図である。この係数決定部２０Ｄは、帯域通過フィルタ（ＢＰＦ）２４Ａ，２４Ｂと、２乗和算出部２５Ａ，２５Ｂと、係数演算部２６とで構成されている。一方の帯域通過フィルタ２４Ａは、低域端の周波数ω＝π／２を含む所定の狭帯域のみを通過帯域として有し、他方の帯域通過フィルタ（ＢＰＦ）２４Ｂは、高域端の周波数ω＝πを含む所定の狭帯域のみを通過帯域して有している。２乗和算出部２５Ａ，２５Ｂは、それぞれ、帯域通過フィルタ２４Ａ，２４Ｂの出力の２乗和＜Ａ^２＞，＜Ｂ^２＞を所定のサンプル期間Ｔ毎に繰り返し算出する。たとえば、帯域通過フィルタ２４Ａの出力値の系列がＡ１，Ａ２，Ａ３，…であれば、２乗和＜Ａ^２＞は、（Ａ１）^２＋（Ａ２）^２＋（Ａ３）^２＋…となる。そして、係数演算部２６は、それら２乗和の比率の平方根値（＜Ｂ^２＞／＜Ａ^２＞）^{（１／２）}を混合係数Ｃ（ｎ）として算出することとなる。
上記係数決定部２０Ｄは、フーリエ変換と周波数スペクトルとを必要とせず、帯域通過フィルタ２４Ａ，２４Ｂは、必ずしもＦＩＲフィルタ（有限長インパルス応答フィルタ）で構成する必要はないのでタップ数の少ないＩＩＲフィルタ（無限長インパルス応答フィルタ）で構成することが可能である。したがって、係数決定部２０Ｄは、図３の係数決定部２０と比べて少ない演算量で混合係数Ｃ（ｎ）を算出することができ、低消費電力化を実現できる。
２．第２実施例
次に、本発明に係る第２実施例の帯域拡張装置および帯域拡張方法を説明する。図１０は、第２実施例の帯域拡張装置２の構成を概略的に示す機能ブロック図である。なお、図１０と図３とで同一符号を付された構成要素は、同一機能または同一構成を有するものとして、その詳細な説明を省略する。
図１０を参照すると、第２実施例の帯域拡張装置２は、ベースバンド処理部１０、係数決定部２０Ａおよび拡張帯域処理部３０Ａを有している。ベースバンド処理部１０は、上記の通り、入力信号ｘ（ｎ）の系列にアップサンプリングを施し、当該アップサンプリングされた信号系列にフィルタリングを施すことで所定の周波数帯域に制限されたベースバンド信号ｘ_Ｌ（ｎ）を生成する。
拡張帯域処理部３０Ａは、入力信号ｘ（ｎ）の系列から拡張帯域成分ｚ_Ｈ（ｎ）を生成する機能を有する。この拡張帯域処理部３０Ａは、スペクトル反転部３３、アップサンプラ３６および帯域通過フィルタ３７を有している。スペクトル反転部３３は、入力信号ｘ（ｎ）のスペクトルを反転して反転信号ｚ（ｎ）を生成する。アップサンプラ３６は、Ｎ倍のレートすなわちＮのファクタで反転信号ｚ（ｎ）の系列をアップサンプリングしてアップサンプリング信号ｚ_Ｕ（ｎ）の系列を生成する。帯域通過フィルタ３７は、π／Ｎ＜｜ω｜＜２π／Ｎの通過帯域を有し、アップサンプリング信号ｚ_Ｕ（ｎ）の系列にフィルタリングを施して拡張帯域成分であるフィルタ信号ｚ_Ｂ（ｎ）の系列を生成する。この拡張帯域成分ｚ_Ｂ（ｎ）は混合部４０に供給される。
図１１Ａ〜図１１Ｄに、Ｎが「２」である場合の周波数スペクトルを例示する。かかる場合、図１１Ａに示される入力信号ｘ（ｎ）が供給された場合、スペクトル反転部３３は、入力信号ｘ（ｎ）のスペクトルを反転して、図１１Ｂに示される周波数スペクトルを持つ反転信号ｚ（ｎ）を生成する。アップサンプラ３６は、２倍のレートすなわち２のファクタで反転信号ｚ（ｎ）の系列にアップサンプリングを施して、図１１Ｃに示されるスペクトルを持つアップサンプリング信号ｚ_Ｕ（ｎ）を生成する。また、帯域通過フィルタ３７は、π／２＜｜ω｜＜πの通過帯域を有し、図１１Ｄに示される周波数スペクトルを持つ拡張帯域成分ｚ_Ｂ（ｎ）を生成することとなる。
一方、係数決定部２０Ａは、ＦＦＴ部２１、スペクトル算出部２２および係数算出部２３Ａを有している。係数算出部２３Ａは、スペクトル算出部２２で算出された周波数スペクトルに基づいて、０〜πの周波数範囲で線形または非線形の回帰曲線を算出し、この回帰曲線を利用して混合係数を算出することができる。具体的には、回帰曲線として、１次関数で表現された線形の回帰直線を使用すればよい。
第２実施例の場合、入力信号ｘ（ｎ）の全周波数帯域の成分にアップサンプリングおよびフィルタリングを施すことで、図１１Ｄに示されるような拡張帯域成分ｚ_Ｂ（ｎ）が生成される。よって、混合係数Ｃ（ｎ）は、入力信号ｘ（ｎ）の周波数帯域の低域端の周波数ω＝０またはその近傍の点での回帰曲線のスペクトル値Ｌ_Ａに対する、高域端の周波数ω＝πまたはその近傍の点での回帰曲線のスペクトル値Ｌ_Ｂの比率Ｌ_Ｂ／Ｌ_Ａで与えられる。
図１２Ａは、入力信号ｘ（ｎ）の周波数スペクトルと回帰曲線Ｌ_Ｒとを例示する図である。スペクトル曲線Ｃ_Ｓのうち、０〜πの周波数範囲の回帰曲線Ｌ_Ｒが算出されている。この周波数スペクトルはデシベル（ｄＢ）で示されているので、ω＝０の点での振幅をＶＡ、ω＝πの点での振幅をＶＢで表すとき、混合係数Ｃ（ｎ）は、Ｃ（ｎ）＝Ｌ_Ｂ／Ｌ_Ａ、Ｌ_Ｂ＝１０^{（ＶＢ／２０）}、Ｌ_Ａ＝１０^{（ＶＡ／２０）}、で与えられる。
なお、上記係数決定部２０Ａの算出方法の代わりに、図８および図９でそれぞれ示した算出法を適用して混合係数Ｃ（ｎ）を算出してもよい。かかる場合、低域端の周波数をω＝０に設定し、高域端の周波数をω＝πに設定して、図８および図９で示した算出法を適用すればよい。
混合部４０では、乗算器４１は、係数決定部２０Ａから供給された混合係数Ｃ（ｎ）を拡張帯域成分ｚ_Ｂ（ｎ）に乗算し、加算器４２は、その乗算結果を、ベースバンド処理部１０から供給されたベースバンド信号に加算する。その加算結果として帯域拡張信号ｙ（ｎ）が生成される。図１２Ｂに、Ｎ＝２の場合の帯域拡張信号ｙ（ｎ）の周波数スペクトルを示す。この帯域拡張信号ｙ（ｎ）は、ベースバンド信号に図１１Ｄの拡張帯域成分ｚ_Ｂ（ｎ）が混合された信号である。
上記の通り、第２実施例の帯域拡張装置２は、入力信号ｘ（ｎ）のスペクトルを反転し、その反転信号ｚ（ｎ）をアップサンプリングし、当該アップサンプリングされた信号ｚ_Ｕ（ｎ）にフィルタリングを施すことで、ベースバンド信号ｘ_Ｌ（ｎ）の周波数帯域の高域端の周波数π／Ｎと略同一の周波数を低域端とする拡張帯域の成分ｚ_Ｂ（ｎ）を生成しており、かかる拡張帯域成分ｚ_Ｂ（ｎ）がベースバンド信号ｘ_Ｌ（ｎ）に混合される。このような帯域拡張装置２は、入力信号ｘ（ｎ）のスペクトルを反転しているので、スペクトル反転を行わない従来技術と比べると、拡張帯域成分ｚ_Ｂ（ｎ）とベースバンド信号ｘ_Ｌ（ｎ）との接続性が良好になり、出力信号ｙ（ｎ）の品質向上が可能になる。
さらに、帯域拡張装置２は、入力信号ｘ（ｎ）の周波数スペクトルに基づいて拡張帯域成分ｚ_Ｂ（ｎ）に乗ずるべき混合係数Ｃ（ｎ）を算出しているので、拡張帯域成分をベースバンド信号に滑らかに接続することができ、高品質で自然な周波数スペクトルを持つ帯域拡張信号ｙ（ｎ）を生成することが可能である。
３．第３実施例
次に、本発明に係る第３実施例の帯域拡張装置および帯域拡張方法を説明する。図１３は、第３実施例の帯域拡張装置３の構成を概略的に示す機能ブロック図である。なお、図１３と図３とで同一符号を付された構成要素は、同一機能または同一構成を有するものとして、その詳細な説明を省略する。
図１３を参照すると、第３実施例の帯域拡張装置３は、ベースバンド処理部１０、係数決定部２０Ｂ、第１拡張帯域処理部３０および第２拡張帯域処理部５０を有している。ベースバンド処理部１０は、上記の通り、入力信号ｘ（ｎ）の系列にアップサンプリングを施し、当該アップサンプリングされた信号系列にフィルタリングを施すことで所定の周波数帯域に制限されたベースバンド信号ｘ_Ｌ（ｎ）を生成するものである。第１拡張帯域処理部３０は、入力信号ｘ（ｎ）の系列から第１の拡張帯域成分ｚ_Ｂ（ｎ）を生成する機能を有する。
第２拡張帯域処理部５０は、アップサンプラ５１と帯域通過フィルタ（ＢＰＦ）５２とを有している。アップサンプラ５１は、第１拡張帯域処理部３０のダウンサンプラ３２から供給されたダウンサンプリング信号ｘ_Ｄ（ｎ）の系列に、２×Ｎ倍のレートすなわち２×Ｎのファクタでアップサンプリングを施してアップサンプリング信号ｘ_Ｕ２（ｎ）の系列を生成する。帯域通過フィルタ（ＢＰＦ）５２は、３π／（２×Ｎ）＜｜ω｜＜４π／（２×Ｎ）の通過帯域を有し、アップサンプリング信号ｘ_Ｕ２（ｎ）の系列にフィルタリングを施して第２の拡張帯域成分ｘ_Ｂ（ｎ）を生成する。
図１４Ａ〜図１４Ｅに、Ｎ＝２の場合の周波数スペクトルを例示する。図４Ａに示されるスペクトルを持つ入力信号ｘ（ｎ）が供給された場合、ベースバンド処理部１０は、図１４Ａに示されるスペクトルを持つベースバンド信号ｘ_Ｌ（ｎ）を出力し、第１拡張帯域処理部３０は、図１４Ｂに示されるスペクトルを持つ第１の拡張帯域成分ｚ_Ｂ（ｎ）を出力する。第１拡張帯域処理部３０のダウンサンプラ３２は、図１４Ｃに示されるスペクトルを持つダウンサンプリング信号ｘ_Ｄ（ｎ）を、第２拡張帯域処理部５０のアップサンプラ５１に供給する。よって、アップサンプラ５１は、図１４Ｄに示されるスペクトルを持つアップサンプリング信号ｘ_Ｕ２（ｎ）を生成する。帯域通過フィルタ５２は、３π／４＜｜ω｜＜πの通過帯域を有し、図１４Ｅに示されるスペクトルを持つ第２の拡張帯域成分ｘ_Ｂ（ｎ）を出力することとなる。
係数決定部２０Ｂは、図３の係数算出部２３と略同一の構成および機能を有し、第１混合係数Ｃ（ｎ）と、その２乗の値である第２混合係数Ｃ（ｎ）^２とを混合部４０Ｂに供給する。混合部４０Ｂにおいて、第１乗算器４１Ａは、第１の拡張帯域成分ｚ_Ｂ（ｎ）に第１混合係数Ｃ（ｎ）を乗算してその乗算結果を加算器４３に供給する一方、第２乗算器４１Ｂは、第２の拡張帯域成分ｘ_Ｂ（ｎ）に第２混合係数Ｃ（ｎ）^２を乗算してその乗算結果を加算器４３に供給する。加算器４３は、ベースバンド処理部１０から供給されたベースバンド信号にそれら乗算結果を加算することで帯域拡張信号ｙ（ｎ）を生成する。図１４Ｆに、Ｎ＝２の場合の帯域拡張信号ｙ（ｎ）の周波数スペクトルを示す。図１４Ｆの帯域拡張信号ｙ（ｎ）は、図１４Ａのベースバンド信号ｘ_Ｌ（ｎ）に、図１４Ｂの第１の拡張帯域成分ｚ_Ｂ（ｎ）と図１４Ｅの第２の拡張帯域成分ｘ_Ｂ（ｎ）とが混合された信号である。
上記の通り、第３実施例の帯域拡張装置３は、第１実施例の帯域拡張装置１（図３）の構成に加えて、第２の拡張帯域成分ｘ_Ｂ（ｎ）を生成する第２拡張帯域処理部５０を有するので、周波数帯域がさらに拡張された出力信号ｙ（ｎ）を得ることが可能になる。
４．第４実施例
次に、本発明に係る第４実施例の帯域拡張装置および帯域拡張方法を説明する。図１５は、第４実施例の帯域拡張装置４の構成を概略的に示す機能ブロック図である。なお、図１５と図３とで同一符号を付された構成要素は、同一機能または同一構成を有するものとして、その詳細な説明を省略する。
図１５を参照すると、第４実施例の帯域拡張装置４は、ベースバンド処理部１０、係数決定部２０Ｃ、第１拡張帯域処理部３０Ｃ、第２拡張帯域処理部５０Ｃおよび混合部４０Ｃを有している。図１６には混合部４０Ｃの構成が概略的に示されている。ベースバンド処理部１０は、上記の通り、入力信号ｘ（ｎ）の系列にアップサンプリングを施し、当該アップサンプリングされた信号系列にフィルタリングを施すことで所定の周波数帯域に制限されたベースバンド信号ｘ_Ｌ（ｎ）を生成するものである。
第１拡張帯域処理部３０Ｃは、入力信号ｘ（ｎ）の系列から、複数の第１サブ帯域成分ｚ_１，…，ｚ_Ｎ−１を拡張帯域成分として生成する機能を有する。第１拡張帯域処理部３０Ｃは、高域通過フィルタ（ＨＰＦ）３１、ダウンサンプラ３２、スペクトル反転部３３およびアップサンプラ３４を有し、さらに、各々が互いに異なる通過帯域を有する複数の帯域通過フィルタ（ＢＰＦ）３５_１，…，３５_Ｎ−１からなるフィルタバンクを有している。ｋ番目（ｋは１〜Ｎ−１の整数）の帯域通過フィルタ３５_ｋは、２ｋπ／（２Ｎ）＜｜ω｜＜（２ｋ＋１）π／（２Ｎ）の通過帯域を有している。これら帯域通過フィルタ３５_１，…，３５_Ｎ−１は、アップサンプラ３４からのアップサンプリング信号ｚ_Ｕ（ｎ）にフィルタリングを施して、周波数帯域が制限された複数の第１サブ帯域成分ｚ_１，…，ｚ_Ｎ−１を生成し乗算部４４に供給する。
一方、第２拡張帯域処理部５０Ｃは、アップサンプラ５１を有し、さらに、各々が互いに異なる通過帯域を有する複数の帯域通過フィルタ（ＢＰＦ）５２_１，…，５２_Ｎ−１からなるフィルタバンクを有している。ｋ番目（ｋは１〜Ｎ−１の整数）の帯域通過フィルタ５２_ｋは、（２ｋ＋１）π／（２Ｎ）＜｜ω｜＜（２ｋ＋２）π／（２Ｎ）の通過帯域を有している。アップサンプラ５１は、ダウンサンプラ３２からのダウンサンプリング信号ｘ_Ｄ（ｎ）の系列に、２×Ｎ倍のレートすなわち２×Ｎのファクタでアップサンプリングを施してアップサンプリング信号ｘ_Ｕ２（ｎ）の系列を生成する。帯域通過フィルタ５２_１，…，５２_Ｎ−１は、アップサンプリング信号ｘ_Ｕ２（ｎ）の系列にフィルタリングを施して、周波数帯域が制限された複数の第２サブ帯域成分ｘ_１，…，ｘ_Ｎ−１を生成し乗算部４４に供給する。
帯域通過フィルタ３５_１，…，３５_Ｎ−１のフィルタ特性と帯域通過フィルタ５２_１，…，５２_Ｎ−１のフィルタ特性とは、第１および第２サブ帯域成分ｚ_１，…，ｚ_Ｎ−１，ｘ_１，…，ｘ_Ｎ−１の周波数帯域が略連続するように構成される。帯域通過フィルタ３５_１，５２_１，３５_２，５２_２，…，３５_Ｎ−１および５２_Ｎ−１の通過帯域は連続している。これより、帯域拡張信号ｙ（ｎ）の周波数帯域が不連続にならず、滑らかに連続することとなる。
図１７Ａ〜図１７Ｄおよび図１８Ａ〜図１８Ｇに、Ｎ＝４の場合の周波数スペクトルを例示する。これら周波数スペクトルは、０〜πの周波数範囲でのみ示されているが、−π〜０の周波数範囲においても、それら周波数スペクトルとω＝０の点に関して鏡像関係にある周波数スペクトル（図示せず）が存在するものとする。図１７Ａに示されるようにスペクトルを持つ入力信号ｘ（ｎ）が供給された場合、ベースバンド処理部１０は、図１７Ｂに示されるように０〜π／４に帯域制限されたスペクトルを持つベースバンド信号ｘ_Ｌ（ｎ）を出力する。第１拡張帯域処理部３０Ｃにおいて、ダウンサンプラ３２は、図１７Ｃに示されるスペクトルを持つダウンサンプリング信号ｘ_Ｄ（ｎ）を出力し、スペクトル反転部３３は、図１７Ｄに示されるスペクトルを持つ反転信号ｚ_Ｄ（ｎ）を出力する。
かかる場合、帯域通過フィルタ３５_１，３５_２，３５_３は、それぞれ、図１８Ａ，図１８Ｂ，図１８Ｃに示されるスペクトルを持つ第１サブ帯域成分ｚ_１，ｚ_２，ｚ_３を出力する。他方、帯域通過フィルタ５２_１，５２_２，５２_３は、それぞれ、図１８Ｄ，図１８Ｅ，図１８Ｆに示されるスペクトルを持つ第２サブ帯域成分ｘ_１，ｘ_２，ｘ_３を出力する。図１８Ａ〜図１８Ｆに示される第１および第２サブ帯域成分ｚ_１，ｘ_１，ｚ_２，ｘ_２，ｚ_３，ｘ_３の周波数帯域は連続していることが分かる。
係数決定部２０Ｃは、図３の係数決定部２０が与える混合係数Ｃ（ｎ）と同じ混合係数Ｃを生成するとともに、Ｃ^２，Ｃ^３，…，Ｃ^２Ｎ−２の混合係数を算出する。これら混合係数のデータＣＤは混合部４０Ｃに供給される。
混合部４０Ｃは、図１６に例示されるように、第１サブ帯域成分用の乗算器４５_１，…，４５_Ｎ−１と、第２サブ帯域成分用の乗算器４６_１，…，４６_Ｎ−１とを有する。第１サブ帯域成分用の乗算器４５_１，４５_２，…，４５_Ｎ−１には、それぞれ、第１サブ帯域成分ｚ_１，ｚ_２，…，ｚ_Ｎ−１が入力し、且つ、これら乗算器４５_１，４５_２，…，４５_Ｎ−１には、それぞれ、乗算係数Ｃ，Ｃ^３，…，Ｃ^２Ｎ−３が与えられている。他方、第２サブ帯域成分用の乗算器４６_１，４６_２，…，４６_Ｎ−１には、それぞれ、第２サブ帯域成分ｘ_１，ｘ_２，…，ｘ_Ｎ−１が入力し、且つ、これら乗算器４６_１，４６_２，…，４６_Ｎ−１には、それぞれ、乗算係数Ｃ^２，Ｃ^４，…，Ｃ^２Ｎ−２が与えられている。加算器４３は、乗算器４５_１，…，４５_Ｎ−１，４６_１，…，４６_Ｎ−１の全出力をベースバンド信号に加算して帯域拡張信号ｙ（ｎ）を生成する。
結果として、図１８Ｇに例示されるように、帯域拡張信号ｙ（ｎ）の周波数スペクトルにおいて、第１サブ帯域成分と第２サブ帯域成分とが交互に配列した拡張帯域成分が形成されることとなる。
上記の通り、第４実施例の帯域拡張装置４は、第１拡張帯域処理部３０Ｃおよび第２拡張帯域処理部５０Ｃがそれぞれフィルタバンク３５_１〜３５_Ｎ−１および５２_１，…，５２_Ｎ−１を有するので、高品質で自然な周波数スペクトルを持つ帯域拡張信号ｙ（ｎ）を生成することが可能である。
５．適用例
上記第１〜第４実施例の帯域拡張装置１〜４を種々の機器に適用することができる。図１９Ａ〜図１９Ｃは、帯域拡張装置１の適用例を概略的に示す図である。図１９Ａを参照すると、帯域拡張装置１（または帯域拡張装置２〜４）は、音声信号処理部６０とＤ／Ａ変換器（ＤＡＣ）６１との間に接続されている。音声信号処理部６０は、たとえば、ＰＣＭ符号化などの音声信号処理を行うブロックである。帯域拡張装置１は、音声信号処理部６０から供給された信号の周波数帯域を拡張し、その帯域拡張信号をＤ／Ａ変換器６１に供給することができる。
図１９Ｂの例では、帯域拡張装置１（または帯域拡張装置２〜４）は、携帯電話機や光ディスク再生装置などに組み込まれたデコーダ６２と、Ｄ／Ａ変換器（ＤＡＣ）６３との間に接続されている。帯域拡張装置１は、デコーダ６２から供給された復号化信号の周波数帯域を拡張し、その帯域拡張信号をＤ／Ａ変換器６３に供給することができる。
図１９Ｃの例では、帯域拡張装置１（または帯域拡張装置２〜４）は、ＡＭチューナやＦＭチューナなどのチューナ６４と、Ｄ／Ａ変換器（ＤＡＣ）６７との間に接続されている。チューナ６４の出力は、低域通過フィルタ（ＬＰＦ）６５でフィルタリングを施され、Ａ／Ｄ変換器（ＡＤＣ）６６でＡ／Ｄ変換される。帯域拡張装置１は、Ａ／Ｄ変換器６６のデジタル出力の周波数帯域を拡張し、その帯域拡張信号をＤ／Ａ変換器６７に供給する。
また、上記の第１〜第４実施例の帯域拡張装置１〜４の全構成または一部構成をハードウェアで実現してもよいし、或いは、ＣＰＵなどのプロセッサで実行されるプログラムで実現してもよい。1. First embodiment
FIG. 3 is a functional block diagram schematically showing the configuration of the bandwidth extension device 1 according to the first embodiment of the present invention. The band extension device 1 includes a baseband processing unit 10, a coefficient determination unit 20, an extension band processing unit 30, and a mixing unit 40. The baseband processing unit 10 performs upsampling on a sequence of input signals x (n) (n is an integer), and baseband signal limited to a predetermined frequency band by filtering the upsampled signal sequence. x _L (N) is generated. The input signal x (n) is a digital signal such as an audio signal.
The baseband processing unit 10 includes an upsampler 11, a low-pass filter (LPF) 12, and a delay device 13. The upsampler 11 performs upsampling on the sequence of the input signal x (n) at a rate N times (N is an integer equal to or greater than 2), that is, a factor of N, and performs an upsampling signal x _U The sequence (n) is generated. The low-pass filter 12 has a pass band of | ω | <π / N (ω is an angular frequency), and the upsampling signal x _U Filter signal x which is filtered to the frequency band of −π / N to + π / N by filtering the sequence (n) _L The sequence (n) is generated. The delay unit 13 outputs the filter signal x for a predetermined delay time. _L (N) is delayed and the delayed signal is output as a baseband signal. FIG. 4A to FIG. 4C illustrate frequency spectra when N is “2”. When the input signal x (n) shown in FIG. 4A is supplied, the upsampler 11 up-samples the input signal x (n) at a double rate, that is, a factor of 2 to obtain the frequency shown in FIG. 4B. Upsampling signal x with spectrum _U (N) is generated. The low-pass filter 12 generates the upsampling signal x in FIG. 4B. _U Filter signal x which is filtered to (n) and limited to a frequency band of −π / 2 to + π / 2 _L (N) will be generated.
On the other hand, the extension band processing unit 30 calculates the extension band component z from the sequence of the input signal x (n). _B (N) has a function of generating. The extended band processing unit 30 includes a high pass filter (HPF) 31, a down sampler 32, a spectrum inversion unit 33, an up sampler 34, and a band pass filter (BPF) 35. The high-pass filter 31 has a pass band (second frequency band) of | ω |> π / 2, and performs filtering on the sequence of the input signal x (n) to filter signal x _H The sequence (n) is generated. The downsampler 32 then passes the filter signal x at a rate of 1/2, or a factor of two. _H The downsampling signal, that is, the band signal x is applied to the series (n) by downsampling. _D The sequence (n) is generated. 5A to 5B illustrate frequency spectra. When the input signal x (n) shown in FIG. 4A is supplied, the high-pass filter 31 having a pass band of | ω |> π / 2 is used as the filter signal x having the frequency spectrum shown in FIG. 5A. _H (N) is generated. The downsampler 32 also outputs the filter signal x at a rate of ½, that is, a factor of 2. _H The downsampling signal x having the frequency spectrum shown in FIG. _D (N) will be generated.
The spectrum reversing unit 33 receives the downsampling signal x _D Invert the signal z by inverting the spectrum of (n) _D (N) is generated. As will be described below using mathematical expressions, “spectrum inversion” means that the frequency spectrum of a signal is shifted by an angular frequency π. Downsampling signal x _D A signal obtained by inverting every other polarity of (n) is an inverted signal z. _D Since (n), the downsampling signal x _D (N) and inverted signal z _D The relationship between (n) is expressed by the following equation (1).
z _D (N) = x _D (N) x (-1) ⁿ = X _D (N) exp (-jnπ). (1)
Where j is an imaginary unit (j ² = −1), and L is the number of samples of Fourier transform and is a positive integer. Therefore, the inverted signal z _D Discrete Fourier transform Z of (n) _D (K) is expanded as follows.

Therefore, X _D (K) downsampling signal x _D When the discrete Fourier transform of (n) is used, the following equation (2) is established.

According to Equation (2), in the frequency domain, Z _D (K) is X _D (K) is shifted by L / 2 point, that is, the angular frequency π. Therefore, “spectrum inversion” means that the frequency spectrum of the signal is shifted by the angular frequency π.
Referring now to FIG. 3, the upsampler 34 receives the inverted signal z at a rate of 2 × N, that is, a factor of 2 × N. _D The upsampling signal z is obtained by upsampling the series (n). _U The sequence (n) is generated. Further, the bandpass filter 35 has a passband of 2π / (2 × N) <| ω | <3π / (2 × N), and the upsampling signal z _U By applying filtering to the sequence (n), the extended band component z _B The sequence (n) is generated. This extended band component z _B The frequency at the lower end (lower end) of the band (n) is the baseband signal x _L The filter characteristics (pass band, stop band, cutoff frequency, etc.) of the band pass filter 35 are configured so as to be substantially the same as the frequency of the high band end (high band side end) of the band (n). In this embodiment, the extended band component z _B The frequency at the lower end of (n) is approximately ± 2π / (2 × N), and the baseband signal x _L Since the frequency at the high end of (n) is also ± π / N, both frequencies are substantially the same.
FIG. 5C to FIG. 5E illustrate frequency spectra when N is “2”. At this time, the spectrum inversion unit 33 performs the downsampling signal x in FIG. 5B. _D Inverted signal z having the spectrum shown in FIG. 5C by inverting the spectrum of (n) _D (N) is generated. The upsampler 34 also has an inverted signal z at a quadruple rate, that is, a factor of 4. _D Upsampling signal z having the spectrum shown in FIG. _U (N) is generated. The bandpass filter 35 having a passband of 2π / 4 <| ω | <3π / 4 is connected to the upsampling signal z. _U (N) is subjected to filtering, and a filter signal z consisting of an extended band component having the spectrum shown in FIG. 5E _B (N) will be generated. In such a case, the baseband signal x shown in FIG. _L The frequency ± π / 2 at the high end of the band (n) is the extension band component z in FIG. 5E. _B This coincides with the frequency ± π / 2 at the lower end of (n).
The above baseband signal x _L (N) and extended band component z _B (N) is mixed in the mixing unit 40. The coefficient determination unit 20 calculates the frequency spectrum of the input signal x (n), and based on this, the expansion band component z _B It is a processing block for determining a mixing coefficient to be multiplied by (n). The coefficient determination unit 20 includes an FFT unit (fast Fourier transform unit) 21, a spectrum calculation unit 22, and a coefficient calculation unit 23. The FFT unit 21 performs a fast Fourier transform on the input signal x (n) sequence to calculate a frequency domain signal sequence. The spectrum calculation unit 22 calculates the frequency spectrum of the input signal x (n) based on the frequency domain signal sequence supplied from the FFT unit 21. The coefficient calculation unit 23 calculates a linear or non-linear regression curve in a frequency range of π / 2 to π based on the frequency spectrum calculated by the spectrum calculation unit 22, and calculates a mixing coefficient using the regression curve. can do. Specifically, a linear regression line expressed by a linear function may be used as the regression curve.
In the case of the first embodiment, components of the frequency band (second frequency band π / 2 to π) as shown in FIG. 5A are extracted from the input signal x (n), and these components are down-sampled and spectrum inverted. Is subjected to upsampling and filtering, so that the extended band component z as shown in FIG. _B (N) is generated. Therefore, the mixing coefficient C (n) is the spectral value L of the regression curve at the low-frequency end frequency ω = π / 2 or a point in the vicinity thereof. _A Is the spectral value L of the regression curve at the frequency ω = π at the high end or near the point _B Ratio L _B / L _A Given in.
FIG. 6A shows the frequency spectrum of the input signal x (n) and the regression curve L. _R FIG. Spectral curve C _S Regression curve L in the frequency range of π / 2 to π based on _R Is calculated. Since this frequency spectrum is expressed in decibels (dB), when the amplitude at the point of ω = π / 2 is expressed as VA and the amplitude at the point of ω = π is expressed as VB, the mixing coefficient C (n) is C (n) = L _B / L _A , L _B = 10 ^{(VB / 20)} , L _A = 10 ^{(VA / 20)} , Given in.
The mixing unit 40 includes a multiplier 41 and an adder 42. The multiplier 41 converts the mixing coefficient C (n) supplied from the coefficient determination unit 20 into the extension band component z. _B Multiplying (n), the adder 42 adds the multiplication result to the baseband signal supplied from the baseband processing unit 10. As a result of the addition, a band extension signal y (n) is generated. FIG. 6B shows the frequency spectrum of the band extension signal y (n) when N is “2”. This band extension signal y (n) is the baseband signal x in FIG. 4C. _L (N) shows the extended band component z in FIG. 5E. _B (N) is a mixed signal.
FIG. 7A is a graph illustrating the measured value of the frequency spectrum (horizontal axis: Hz) of the original signal when the sampling frequency fs is 11025 Hz, and FIG. 7B uses the band extension device 1 of the first embodiment. It is a graph which illustrates the measured value of the frequency spectrum of the band extension signal obtained by extending the frequency band of the original signal. FIG. 7B shows that the baseband signal and the extended band component are smoothly connected.
As described above, the band extending apparatus 1 according to the first embodiment uses the band signal x whose frequency band is limited. _H Band signal x obtained by down-sampling (n) _D The spectrum of (n) is inverted. The band extension device 1 further includes the inverted signal z _D (N) is upsampled, and the upsampled signal z _U By applying filtering to (n), the baseband signal x _L The component z of the extension band having the low frequency end substantially the same as the frequency π / N at the high frequency end of the frequency band (n) _B (N), and such an extended band component z _B (N) is the baseband signal x _L (N) is mixed. Such a band extension device 1 is configured to transmit the band signal x _D Since the spectrum of (n) is inverted, compared with the prior art that does not perform spectrum inversion, the extended band component z _B (N) and baseband signal x _L The connectivity with (n) becomes good, and the quality of the output signal y (n) can be improved.
Further, the band extension device 1 determines the extension band component z based on the frequency spectrum of the input signal x (n). _B Since the mixing coefficient C (n) to be multiplied by (n) is calculated, the extension band component can be smoothly connected to the baseband signal, and the band extension signal y (n having a high-quality and natural frequency spectrum can be obtained. ) Can be generated.
Next, another example of the method for calculating the mixing coefficient C (n) will be described. In this calculation method, a narrow band Δω including the low-frequency end frequency ω = π / 2. _A First average of spectral values of multiple points at <L _A > And a narrow band Δω including the frequency ω = π at the high end. _B Second average of spectral values at multiple points in <L _B > And then these first averages <L _A > And second average <L _B > Is calculated as the mixing coefficient C (n). FIG. 8 is a graph illustrating a frequency spectrum for explaining the calculation method. In such a case, the narrow band Δω _A The first average <VA> of spectral values at and the narrow band Δω _B A second average <VB> of the spectrum values at is calculated. Since the frequency spectrum is shown in decibels (dB), the mixing coefficient C (n) is C (n) = <L _B >> / <L _A >, <L _B > = 10 ^{(<VB> / 20)} , <L _A > = 10 ^{(<VA> / 20)} , Given in.
If the above calculation method is employed, the coefficient calculation unit 23 does not need a frequency curve regression curve, so the mixing coefficient C (n) can be calculated with a small amount of calculation, and low power consumption can also be realized.
Further, still another method for calculating the mixing coefficient C (n) will be described with reference to FIG. FIG. 9 is a functional block diagram schematically showing the configuration of a coefficient determination unit 20D that replaces the coefficient determination unit 20 shown in FIG. The coefficient determination unit 20D includes band pass filters (BPF) 24A and 24B, square sum calculation units 25A and 25B, and a coefficient calculation unit 26. One band-pass filter 24A has only a predetermined narrow band including a low-frequency end frequency ω = π / 2 as a pass band, and the other band-pass filter (BPF) 24B has a high-frequency end frequency ω = It has only a predetermined narrow band including π as a pass band. The sum of squares calculation units 25A and 25B respectively calculate the sum of squares of the outputs of the bandpass filters 24A and 24B <A ² >, <B ² > Is repeatedly calculated every predetermined sample period T. For example, if the output value series of the band pass filter 24A is A1, A2, A3,. ² > (A1) ² + (A2) ² + (A3) ² + ... Then, the coefficient calculation unit 26 calculates the square root value (<B ² >> / <A ² >) ^(1/2) Is calculated as a mixing coefficient C (n).
The coefficient determination unit 20D does not require Fourier transform and frequency spectrum, and the band-pass filters 24A and 24B do not necessarily have to be configured with FIR filters (finite-length impulse response filters). Infinite length impulse response filter). Therefore, the coefficient determination unit 20D can calculate the mixing coefficient C (n) with a smaller amount of calculation than the coefficient determination unit 20 of FIG. 3, and can realize low power consumption.
2. Second embodiment
Next, a band extending apparatus and a band extending method according to the second embodiment of the present invention will be described. FIG. 10 is a functional block diagram schematically showing the configuration of the band extending apparatus 2 of the second embodiment. It should be noted that the components denoted by the same reference numerals in FIG. 10 and FIG. 3 have the same function or the same configuration, and detailed description thereof is omitted.
Referring to FIG. 10, the band extending apparatus 2 of the second embodiment includes a baseband processing unit 10, a coefficient determining unit 20A, and an extended band processing unit 30A. As described above, the baseband processing unit 10 performs upsampling on the sequence of the input signal x (n) and performs filtering on the upsampled signal sequence to thereby limit the baseband signal x limited to a predetermined frequency band. _L (N) is generated.
The extension band processing unit 30A extracts the extension band component z from the sequence of the input signal x (n). _H (N) has a function of generating. The extended band processing unit 30A includes a spectrum inversion unit 33, an upsampler 36, and a band pass filter 37. The spectrum inversion unit 33 inverts the spectrum of the input signal x (n) to generate an inversion signal z (n). The upsampler 36 upsamples the sequence of the inverted signal z (n) at a rate N times, that is, a factor of N, to upsample the signal z _U The sequence (n) is generated. The bandpass filter 37 has a passband of π / N <| ω | <2π / N, and the upsampling signal z _U Filter signal z which is an extension band component by filtering the sequence of (n) _B The sequence (n) is generated. This extended band component z _B (N) is supplied to the mixing unit 40.
FIG. 11A to FIG. 11D illustrate frequency spectra when N is “2”. In such a case, when the input signal x (n) shown in FIG. 11A is supplied, the spectrum inversion unit 33 inverts the spectrum of the input signal x (n) and has the frequency spectrum shown in FIG. 11B. z (n) is generated. The upsampler 36 upsamples the series of inverted signals z (n) at a double rate or factor of 2 to produce an upsampled signal z having the spectrum shown in FIG. 11C. _U (N) is generated. The bandpass filter 37 has a passband of π / 2 <| ω | <π, and an extended band component z having a frequency spectrum shown in FIG. 11D. _B (N) will be generated.
On the other hand, the coefficient determination unit 20A includes an FFT unit 21, a spectrum calculation unit 22, and a coefficient calculation unit 23A. The coefficient calculation unit 23A calculates a linear or non-linear regression curve in a frequency range of 0 to π based on the frequency spectrum calculated by the spectrum calculation unit 22, and calculates a mixing coefficient using the regression curve. Can do. Specifically, a linear regression line expressed by a linear function may be used as the regression curve.
In the case of the second embodiment, by performing upsampling and filtering on the components of the entire frequency band of the input signal x (n), the extended band component z as shown in FIG. 11D. _B (N) is generated. Therefore, the mixing coefficient C (n) is the spectral value L of the regression curve at a frequency ω = 0 at the lower end of the frequency band of the input signal x (n) or a point in the vicinity thereof. _A Is the spectral value L of the regression curve at the frequency ω = π at the high end or near the point _B Ratio L _B / L _A Given in.
FIG. 12A shows the frequency spectrum of the input signal x (n) and the regression curve L. _R FIG. Spectral curve C _S Among them, the regression curve L in the frequency range of 0 to π _R Is calculated. Since this frequency spectrum is expressed in decibels (dB), when the amplitude at the point of ω = 0 is expressed as VA and the amplitude at the point of ω = π is expressed as VB, the mixing coefficient C (n) is expressed as C ( n) = L _B / L _A , L _B = 10 ^{(VB / 20)} , L _A = 10 ^{(VA / 20)} , Given in.
Note that the mixing coefficient C (n) may be calculated by applying the calculation methods shown in FIGS. 8 and 9 instead of the calculation method of the coefficient determination unit 20A. In such a case, the calculation method shown in FIGS. 8 and 9 may be applied by setting the frequency at the low band end to ω = 0 and the frequency at the high band end to ω = π.
In the mixing unit 40, the multiplier 41 converts the mixing coefficient C (n) supplied from the coefficient determination unit 20A into the extension band component z. _B Multiplying (n), the adder 42 adds the multiplication result to the baseband signal supplied from the baseband processing unit 10. As a result of the addition, a band extension signal y (n) is generated. FIG. 12B shows the frequency spectrum of the band extension signal y (n) when N = 2. This band extension signal y (n) is added to the baseband signal as an extension band component z in FIG. 11D. _B (N) is a mixed signal.
As described above, the band extending apparatus 2 of the second embodiment inverts the spectrum of the input signal x (n), up-samples the inverted signal z (n), and the up-sampled signal z _U By applying filtering to (n), the baseband signal x _L The component z of the extension band having the low frequency end substantially the same as the frequency π / N at the high frequency end of the frequency band (n) _B (N), and such an extended band component z _B (N) is the baseband signal x _L (N) is mixed. Since such a band extending apparatus 2 inverts the spectrum of the input signal x (n), compared to the conventional technique that does not perform spectrum inversion, the expanded band component z _B (N) and baseband signal x _L The connectivity with (n) becomes good, and the quality of the output signal y (n) can be improved.
Further, the band extension device 2 generates an extension band component z based on the frequency spectrum of the input signal x (n). _B Since the mixing coefficient C (n) to be multiplied by (n) is calculated, the extension band component can be smoothly connected to the baseband signal, and the band extension signal y (n having a high-quality and natural frequency spectrum can be obtained. ) Can be generated.
3. Third embodiment
Next, a band extending apparatus and a band extending method according to a third embodiment of the present invention will be described. FIG. 13 is a functional block diagram schematically showing the configuration of the band extending apparatus 3 of the third embodiment. It should be noted that the components denoted by the same reference numerals in FIGS. 13 and 3 have the same function or the same configuration, and detailed description thereof is omitted.
Referring to FIG. 13, the bandwidth extension device 3 of the third embodiment includes a baseband processing unit 10, a coefficient determination unit 20 </ b> B, a first extension band processing unit 30, and a second extension band processing unit 50. As described above, the baseband processing unit 10 performs upsampling on the sequence of the input signal x (n) and performs filtering on the upsampled signal sequence to thereby limit the baseband signal x limited to a predetermined frequency band. _L (N) is generated. The first extension band processing unit 30 calculates the first extension band component z from the sequence of the input signal x (n). _B (N) has a function of generating.
The second extension band processing unit 50 includes an up sampler 51 and a band pass filter (BPF) 52. The upsampler 51 includes a downsampling signal x supplied from the downsampler 32 of the first extension band processing unit 30. _D The upsampling signal x is obtained by upsampling the sequence (n) at a rate of 2 × N, that is, a factor of 2 × N. _U2 The sequence (n) is generated. The band pass filter (BPF) 52 has a pass band of 3π / (2 × N) <| ω | <4π / (2 × N), and the upsampling signal x _U2 The second extension band component x is obtained by filtering the sequence (n). _B (N) is generated.
14A to 14E illustrate frequency spectra in the case of N = 2. When the input signal x (n) having the spectrum shown in FIG. 4A is supplied, the baseband processing unit 10 performs the baseband signal x having the spectrum shown in FIG. 14A. _L (N) is output, and the first extension band processing unit 30 outputs the first extension band component z having the spectrum shown in FIG. 14B. _B (N) is output. The downsampler 32 of the first extension band processing unit 30 includes a downsampling signal x having the spectrum shown in FIG. 14C. _D (N) is supplied to the upsampler 51 of the second extension band processing unit 50. Therefore, the upsampler 51 has the upsampling signal x having the spectrum shown in FIG. 14D. _U2 (N) is generated. The band pass filter 52 has a pass band of 3π / 4 <| ω | <π, and has a second extension band component x having a spectrum shown in FIG. 14E. _B (N) is output.
The coefficient determination unit 20B has substantially the same configuration and function as the coefficient calculation unit 23 of FIG. 3, and includes a first mixing coefficient C (n) and a second mixing coefficient C (n) that is a square value thereof. ² Are supplied to the mixing unit 40B. In the mixing unit 40B, the first multiplier 41A includes a first extension band component z _B (N) is multiplied by the first mixing coefficient C (n) and the result of the multiplication is supplied to the adder 43, while the second multiplier 41B includes the second extension band component x. _B (N) to the second mixing coefficient C (n) ² And the multiplication result is supplied to the adder 43. The adder 43 adds the multiplication results to the baseband signal supplied from the baseband processing unit 10 to generate the band extension signal y (n). FIG. 14F shows the frequency spectrum of the band extension signal y (n) when N = 2. The band extension signal y (n) in FIG. 14F is the baseband signal x in FIG. 14A. _L (N) includes the first extension band component z in FIG. 14B. _B (N) and the second extension band component x of FIG. 14E _B (N) is a mixed signal.
As described above, the band extending apparatus 3 of the third embodiment has the second extended band component x in addition to the configuration of the band extending apparatus 1 (FIG. 3) of the first embodiment. _B Since the second extension band processing unit 50 that generates (n) is included, it is possible to obtain an output signal y (n) whose frequency band is further extended.
4). Fourth embodiment
Next, a band extending apparatus and a band extending method according to the fourth embodiment of the present invention will be described. FIG. 15 is a functional block diagram schematically showing the configuration of the band extending device 4 of the fourth embodiment. It should be noted that the components denoted by the same reference numerals in FIG. 15 and FIG. 3 have the same function or the same configuration and will not be described in detail.
Referring to FIG. 15, the bandwidth expansion device 4 of the fourth embodiment includes a baseband processing unit 10, a coefficient determination unit 20C, a first expansion band processing unit 30C, a second expansion band processing unit 50C, and a mixing unit 40C. ing. FIG. 16 schematically shows the configuration of the mixing unit 40C. As described above, the baseband processing unit 10 performs upsampling on the sequence of the input signal x (n) and performs filtering on the upsampled signal sequence to thereby limit the baseband signal x limited to a predetermined frequency band. _L (N) is generated.
The first extension band processing unit 30C receives a plurality of first subband components z from the sequence of input signals x (n). ₁ , ..., z _N-1 Is generated as an extended band component. The first extended band processing unit 30C includes a high pass filter (HPF) 31, a down sampler 32, a spectrum inversion unit 33, and an up sampler 34, and a plurality of band pass filters (each having a different pass band). BPF) 35 ₁ , ..., 35 _N-1 A filter bank consisting of The kth (k is an integer from 1 to N-1) bandpass filter 35 _k Has a passband of 2kπ / (2N) <| ω | <(2k + 1) π / (2N). These band pass filters 35 ₁ , ..., 35 _N-1 Is the upsampling signal z from the upsampler 34 _U A plurality of first subband components z whose frequency band is limited by filtering (n) ₁ , ..., z _N-1 Is generated and supplied to the multiplication unit 44.
On the other hand, the second extension band processing unit 50C includes an upsampler 51, and a plurality of band pass filters (BPF) 52 each having a different pass band. ₁ , ..., 52 _N-1 A filter bank consisting of k-th band-pass filter 52 (k is an integer of 1 to N−1) _k Has a passband of (2k + 1) π / (2N) <| ω | <(2k + 2) π / (2N). The upsampler 51 receives the downsampling signal x from the downsampler 32. _D The upsampling signal x is obtained by upsampling the sequence (n) at a rate of 2 × N, that is, a factor of 2 × N. _U2 The sequence (n) is generated. Band pass filter 52 ₁ , ..., 52 _N-1 Is the upsampling signal x _U2 A plurality of second subband components x whose frequency band is limited by filtering the sequence (n) ₁ , ..., x _N-1 Is generated and supplied to the multiplication unit 44.
Band pass filter 35 ₁ , ..., 35 _N-1 Filter characteristics and bandpass filter 52 ₁ , ..., 52 _N-1 Filter characteristics of the first and second subband components z ₁ , ..., z _N-1 , X ₁ , ..., x _N-1 Are configured to be substantially continuous. Band pass filter 35 ₁ , 52 ₁ , 35 ₂ , 52 ₂ , ..., 35 _N-1 And 52 _N-1 The passband is continuous. As a result, the frequency band of the band extension signal y (n) is not discontinuous and is continuously continuous.
17A to 17D and FIGS. 18A to 18G illustrate frequency spectra in the case of N = 4. These frequency spectra are shown only in the frequency range of 0 to π, but also in the frequency range of -π to 0, the frequency spectrum (not shown) that has a mirror image relationship with respect to the frequency spectrum and the point of ω = 0. Shall exist. When an input signal x (n) having a spectrum is supplied as shown in FIG. 17A, the baseband processing unit 10 has a base having a spectrum whose band is limited to 0 to π / 4 as shown in FIG. 17B. Band signal x _L (N) is output. In the first extension band processing unit 30C, the downsampler 32 receives the downsampling signal x having the spectrum shown in FIG. 17C. _D (N) is output, and the spectrum inversion unit 33 outputs the inverted signal z having the spectrum shown in FIG. 17D. _D (N) is output.
In such a case, the band pass filter 35 ₁ , 35 ₂ , 35 ₃ Are the first subband components z having the spectra shown in FIGS. 18A, 18B, and 18C, respectively. ₁ , Z ₂ , Z ₃ Is output. On the other hand, the bandpass filter 52 ₁ , 52 ₂ , 52 ₃ Are the second subband components x having the spectra shown in FIGS. 18D, 18E, and 18F, respectively. ₁ , X ₂ , X ₃ Is output. First and second subband components z shown in FIGS. 18A to 18F ₁ , X ₁ , Z ₂ , X ₂ , Z ₃ , X ₃ It can be seen that the frequency band of is continuous.
The coefficient determination unit 20C generates the same mixing coefficient C as the mixing coefficient C (n) given by the coefficient determination unit 20 in FIG. ² , C ³ , ..., C ^2N-2 The mixing coefficient is calculated. The mixing coefficient data CD is supplied to the mixing unit 40C.
As illustrated in FIG. 16, the mixing unit 40C includes a multiplier 45 for the first subband component. ₁ , ..., 45 _N-1 And a multiplier 46 for the second subband component ₁ , ..., 46 _N-1 And have. Multiplier 45 for the first subband component ₁ , 45 ₂ , ..., 45 _N-1 Respectively, the first subband component z ₁ , Z ₂ , ..., z _N-1 And the multiplier 45 ₁ , 45 ₂ , ..., 45 _N-1 Includes multiplication coefficients C and C, respectively. ³ , ..., C ^2N-3 Is given. On the other hand, the multiplier 46 for the second subband component ₁ , 46 ₂ , ..., 46 _N-1 Respectively, the second subband component x ₁ , X ₂ , ..., x _N-1 And the multiplier 46 ₁ , 46 ₂ , ..., 46 _N-1 Respectively, the multiplication coefficient C ² , C ⁴ , ..., C ^2N-2 Is given. The adder 43 is a multiplier 45. ₁ , ..., 45 _N-1 , 46 ₁ , ..., 46 _N-1 Are added to the baseband signal to generate a band extension signal y (n).
As a result, as illustrated in FIG. 18G, an extension band component in which the first subband component and the second subband component are alternately arranged is formed in the frequency spectrum of the band extension signal y (n). Become.
As described above, in the band expansion device 4 of the fourth embodiment, the first expansion band processing unit 30C and the second expansion band processing unit 50C are respectively connected to the filter bank 35. ₁ ~ 35 _N-1 And 52 ₁ , ..., 52 _N-1 Therefore, it is possible to generate the band extension signal y (n) having a high-quality and natural frequency spectrum.
5. Application examples
The band extending apparatuses 1 to 4 of the first to fourth embodiments can be applied to various devices. 19A to 19C are diagrams schematically illustrating an application example of the band extending device 1. Referring to FIG. 19A, the bandwidth extension device 1 (or bandwidth extension devices 2 to 4) is connected between the audio signal processing unit 60 and the D / A converter (DAC) 61. The audio signal processing unit 60 is a block that performs audio signal processing such as PCM encoding. The band extension device 1 can extend the frequency band of the signal supplied from the audio signal processing unit 60 and supply the band extension signal to the D / A converter 61.
In the example of FIG. 19B, the bandwidth extension device 1 (or bandwidth extension devices 2 to 4) is connected between a decoder 62 incorporated in a mobile phone, an optical disk playback device or the like, and a D / A converter (DAC) 63. Has been. The band extension device 1 can extend the frequency band of the decoded signal supplied from the decoder 62 and supply the band extension signal to the D / A converter 63.
In the example of FIG. 19C, the band extension device 1 (or the band extension devices 2 to 4) is connected between a tuner 64 such as an AM tuner or an FM tuner and a D / A converter (DAC) 67. The output of the tuner 64 is filtered by a low-pass filter (LPF) 65 and A / D converted by an A / D converter (ADC) 66. The band extension device 1 extends the frequency band of the digital output of the A / D converter 66 and supplies the band extension signal to the D / A converter 67.
Further, all or part of the bandwidth extension apparatuses 1 to 4 of the first to fourth embodiments may be realized by hardware, or may be realized by a program executed by a processor such as a CPU. May be.

Claims (22)

A bandwidth expansion device for expanding the frequency band of an input signal,
A baseband processing unit that performs upsampling on the input signal and generates a baseband signal limited to a predetermined frequency band by filtering the upsampled signal;
An extension band processing unit that generates an extension band component from the input signal;
A mixing unit that mixes the extension band component with the baseband signal,
The extended bandwidth processing unit
A spectrum inversion unit for generating an inverted signal by inverting the spectrum of the input signal or a band signal obtained from the input signal;
An upsampler for upsampling the inverted signal;
A band pass filter that generates, as the extension band component, a frequency band component having a frequency that is substantially the same as the frequency at the high frequency end of the frequency band of the baseband signal by filtering the output of the upsampler When,
A band extending apparatus comprising: The bandwidth extension apparatus according to claim 1,
The spectrum inversion unit generates the inverted signal by inverting the spectrum of the band signal,
The extended bandwidth processing unit
A second band pass filter that filters the input signal to generate a filter signal limited to a second frequency band;
And a downsampler for downsampling the filter signal to obtain the band signal. 3. The band extending apparatus according to claim 2, wherein the down sampler performs the down sampling with a factor of two. 4. The band extending apparatus according to claim 2, wherein a frequency spectrum of the input signal and a regression curve of the frequency spectrum are calculated, and a mixing coefficient to be multiplied by the extension band component is determined using the regression curve. A coefficient determination unit;
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. 5. The band extension apparatus according to claim 4, wherein the coefficient determination unit uses the regression curve, and a first spectrum value at a frequency at a high frequency end of the second frequency band or a frequency in the vicinity thereof, Obtaining a second spectral value at a frequency at a lower end of the second frequency band or a frequency in the vicinity thereof, and calculating a ratio of the first and second spectral values as the mixing coefficient, To expand the bandwidth. The band extension apparatus according to claim 2 or 3, further comprising a coefficient determination unit that calculates a frequency spectrum of the input signal and determines a mixing coefficient to be multiplied by the extension band component using the frequency spectrum.
The coefficient determination unit uses the frequency spectrum to determine a first spectrum value at a frequency at or near the high frequency end of the second frequency band and a frequency at the low frequency end of the second frequency band or A second spectral value at a frequency in the vicinity thereof, and a ratio of the first and second spectral values is calculated as the mixing coefficient,
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. The band extension apparatus according to claim 2 or 3, further comprising a coefficient determination unit that calculates a frequency spectrum of the input signal and determines a mixing coefficient to be multiplied by the extension band component using the frequency spectrum.
The coefficient determination unit uses the frequency spectrum to calculate a first average of spectrum values in a predetermined narrow band including a frequency at a high frequency end of the second frequency band and a frequency at a low frequency end of the second frequency band. And calculating a second average of spectral values in a predetermined narrow band including the ratio of the first and second averages as the mixing coefficient,
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. The band extension apparatus according to claim 2 or 3, further comprising a coefficient determination unit that determines a mixing coefficient to be multiplied by the extension band component,
The coefficient determination unit
A first bandpass filter for filtering the input signal having a predetermined narrowband including a frequency at the high end of the second frequency band as a passband;
A second bandpass filter that filters the input signal with a predetermined narrowband including a low-end frequency of the second frequency band as a passband;
The sum of squares of the output of the first bandpass filter is calculated over a predetermined sample period, and the sum of squares of the output of the second bandpass filter is calculated over the sample period to calculate the sum of squares. An arithmetic unit that calculates a square root value of the ratio as the mixing coefficient,
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. 2. The band extension apparatus according to claim 1, wherein a frequency spectrum of the input signal and a regression curve of the frequency spectrum are calculated, and a coefficient determination for determining a mixing coefficient to be multiplied by the extension band component using the regression curve. Further comprising
The spectrum inversion unit inverts the spectrum of the input signal to generate the inverted signal,
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. 10. The band extending apparatus according to claim 9, wherein the coefficient determination unit uses the regression curve to calculate a first spectrum value at a frequency at a high frequency end of the frequency band of the input signal or a frequency in the vicinity thereof. Obtaining a second spectral value at a frequency at or near a lower end of the frequency band of the input signal, and calculating a ratio of the first and second spectral values as the mixing coefficient. A bandwidth extension device. The band extending apparatus according to claim 1, further comprising a coefficient determining unit that calculates a frequency spectrum of the input signal and determines a mixing coefficient to be multiplied by the extended band component using the frequency spectrum.
The spectrum inversion unit inverts the spectrum of the input signal to generate the inverted signal,
The coefficient determination unit uses the frequency spectrum to determine a first spectrum value at a frequency at or near a high frequency end of the frequency band of the input signal and a low frequency end of the frequency band of the input signal. A second spectral value at a frequency or a frequency in the vicinity thereof, and a ratio of the first and second spectral values is calculated as the mixing coefficient,
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. The band extending apparatus according to claim 1, further comprising a coefficient determining unit that calculates a frequency spectrum of the input signal and determines a mixing coefficient to be multiplied by the extended band component using the frequency spectrum.
The spectrum inversion unit inverts the spectrum of the input signal to generate the inverted signal,
The coefficient determination unit uses the frequency spectrum, and uses a first average of spectrum values in a predetermined narrow band including a frequency at a high band end of the frequency band of the input signal, and a low band end of the frequency band of the input signal. And calculating a second average of the spectrum values in a predetermined narrow band including the frequency, and calculating a ratio of the first and second averages as the mixing coefficient,
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. The band extending apparatus according to claim 1, further comprising a coefficient determining unit that determines a mixing coefficient to be multiplied by the extended band component,
The spectrum inversion unit inverts the spectrum of the input signal to generate the inverted signal,
The coefficient determination unit
A first band pass filter that filters the input signal with a predetermined narrow band including a frequency at the high end of the frequency band of the input signal as a pass band;
A second bandpass filter for filtering the input signal having a predetermined narrowband including a frequency at a lower end of the frequency band of the input signal as a passband;
The sum of squares of the output of the first bandpass filter is calculated over a predetermined sample period, and the sum of squares of the output of the second bandpass filter is calculated over the sample period to calculate the sum of squares. An arithmetic unit that calculates a square root value of the ratio as the mixing coefficient,
The band extension device, wherein the mixing unit multiplies the extension band component by the mixing coefficient and adds the multiplication result to the baseband signal. The band extension apparatus according to claim 2 or 3, further comprising a second extension band processing unit that generates a second extension band component from the input signal,
The second extension bandwidth processing unit
An upsampler for upsampling the band signal supplied from the downsampler included in the extended band processing unit;
A band pass that generates a frequency band component having a frequency that is substantially the same as the frequency of the high band edge of the extension band component as the low band edge by filtering the output of the upsampler as the second band extension component. A filter, and
The mixing unit mixes the extension band component and the second extension band component with the baseband signal. 15. The band extending apparatus according to claim 14, wherein a frequency spectrum of the input signal and a regression curve of the frequency spectrum are calculated, and using the regression curve, the first mixing coefficient to be multiplied by the extension band component and the A coefficient determination unit for determining a second mixing coefficient to be multiplied by the second extension band component;
The mixing unit multiplies the extension band component by the first mixing coefficient and multiplies the second extension band component by the second mixing coefficient and adds the multiplication result to the baseband signal. Bandwidth expansion device. 15. The band extending apparatus according to claim 14, wherein a frequency spectrum of the input signal is calculated, and the first mixing coefficient to be multiplied by the extension band component and the second extension band component are multiplied by using the frequency spectrum. A coefficient determination unit for determining a power second mixing coefficient;
The mixing unit multiplies the extension band component by the first mixing coefficient and multiplies the second extension band component by the second mixing coefficient and adds the multiplication result to the baseband signal. Bandwidth expansion device. The band extension apparatus according to claim 2 or 3, further comprising a second extension band processing unit that generates a second extension band component from the input signal,
The bandpass filter included in the extension band processing unit generates a plurality of first subband components limited to different frequency bands as the extension band components by filtering the output of the upsampler. Consists of banks,
The second extension bandwidth processing unit
An upsampler for upsampling the band signal supplied from the downsampler included in the extended band processing unit;
A filter bank that generates a plurality of second subband components limited to different frequency bands as the second extension band components by filtering the output of the upsampler,
The band extender characterized in that the mixing unit mixes the first subband component and the second subband component with the baseband signal. 18. The band extending apparatus according to claim 17, wherein the frequency band of the first subband component and the frequency band of the second subband component are continuous. The band extending apparatus according to claim 17 or 18, wherein a frequency spectrum of the input signal and a regression curve of the frequency spectrum are calculated, and a plurality of the first subband components to be multiplied by using the regression curve. A coefficient determining unit that determines a first mixing coefficient and a plurality of second mixing coefficients to be respectively multiplied by the second subband component;
The mixing unit multiplies the first subband component by the first mixing coefficient, and multiplies the second subband component by the second mixing coefficient, and adds the multiplication result to the baseband signal. A bandwidth expansion apparatus characterized by that. 19. The band extending apparatus according to claim 17, wherein a frequency spectrum of the input signal is calculated, and a plurality of first mixing coefficients to be multiplied by the first subband component are calculated using the frequency spectrum. A coefficient determining unit that determines a plurality of second mixing coefficients to be multiplied by the two subband components,
The mixing unit multiplies the first subband component by the first mixing coefficient, and multiplies the second subband component by the second mixing coefficient, and adds the multiplication result to the baseband signal. A bandwidth expansion apparatus characterized by that. A bandwidth expansion method for extending the frequency band of an input signal,
(A) performing upsampling on the input signal and filtering the upsampled signal to generate a baseband signal limited to a predetermined frequency band;
(B) generating an extension band component from the input signal;
(C) mixing the extension band component with the baseband signal,
The step (b)
(B-1) inverting the spectrum of the input signal or a band signal obtained from the input signal to generate an inverted signal;
(B-2) performing upsampling on the inverted signal;
(B-3) A frequency having a low frequency end that is substantially the same as the frequency at the high frequency end of the frequency band of the baseband signal by filtering the inverted signal upsampled in step (b-2). Generating a band component as the extension band component. A bandwidth extension program for causing a processor to execute a bandwidth extension process for extending a frequency band of an input signal,
The bandwidth extension process includes:
Baseband processing for performing upsampling on the input signal and generating a baseband signal limited to a predetermined frequency band by filtering the upsampled signal;
Extended band processing for generating an extended band component from the input signal;
A mixing process for mixing the baseband signal with the extension band component,
The extended bandwidth processing is
Spectrum inversion processing for generating an inverted signal by inverting the spectrum of the input signal or a band signal obtained from the input signal;
An upsampling process for upsampling the inverted signal;
Filter processing for generating, as the extension band component, a frequency band component having a frequency that is substantially the same as the frequency at the high band edge of the frequency band of the baseband signal by filtering the output of the upsampler. And a bandwidth extension program.

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