Complex exponential modulated filter bank for high frequency reconstruction

• US 9,667,229 B1
• Filed: 03/24/2017
• Issued: 05/30/2017
• Est. Priority Date: 02/18/2009
• Status: Active Grant

• Current Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Original Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Inventors: Ekstrand, Per
• Primary Examiner(s): Yaary, Michael D
• Application Number:
  US15/468,689
• Time in Patent Office: 67 Days
• Field of Search: None
• CPC Class Codes: G06F 17/10 Complex mathematical operations function generation by table look-up G06F1/03; evaluation of elementary functions by calculation G06F7/544
• CPC Class Codes: G06F 17/11 for solving equations , e.g. nonlinear equations, general mathematical optimization problems optimization specially adapted for a specific administrative, business or logistic context G06Q10/04
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: G10L 19/0204 using subband decomposition
• CPC Class Codes: G10L 19/0208 Subband vocoders
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering
• CPC Class Codes: G10L 19/265 Pre-filtering, e.g. high frequency emphasis prior to encoding
• CPC Class Codes: G10L 21/00 Speech or voice signal processing techniques to produce another audible or non-audible signal, e.g. visual or tactile, in order to modify its quality or its intelligibility G10L19/00 takes precedence
• CPC Class Codes: H03H 17/0201 Wave digital filters
• CPC Class Codes: H03H 17/0248 Filters characterised by a particular frequency response or filtering method
• CPC Class Codes: H03H 17/0266 Filter banks
• CPC Class Codes: H03H 17/0275 comprising non-recursive filters
• CPC Class Codes: H03H 17/0294 Variable filters; Programmable filters
• CPC Class Codes: H03H 2017/0297 Coefficients derived from input parameters
• CPC Class Codes: H04R 3/04 for correcting frequency response
• CPC Class Codes: H04S 7/307 Frequency adjustment, e.g. tone control H04S7/301 takes precedence

Abstract

An apparatus and method are disclosed for filtering and performing high frequency reconstruction of an audio signal. The apparatus includes an analysis filter bank, a phase shifter, a high frequency reconstructor, and a synthesis filter bank. The analysis filterbank receives real-valued time domain input audio samples and generates complex valued subband samples. The phase shifter shifts a phase of the complex-valued subband samples by an arbitrary amount. The high frequency reconstructor modifies at least some of the complex valued subband samples. A phase shifter unshifts a phase of the modified complex-valued subband samples by the arbitrary amount. The synthesis filter bank receives the modified complex valued subband samples and generates time domain output audio samples. The analysis filter bank comprises analysis filters that are complex exponential modulated versions of a prototype filter with an arbitrary phase shift.

Claims

1. A signal processing device for filtering and performing high frequency reconstruction of an audio signal, the signal processing device comprising:

an analysis filter bank that receives real valued time domain input audio samples and generates complex-valued subband samples;

a phase shifter that shifts a phase of the complex-valued subband samples by an amount;

a high frequency reconstructor that generates modified complex-valued subband samples through a high frequency reconstruction process;

a phase shifter that unshifts a phase of the modified complex-valued subband samples by the amount; and

a synthesis filter bank that receives the modified complex-valued subband samples and generates time domain output audio samples,

wherein the analysis filter bank comprises analysis filters (h_k(n)) and the synthesis filter bank comprises synthesis filters (f_k(n)) that are complex exponential modulated versions of a prototype filter (p₀(n)) according to;

2. The signal processing device of claim 1 wherein the prototype filter (p₀(n)) is a symmetric low pass prototype filter or an asymmetric low pass prototype filter.

3. The signal processing device of claim 1 wherein the analysis filter bank is a pseudo QMF bank.

4. The signal processing device of claim 1 wherein an order of the prototype filter (p₀(n)) equals the system delay D.

5. The signal processing device of claim 1 wherein the high frequency reconstructor performs spectral band replication (SBR).

6. The signal processing device of claim 1 wherein a value of the amount is chosen to reduce a complexity of an implementation of the apparatus.

7. The signal processing device of claim 1 wherein the one or more hardware elements comprise a digital signal processor, a microprocessor, or a memory.

8. The signal processing device of claim 1 wherein the number of channels in the analysis filter bank is different than a number of channels in the synthesis filter bank.

9. The signal processing device of claim 8 wherein the number of channels in the analysis filter bank is 32 and the number of channels in the synthesis filter bank is 64.

10. A method performed by an signal processing device for filtering and performing high frequency reconstruction of an audio signal, the method comprising:

receiving real-valued time domain input audio samples;

filtering the real-valued time domain input audio samples with an analysis filter bank to generate complex valued subband samples;

shifting a phase of the complex-valued subband samples by an amount;

generating modified complex valued subband samples through a high frequency reconstruction process;

unshifting a phase of the modified complex-valued subband samples by the amount;

receiving the modified complex valued subband samples;

filtering the modified complex valued subband samples with a synthesis filter bank to generate time domain output audio samples,

wherein the analysis filter bank comprises analysis filters (h_k(n)) and the synthesis filter bank comprises synthesis filters (f_k(n)) that are complex exponential modulated versions of a prototype filter (p₀(n)) according to;

11. A non-transitory computer readable medium containing instructions that when executed by a processor perform the method of claim 10.