Digital encoder with dynamic quantization bit allocation

• US 5,664,056 A
• Filed: 07/08/1994
• Issued: 09/02/1997
• Est. Priority Date: 08/02/1991
• Status: Expired due to Term

• Current Assignee: Sony Corporation (Sony Group Corp.)
• Original Assignee: Sony Corporation (Sony Group Corp.)
• Inventors: Akagiri, Kenzo
• Primary Examiner(s): MacDonald, Allen R.
• Assistant Examiner(s): Dorvil, Richemond
• Application Number:
  US08/272,872
• Time in Patent Office: 1,152 Days
• Field of Search: 395/2.38, 395/2.39, 395/2.14, 395/2.15, 395/2.13, 381/29-51
• US Class Current: 704/200.1
• CPC Class Codes: H04B 1/665 using psychoacoustic properties of the ear, e.g. masking effect

Abstract

An encoder apparatus and method for compressing a digital input signal derived from an analog signal to reduce the number of bits required to represent the analog signal with low quantizing noise. In the encoder, a digital input signal derived from the analog signal is divided into frequency ranges. The digital signal in each of the frequency ranges is divided in time into blocks, the time duration of which may be adaptively varied. The blocks are orthogonally transformed into spectral coefficients, which are grouped into critical bands. The total number of bits available for quantizing the spectral coefficients is allocated among the critical bands. In a first embodiment and a second embodiment, fixed bits are allocated among the critical bands according to a selected one of a plurality of predetermined bit allocation patterns and variable bits are allocated among the critical bands according to the energy in the critical bands. In the first embodiment, the apportionment between fixed bits and variable bits is fixed. In a second embodiment, the apportionment between fixed bits and variable bits is varied according to the smoothness of the spectrum of the input signal. Both embodiments give low quantizing noise levels with both broad spectrum signals and highly tonal signals.

Claims

1. A digital encoding apparatus for compressing a digital input signal to provide a compressed output signal, the apparatus comprising:

frequency dividing means for receiving the digital input signal and for dividing the digital input signal into a plurality of frequency ranges;

time dividing means for dividing in time at least one of the frequency ranges of the digital input signal into a plurality of blocks;

orthogonal transform means for orthogonally transforming each block to provide a plurality of spectral coefficients;

means for grouping by frequency the plurality of spectral coefficients from the orthogonal transform means into critical bands, each critical band having a band energy and a band frequency;

bit allocation pattern selecting means for selecting a predetermined bit allocation pattern from a plurality of predetermined bit allocation patterns;

first bit allocating means for allocating a total number of fixed bits among the critical bands to allocate to each critical band a number of fixed bits for quantizing each spectral coefficient in the critical band, the number of fixed bits allocated to each critical band being determined according to the selected predetermined bit allocation pattern;

second bit allocating means for allocating a total of number of variable bits among the critical bands to allocate to each critical band a number of variable bits for quantizing each spectral coefficient in the critical band, the number of variable bits allocated to each critical band being determined according to the band energy of the critical band; and

means for quantizing each spectral coefficient in each band using a number of bits equal to a sum of the number of fixed bits allocated for quantizing each spectral coefficient in the critical band and the number of variable bits allocated for quantizing each spectral coefficient in the critical band, the spectral coefficients in at least one of the critical bands being quantized using a number of fixed bits greater than zero and a number of variable bits greater than zero.

2. The digital encoding apparatus of claim 1, wherein the total number of fixed bits and the total number of variable bits each remain constant with time.

3. The digital encoding apparatus of claim 2, wherein:

the apparatus additionally comprises energy determining means for determining an energy of the digital input signal; and

the bit pattern selecting means selects a predetermined bit allocation pattern wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies when the energy determining means determines that the energy of the digital input signal is low.

4. The digital encoding apparatus of claim 2, wherein:

the apparatus additionally comprises energy determining means for determining the band energy of one of the critical bands; and

the bit pattern selecting means selects one of the plurality of predetermined bit patterns wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies when the energy determining means determines that the band energy in the one of the critical bands is low.

5. The digital encoding apparatus of claim 2, wherein:

the apparatus additionally comprises energy determining means for deriving a derived signal by further dividing one of the frequency ranges of the digital input signal in frequency, and for determining an energy of the derived signal; and

the bit allocation pattern selecting means selects one of the plurality of predetermined bit allocation patterns wherein fewer bits are allocated to critical bands towards higher frequencies than to critical bands towards middle frequencies when the energy determining means determines that the energy of the derived signal is low.

6. The digital encoding apparatus of claim 2, wherein:

each block has a duration in time, and

the time dividing means is additionally for changing the duration in time of the blocks in at least one of the frequency ranges in response to the digital input signal.

7. The digital encoding apparatus of claim 1, wherein:

each block has a duration in time, and

the time dividing means is additionally for changing the duration in time of the blocks in at least one of the frequency ranges in response to the digital input signal.

8. The digital encoding apparatus of claim 1, wherein:

a grand total number of bits is available for quantizing all the spectral coefficients; and

the apparatus additionally comprises means for dynamically apportioning the grand total number of bits between the total number of fixed bits and the total number of variable bits in response to the digital input signal.

9. The digital encoding apparatus of claim 8, wherein:

the apparatus additionally comprises energy determining means for determining an energy of the digital input signal; and

the bit pattern selecting means selects a predetermined bit pattern wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies when the energy determining means determines that the energy of the digital input signal is low.

10. The digital encoding apparatus of claim 8, wherein:

the apparatus additionally comprises energy determining means for determining the band energy of one of the critical bands; and

the bit pattern selecting means selects one of the plurality of predetermined bit allocation patterns wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies when the energy determining means determines that the band energy the one of the critical bands is low.

11. The digital encoding apparatus of claim 8, wherein:

the apparatus additionally comprises energy determining means for deriving a derived signal by further dividing one of the frequency ranges of the digital input signal in frequency, and for determining an energy of the derived signal; and

the bit allocation pattern selecting means selects one of the plurality of predetermined bit allocation patterns wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies when the energy determining means determines that the energy of the derived signal is low.

12. The digital encoding apparatus of claim 8, wherein:

the digital input signal has a spectrum having a smoothness,

the apparatus includes a spectral smoothness index generating means for generating a spectral smoothness index in response to the smoothness of the spectrum of the digital input signal, and

the means for apportioning the grand total number of quantizing bits between the total number of fixed bits and the total number of variable bits increases the total number of fixed bits and decreases the total number of variable bits in response to an increase in the spectral smoothness index.

13. The apparatus of claim 12, wherein the spectral smoothness index generating means derives the spectral smoothness index in response to a measured difference in energy between adjacent critical bands.

14. The apparatus of claim 12, wherein:

the apparatus additionally comprises a floating point processing means for floating point processing the spectral coefficients in each critical band, and for generating floating point data for each critical band, and

the spectral smoothness index generating means derives the spectral smoothness index in response to a difference in floating point data between adjacent critical bands.

15. The digital encoding apparatus of claim 8, wherein:

each block has a duration in time, and

the time dividing means is additionally for changing the duration in time of the blocks in at least one of the frequency ranges in response to the input signal.

26. The method of claim 5, wherein, in the step of providing a total number of fixed bits and a total number of variable bits, the total number of fixed bits provided and the total number of variable bits provided each remain constant with time.

27. The method of claim 26, wherein:

in the step of dividing in time, each block has a duration in time, and

the step of dividing in time includes the step of changing the duration in time of the blocks in at least one of the frequency ranges in response to the digital input signal.

16. A system for compressing a digital input signal to provide a compressed digital signal, and for expanding the compressed digital signal to provide a digital output signal, the system comprising:

a compressor comprising;

means for dividing the digital input signal into a plurality of frequency ranges;

means for dividing in time each of the frequency ranges of the digital input signal into a plurality of blocks;

orthogonal transform means for orthogonally transforming each block to provide a plurality of spectral coefficients;

means for grouping by frequency the plurality of spectral coefficients from the orthogonal transform means into critical bands, each critical band having a band energy and a band frequency;

bit allocation pattern selecting means for selecting a predetermined bit allocation pattern from a plurality of predetermined bit allocation patterns;

first bit allocating means for allocating a total number of fixed bits among the critical bands to allocate to each critical band a number of fixed bits for quantizing each spectral coefficient in the critical band, the number of fixed bits allocated to each critical band being determined according to the selected predetermined bit allocation pattern;

second bit allocating means for allocating a total number of variable bits among the critical bands to allocate to each critical band a number of variable bits for quantizing each spectral coefficient in the band, the number of variable bits allocated to each critical band being determined according to the band energy of the critical band;

means for quantizing each spectral coefficient in each band using a number of bits equal to a sum of the number of fixed bits allocated for quantizing each spectral coefficient in the critical band and the number of variable bits allocated for quantizing each spectral coefficient in the critical band, and for generating word length data indicating the sum for each critical band, the spectral coefficients in at least one of the critical bands being quantized using a number of fixed bits greater than zero and a number of variable bits greater than zero; and

means for multiplexing the quantized spectral coefficients and the word length data to provide the compressed digital signal, and a decoder, comprising;

demultiplexing means for extracting the word-length data from the compressed digital signal, and for extracting the spectral coefficients from the compressed digital signal using the word-length data;

means for grouping by frequency the extracted spectral coefficients into a plurality of frequency ranges corresponding to the plurality of frequency ranges in the encoder;

means for performing an inverse orthogonal transform on the spectral coefficients in each frequency range to generate blocks of time-dependent data in each frequency range; and

means for combining the blocks of time-dependent data in each frequency range to provide the digital output signal.

17. An apparatus for deriving a compressed digital recording signal from a digital input signal for recording on a recording medium, the apparatus comprising:

means for dividing the digital input signal into a plurality of frequency ranges;

means for dividing in time each of the frequency ranges of the digital input signal into a plurality of blocks;

orthogonal transform means for orthogonally transforming each block to provide a plurality of spectral coefficients;

means for grouping by frequency the plurality of spectral coefficients from the orthogonal transform means into critical bands, each critical band having a band energy and a band frequency;

bit allocation pattern selecting means for selecting a predetermined bit allocation pattern from a plurality of predetermined bit allocation patterns;

first bit allocating means for allocating a total number of fixed bits among the critical bands to allocate to each critical band a number of fixed bits for quantizing each spectral coefficient in the critical band, the number of fixed bits allocated to each critical band being determined according to the selected predetermined bit allocation pattern;

second bit allocating means for allocating a total number of variable bits among the critical bands to allocate to each critical band a number of variable bits for quantizing each spectral coefficient in the band, the number of variable bits allocated to each critical band being determined according to the band energy;

means for quantizing each spectral coefficient in each band using a number of bits equal to a sum of the number of fixed bits allocated for quantizing each spectral coefficient in the critical band and the number of variable bits allocated for quantizing each spectral coefficient in the critical band, the spectral coefficients in at least one of the critical band being quantizing using a number of fixed bits greater than zero and a number of variable bits greater than zero, and for generating word length data indicating the sum for each critical band;

means for multiplexing the quantized spectral coefficients and word length data to provide the compressed digital signal; and

recording means for recording the compressed digital signal on the recording medium.

18. The apparatus of claim 17, wherein the recording medium is an optical disc.

19. The apparatus of claim 17, wherein the recording medium includes a semiconductor memory.

20. The apparatus of claim 17, wherein the total number of fixed bits and the total number of variable bits each remain constant with time.

21. The apparatus of claim 20, wherein:

each block has a duration in time, and

the means for dividing each frequency range by time includes means for changing the duration in time of the blocks in at least one of the frequency ranges in response to the digital input signal.

22. The apparatus of claim 17, wherein:

a grand total number of bits is available for quantizing all the spectral coefficients; and

the apparatus additionally comprises means for dynamically apportioning the grand total number of bits between the total number of fixed bits and the total number of variable bits in response to the digital input signal.

23. The apparatus of claim 22, wherein:

the digital input signal has a spectrum having a smoothness,

the apparatus additionally includes means for generating a spectral smoothness index in response to the smoothness of the spectrum of the digital input signal, and

the means for apportioning the grand total number of bits between the total number of fixed bits and the total number of variable bits increases the total number of fixed bits and decreases the total number of variable bits in response to an increase in the spectral smoothness index.

24. The apparatus of claim 22, wherein:

each block has a duration in time, and

the means for dividing each frequency range by time includes means for changing the duration in time of the blocks in at least one of the frequency ranges in response to the digital input signal.

25. A method for deriving a compressed digital signal data from a digital input signal, the method including the steps of:

dividing the digital input signal into a plurality of frequency ranges;

dividing in time each of the frequency ranges of the non-compressed digital input signal into a plurality of blocks;

orthogonally transforming each block to provide a plurality of spectral coefficients;

grouping by frequency the plurality of spectral coefficients provided by the orthogonal transform step into critical bands, each critical band having a band energy and a band frequency;

providing a total number of variable bits and a total number of fixed bits, the total number of fixed bits being arranged in a plurality of predetermined bit allocation patterns;

selecting one of the plurality of predetermined bit allocation patterns as a selected predetermined bit allocation pattern;

allocating the total number of fixed bits among the critical bands to allocated to each critical band a number of fixed bits for quantizing each spectral coefficient in the critical band, the number of fixed bits allocated to each critical band being determined according to the selected predetermined bit allocation pattern;

allocating the total number of variable bits among the critical bands to allocate to each critical band a number of variable bits for quantizing each spectral coefficient in the critical band, the number of quantizing bits allocated to each critical band being determined according to the band energy of the critical band;

quantizing each spectral coefficient in each critical band using a number of bits equal to a sum of the number of fixed bits allocated for quantizing each spectral coefficient in the critical band and the number of variable bits allocated for quantizing each spectral coefficient in the critical band, the spectral coefficients in at least one of the critical band being quantized using a number of fixed bits greater than zero and a number of variable bits greater than zero;

generating word length data indicating the sum for each critical band; and

multiplexing the quantized spectral coefficients and word length data to provide the compressed digital signal.

28. The method of claim 25, wherein the step of providing a total number of variable bits and a total number of fixed bits includes the steps of:

providing a grand total number of bits for quantizing all the spectral coefficients; and

dynamically apportioning the grand total number of bits between the total number of fixed bits and the total number of variable bits in response to the digital input signal.

29. The method of claim 28, wherein:

the digital input signal has a spectrum having a smoothness,

the method includes the additional step of generating a spectral smoothness index in response to the smoothness of the spectrum of the non-compressed digital input signal, and

in the step of apportioning the grand total number of bits between the total number of fixed bits and the total number of variable bits, the total number of fixed bits is increased and the total number of variable bits is decreased in response to an increase in the spectral smoothness index.

30. The method of claim 28, wherein:

in the step of dividing in time, each block has a duration in time, and

the step of dividing in time includes the step of changing the duration in time of the blocks in at least one of the frequency ranges in response to the digital input signal.

31. The method of claim 25, wherein:

the method additionally comprises the step of determining an energy of the digital input signal; and

in the step of selecting one of the plurality of predetermined bit allocation patterns, one of the plurality of predetermined bit allocation patterns wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies is selected when the energy determining step determines that the energy of the digital input signal is low.

32. The method of claim 25, wherein:

the method additionally comprises the step of determining the band energy of one of the critical bands; and

in the step of selecting one of the plurality of predetermined bit allocation patterns, one of the plurality of predetermined bit patterns wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies is selected when the energy determining step determines that the band energy in the one of the critical bands is low.

33. The method of claim 25, wherein:

the method additionally comprises the steps of;

deriving a derived signal by further dividing one of the frequency ranges of the digital input signal in frequency, and

determining an energy of the derived signal; and

in the step of selecting one of the plurality of predetermined bit allocation patterns, one of the plurality of predetermined bit allocation patterns wherein fewer bits are allocated to critical bands having band frequencies towards higher frequencies than to critical bands having band frequencies towards middle frequencies is selected when the energy determining step determines that the energy of the derived signal is low.