System for subband coding of a digital audio signal

• US 4,896,362 A
• Filed: 04/22/1988
• Issued: 01/23/1990
• Est. Priority Date: 04/27/1987
• Status: Expired due to Term

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Breeuwer, Marcel, Veldhuis, Raymond N. J., van der Waal, Robbert G.
• Primary Examiner(s): Kemeny, Emanuel S.
• Assistant Examiner(s): Knepper, David D.
• Application Number:
  US07/184,746
• Time in Patent Office: 641 Days
• Field of Search: 364/513.5, 381/29-42, 84/1.01
• US Class Current: 704/200.1
• CPC Class Codes: G10L 19/002 Dynamic bit allocation for perceptual audio coders G10L19/032
• CPC Class Codes: G10L 19/0204 using subband decomposition
• CPC Class Codes: G10L 25/18 the extracted parameters being spectral information of each sub-band
• CPC Class Codes: G11B 20/10527 Audio or video recording; Data buffering arrangements G11B20/12 - G11B20/18 take precedence
• CPC Class Codes: H04B 1/665 using psychoacoustic properties of the ear, e.g. masking effect
• CPC Class Codes: H04B 1/667 using a division in frequency subbands for TV signals H04N19/63

Abstract

A system for subband coding of a digital audio signal x(k) includes in the coder (1) a filter bank (3) for splitting the audio signal band, with sampling rate reduction, into subbands (p=1, . . . P) of approximately critical bandwidth and in the decoder (2) a filter bank (5) for merging these subbands, with sampling rate increase. For each subband (p) the coder (1) comprises a detector (7(p)) for determining a parameter G(p;m) representative of the signal level in a block (p;m) of M samples of the subband signal x_p (k) as well as a quantizer (8(p)) for adaptively block quantizing this subband signal in response to parameter G(p;m), and the decoder (2) comprises a dequantizer (9(p)) for adaptively block dequantizing the quantized subband signal s_p (k) in response to parameter G(p;m). The quantizing characteristics are related to the noise-masking curve of the human auditory system, owing to which a high-quality of the replica x(k) of audio signal x(k) is attained with an average number of approximately 2.5 bits per sample for representing the output signals of the coder (1). The occasional audibility of quantizing noise in this replica x(k) is reduced effectively in that the coder (1) and decoder (2) contain identical bit allocation means (23, 24) responsive to a set of parameters G(p;m) for the higher group of subbands (p_im ≦p≦P) within an allocation window (FIG. 5) for allocating a number of B(p;m) bits per sample from a fixed predetermined number of B bits for this allocation window to the quantizer (8(p)) and the dequantizer (9(p)) for the block (p;m) of subband signal x_p (k) and s_p (k), respectively.

Claims

1. A digital system including a coder and a decoder for subband coding of a digital audio music signal having a given sampling rate 1/T, the coder comprising:

analysis filter means responsive to the audio music signal for generating a number of P subband signals, the analysis filter means dividing the audio music signal band according to the quadrature mirror filter technique, with sampling rate reduction into successive subbands of band numbers p (i≦

p≦

P) increasing with the frequency, the bandwidth and the sampling rate for each subband being an integral submultiple of 1/(2T) and 1/T, respectively, and the bandwidths of the subbands approximately corresponding with the critical bandwidths of the human auditory system in the respective frequency ranges,

means responsive to each of the subband signals for determining a characteristic parameter G(p;

m) which is representative of the signal level in a block having a same number of M signal samples for each subband, m being the number of the block,

means for adaptively quantizing the blocks of the respective subband signals in response to the respective characteristic parameters G(p;

m);

and the decoder comprising;

means for adaptively dequantizing the blocks of quantized subband signals in response to the respective characteristic parameters G(p;

m),

synthesis filter responsive to the dequantized subband signals means for constructing a replica of the digital audio music signal, these synthesis filter means merging the subbands to the audio music signal band according to the quadrature mirror filter technique, with the sampling rate increase,

characterized in that

the respective quantizing means in the coder and the respective dequantizing means in the decoder for each of the subbands having a band number p smaller than p_im are arranged for the respective quantizing and dequantizing of the subband signals with a fixed number of B(p) bits, the subband having band number p_im being situated in the portion of the audio music signal band with the lowest thresholds for masking noise in critical bands of the human auditory system by single music tones in the centre of the respective critical bands,

the coder and the decoder each further include bit allocation means responsive to the respective characteristic parameter G(p;

m) of the subbands having a band number p not smaller than p_im within an allocation window having a duration equal to the block length for the subband having the band number p_im, for allocating a number of B(p;

m) bits from a predetermined fixed total number of B bits for the allocation window to the respective quantizing means in the coder and the respective dequantizing means in the decoder for the signal block having block number m of the subband having band number p, the bit allocation means each comprising;

comparator means for comparing within each allocation window the characteristic parameters G(p;

m) to respective thresholds T(p) for the subbands having band number p and for generating respective binary comparator signals C(p;

m) having a first value C(p;

m)="1" for a parameter G(p;

m) not smaller than the threshold T(p) and a second value C(p;

m)="0" in the opposite case, these thresholds T(p) being related to the thresholds of the human auditory system for just perceiving single music tones,

means for storing a predetermined allocation pattern {B(p)} of numbers of B(p) quantizing bits for subbands having respective band numbers p, these numbers B(p) being related to the thresholds for masking noise in the critical bands of the human auditory system by single music tones in the centre of the respective critical bands,

means for determining an allocation pattern {B(p;

m)} of respective numbers of B(p;

m) quantizing bits for the signal block having the block number m of the subband having band number p, in response to the allocation pattern stored {B(p)} and the respective characteristic parameters G(p;

m) and comparator signals C(p;

m), the allocation pattern {B(p;

m)} being equal to the allocation pattern stored {B(p)} if all comparator signals C(p;

m) within an allocation window have the said first value C(p;

m)="1" and, in the opposite case, the allocation pattern {B(p;

m)} being formed by not allocating quantizing bits to blocks within an allocation window having a comparator signal of the said second value C(p;

m)="0" and by allocating the sum S of the numbers of B(p) quantizing bits available within an allocation window for the latter blocks in the allocation pattern stored {B(p)} to the blocks within an allocation window having a comparator signal of the said first value C(p;

m)="1" and having the largest values of the characteristic parameter G(p;

m) for obtaining numbers of B(p;

m) quantizing bits which are greater than the corresponding numbers of B(p) quantizing bits in the allocation pattern stored {B(p)},

means for supplying the allocation pattern {B(p;

m)} determined thus to the respective quantizing means in the coder and the respective dequantizing means in the decoder.

2. A digital system as claimed in claim 1, characterized in that the said bit allocation means also include means which in response to successive characteristic parameters G(p;

m) and G(p;

m+1) of each subband of band number p exceeding p_im ;

do not allocate quantizing bits to block (p;

m+1) and add the numbers of B(p;

m+1) quantizing bits available for this block to the said sum S, if the ratio Q=G(p;

m)/G(p;

m+1) is greater than a predetermined value R(p) of the order of 10² and block (p;

m+1) is situated within the allocation window;

do not allocate quantizing bits to block (p;

m) and add the numbers of B(p;

m) quantizing bits available for this block to the said sum S, if the ratio Q=G(p;

m)/G(p;

m+1) is smaller than the value 1/R(p) and block (p;

m) is situated within the allocation window.

4. An encoder according to claim 1, wherein said bit allocation means further includes means which in response to successive characteristic parameters G(p;

m) and G(p;

m+1) of each subband of band number p exceeding p_im

do not allocate quantizing bits to block (p;

m+1) and add the numbers of B(p;

m+1) quanitizing bits available for this block to the said sum S if the ratio Q=G(p;

m)/G(p;

m+1) is greater than a predetermined value R(p) of the order of ten to the power of two and block (p;

m+1) is situated within the allocation window, and

do not allocate quantizing bits to block (p;

m) and add the numbers of B(p;

m) quantizing bits available for this block to the said sum S, if the ratio Q=G(p;

m)/G(p;

m+1) is smaller than the value 1/R(p) and the block (p;

m) is situated within the allocation window.

3. A coder for subband coding for a digital audio music signal having a given sampling rate 1/T, the coder comprising:

(a) analysis filter means responsive to the audio music signal for generating a number of P subband signals, the analysis filter means dividing the audio music signal band according to the quadrature mirror filter technique, with sampling rate reduction into successive subbands of band numbers p (i≦

p≦

P) increasing with the frequency, the bandwidth and the sampling rate for each subband being an integral submultiple of 1/(2T) and 1/T, respectively, and the bandwidths of the subbands approximately corresponding with the critical bandwidths of the human auditory system in the frequency ranges,

(b) means responsive to each of the subband signals for determining a characteristic parameter G(p;

m) which is representative of the signal level in a block having a same number of M signal samples for each subband, m being the number of the blocks; and

(c) means for adaptively quantizing the blocks of the respective subband signals in response to the respective characteristic parameters G(p;

m), wherein

said means for adaptively quantizing for each of the subbands having a band number p smaller than p_im are arranged for the quantizing of the subband signals with a fixed numbers of B(p) bits, the subband having band numbers p_im being situated in the portion of the audio music signal band with the lowest thresholds for masking noise in critical bands of the human auditory system by single music tones in the center of the respective critical bands, said coder further including

(d) bit allocation means responsive to the respective characteristic parameters G(p;

m) of the subbands having a band number p not smaller than p_im within an allocation window having a duration equal to the block length for the subband having the band number p_im, for allocating a number of B(p;

m) bits for a predetermined fixed total number of B bits for the allocation window to the quantizing means in the coder for the signal block having block number m of the subband having band number p, the bit allocation means each comprising

(i) comparator means for comparing within each allocation window the characteristic parameters G(p;

m) to respective thresholds T(p) for the subbands having band number p and for generating respective binary comparator signals C(p;

m) having a first value C(p;

m)="1" for a parameter G(p;

m) not smaller than the threshold T(p) and a second value C(p;

m)="0" in the opposite case, these thresholds T(p) being related to the thresholds, of the human auditory system for perceiving just single music tones.,

(ii) means for storing a predetermined allocation pattern {B(p)} of numbers of B(p) quantizing bits for subbands having respective band numbers p, these numbers B(p) being related to respective band numbers p, these numbers B(p) being related to the thresholds for masking noise in the critical bands of the human auditory system by single music tones in the center of the respective critical bands,

(iii) means for determining an allocation pattern {B(p;

m)} of respective numbers of B(p;

m) quantizing bits for the signal block having the block number m of the subband having band number p, in response to the allocation pattern stored {B(p)} and the respective characteristic parameters G(p;

m) and comparator signals C(p;

m), the allocation pattern {B(p;

m)} being equal to the allocation pattern stored {B(p)} if all comparator signals C(p;

m) within an allocation window have the said first value C(p;

m)="1" and, in the opposite case, the allocation pattern {B(p;

m)} bing formed by not allocating quantizing bits to blocks within an allocation window having a comparator signal of the said second value C(p;

m)="0" and by allocating the sum S of the numbers of B(p) quantizing bits available within an allocation window for the latter blocks in the allocation pattern stored {B(p)} to the blocks within an allocation window having a comparator signal of the said first value C(p;

m)="1" and having the largest values of the characteristic parameter G(p;

m) for obtaining numbers of B(p;

m) quantizing bits which are greater than the corresponding numbers of B(p) quantizing bits in the allocation patter stored {B(p)}, and

(iv) means for supplying the allocation pattern {B(p;

m)} determined thus to the respective quantizing means in the coder.