Tunable post-filter for tandem coders

• US 5,694,519 A
• Filed: 12/09/1996
• Issued: 12/02/1997
• Est. Priority Date: 02/18/1992
• Status: Expired due to Term

• Current Assignee: Agere Systems Incorporated (Broadcom, Inc.)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Chen, Juin-Hwey, Cox, Richard Vandervoort, Jayant, Nuggehally Sampath
• Primary Examiner(s): Knepper, David D.
• Application Number:
  US08/762,473
• Time in Patent Office: 358 Days
• Field of Search: 395/2, 395/2.12, 395/2.2, 395/2.3-2.39, 395/2.28, 379/88, 379/340, 379/347
• US Class Current: 704/228
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering

Abstract

An adaptive postfilter is used on the decoding side of tandem codecs (coder/decoders). Post-filter parameters are adapted using a backward synthesis filter. The parameters used are 10th order LPC (Linear Predictive Coding) predictor coefficients. The system employed uses Low-Delay Code Excited Linear Predictive codecs (LD-CELP).

Claims

1. A method of processing an encoded signal to generate a postfiltered signal, the method comprising:

(a) decoding the encoded signal to generate a decoded signal; and

(b) postfiltering the decoded signal with a postfilter to generate the postfiltered signal, the postfilter comprising a set of tunable parameters, the set of tunable parameters having preselected values that are based upon an output signal that has been encoded more than once and decoded more than once.

2. The method of claim 1 wherein the decoding is performed by a code excited linear predictive decoder.

3. The method of claim 1 wherein the preselected values are preselected for:

(i) a long term postfiltering parameter, λ

, the long term postfiltering parameter being used to define a long term postfilter having a transfer function of

space="preserve" listing-type="equation">H.sub.1 (z)=g.sub.1 (1+bz.sup.-p)

wherein b is a filter coefficient defined by ##EQU4## and wherein p is a pitch period, g₁ is a scaling factor defined as 1/(1+b), and β

is a tap weight of a single-tap pitch predictor with a pitch period of p samples; and

(ii) a set of short term postfiltering parameters, γ

₁, γ

₂, and γ

₃, the set of short term postfiltering parameters, being used to define a short term postfilter having a transfer function of

space="preserve" listing-type="equation">H(z)=(1-μ

z.sup.-1) (1-Σ

.sub.i=1 to 10 b.sub.i z.sup.-i)/(1-Σ

.sub.i=1 to 10 a.sub.i z.sup.-1)

wherein ##EQU5## and wherein the a_i '"'"'s are a set of predictor coefficients and k₁ is a first reflection coefficient.

4. The method of claim 3 wherein λ

, γ

₁, γ

₂, and γ

₃, are about 0.15, about 0.65, about 0.75, and about 0.15, respectively.

5. The method of claim 1 further comprising the steps of:

(a) determining if a non-voice signal was the encoded signal that was decoded to generate the decoded signal; and

(b) deactivating the postfilter if the encoded signal was a non-voice signal.

6. A device for processing an encoded signal to generate a postfiltered signal, the device comprising:

(a) means for decoding the encoded signal to generate a decoded signal; and

(b) means for postfiltering the decoded signal to generate the postfiltered signal, the postfilter comprising a set of tunable parameters, the set of tunable parameters having preselected values that are based upon an output signal that has been encoded more than once and decoded more than once.

7. The device of claim 6 wherein the decoder is a code excited linear predictive (CELP) decoder.

8. The device of claim 6 wherein the preselected values are preselected for:

(i) a long term postfiltering parameter, λ

, the long term postfiltering parameter being used to define a long term postfilter having a transfer function of

space="preserve" listing-type="equation">H.sub.1 (z)=g.sub.1 (1+bz.sup.-p)

wherein b is a filter coefficient defined by ##EQU6## and wherein p is a pitch period, g₁ is a scaling factor defined as 1/(1+b), and β

is a tap weight of a single-tap pitch predictor with a pitch period of p samples; and

(ii) a set of short term postfiltering parameters, γ

₁, γ

₂, and γ

₃, the set of short term postfiltering parameters, being used to define a short term postfilter having a transfer function of

space="preserve" listing-type="equation">H(z)=(1-μ

z.sup.-1) (1-Σ

.sub.i=1 to 10 b.sub.i z.sup.-i)/(1-Σ

.sub.i=1 to 10 a.sub.i z.sup.-i)

wherein ##EQU7## and wherein the a_i '"'"'s are a set of predictor coefficients and k₁ is a first reflection coefficient.

9. The device of claim 8 wherein λ

, γ

₁, γ

₂, and γ

₃, are about 0.15, about 0.65, about 0.75, and about 0.15, respectively.

10. The device of claim 6 further comprising:

(a) means for determining if a non-voice signal was the encoded signal that was decoded to generate the decoded signal; and

(b) means for deactivating the means for postfiltering if the encoded signal was a non-voice signal.

11. A method of processing a first encoded signal in a telecommunications network having a plurality of nodes, the method comprising:

(a) receiving the first encoded signal in a first of the nodes;

(b) decoding the first encoded signal to form a first decoded signal;

(c) postfiltering the first decoded signal with a postfilter to form a first postfiltered signal, the postfilter comprising a set of tunable parameters, the set of tunable parameters having preselected values that are based upon an output signal that has been encoded more than once and decoded more than once;

(d) encoding the first postfiltered signal to form a second encoded signal;

(e) transmitting the second encoded signal to a second node;

(f) decoding the second encoded signal in the second node to form a second decoded signal;

(g) postfiltering the second decoded signal with the postfilter to form a second postfiltered signal.

12. A method of processing a speech signal encoded by a predetermined type of encoder, the method comprising the steps of:

(a) decoding the encoded signal with a predetermined type of decoder to generate a decoded signal; and

(b) postfiltering the decoded signal with a predetermined type of postfilter to generate a postfiltered signal, the predetermined type of postfilter operating with a set of tunable parameters and being characterized in that;

(1) if the speech signal is subjected to a plurality of cycles, a first signal having a first quality will result, each cycle comprising sequential encoding, decoding, and postfiltering and using the predetermined type of encoder, the predetermined type of decoder, and the predetermined type of postfilter, respectively, the set of tunable parameters of the predetermined type of postfilter having a first set of values;

(2) if the speech signal is subjected to one cycle, a second signal having a second quality will result, the set of tunable parameters having the first set of values; and

(3) the set of tunable parameters for the predetermined type of postfilter has a second set of values such that if the speech signal is subjected to the plurality of cycles, a third signal having a third quality will result, and if the speech signal is subjected to one cycle, a fourth signal having a fourth quality will result, the third quality being greater than the first quality, the fourth quality being less than the second quality.

13. Apparatus comprising

a low-delay code excited linear predictive decoder which generates decoded speech in response to received encoded speech, and

a postfilter for postfiltering said decoded speech, said postfilter comprising a long-term postfilter and a short-term postfilter,

said long-term postfilter having a transfer function of

14. λ

=0. 15, γ

₁ =0.65, γ

₂ =0.75, γ

₃ =0.15.

16. The invention of claim 14 wherein said second set of values is further such that the mean opinion score of the quality of said signal produced at the output of the third cycle is optimized.

15. A method of processing a speech signal encoded by a predetermined type of encoder, the method comprising the steps of

decoding the encoded signal using a predetermined type of decoder to generate a decoded signal;

and

postfiltering the decoded signal with a predetermined type of postfilter to generate a postfiltered signal in which coding noise in said decoded signal is reduced, the postfilter operating with a tunable set of parameters, said postfilter being such that if said tunable set of parameters were to have a first set of values, a mean opinion score of the quality of said postfiltered signal would be substantially optimized and said coding noise would be reduced by a first amount, characterized in that said tunable set of parameters has a second set of values different from said first set of values, said second set of values being such that said postfilter reduces said coding noise by a second amount that is less than said first amount,

wherein said second set of values is further such that when said postfiltered signal is subjected to two additional cycles of encoding, decoding and postfiltering, each using said predetermined types of encoder, decoder and postfilter, respectively, the mean opinion score of the quality of the signal produced at an output of the third cycle is greater than it would be if said tunable set of parameters were to have said first set of values.

17. A method for use in a system in which a speech signal may be subjected to at least first, second and third sequential encoding/decoding/postfiltering cycles each of which uses a) a predetermined type of encoder, b) a predetermined type of decoder, and c) a predetermined type of postfilter operating with a tunable set of said parameters having a set of values, each cycle generating a respective postfiltered signal, the method comprising the steps of

decoding an encoded signal during an individual one of said cycles to generate a decoded signal;

and

postfiltering the decoded signal in said individual one of said cycles to generate one of said postfiltered signals, said set of values being such that a) the speech quality of said first postfiltered signal is less than it would be if said parameters had another set of values and b) the speech quality of said third postfiltered signal is greater than it would be if said parameters had said another set of values.

18. A method of processing a speech signal encoded by a predetermined type of encoder, the method comprising the steps of decoding the encoded signal using a predetermined type of decoder to generate a decoded signal;

and

postfiltering the decoded signal with a predetermined type of postfilter to generate a postfiltered signal, the postfilter operating with a tunable set of parameters, the predetermined type of postfilter being such that if speech signals are subjected to three sequential encoding/decoding/postfiltering cycles each using a) said predetermined type of encoder, b) said predetermined type of decoder, and c) said predetermined type of postfilter operating with a first set of values of said parameters, said postfilter reduces coding noise in the decoded signal produced during each cycle by a substantially maximum amount with tolerable speech distortion, and the postfiltered signal that is produced in the third cycle has a first level of speech distortion, characterized in that in said postfiltering step, said tunable set of parameters has a second set of values, said second set of values being such that if speech signals are subjected to three of said encoding/decoding/postfiltering cycles with said postfilter operating with said second set of values, said postfilter reduces coding noise in the decoded signal produced during each of the cycles by less than said substantially maximum amount, and the postfiltered signal that is produced in the third cycle has a second level of speech distortion which is less than said first level of speech distortion.