Frame error concealment

1. A frame loss concealment method performed by an audio decoder, the method comprising:

analyzing sign changes of transform coefficients in received frames by determining a number of sign changes between corresponding transform coefficients of bands, each comprising a plurality of coefficients, of consecutive frames that do not contain transient;

accumulating the number of sign changes in corresponding bands of consecutive frames; and

reconstructing a lost frame by copying the transform coefficients from a previous frame, but with reversed signs of transform coefficients in bands having an accumulated number of sign changes that exceeds a predetermined threshold, wherein

threshold depends on the predetermined number of consecutive frames that do not contain transient.

2. The method of claim 1 , wherein signs of copied transform coefficients are randomized if any of two previous frames contains transient.

3. The method of claim 1 , wherein the threshold is assigned a first value for 2 consecutive frames that do not contain transient and a second value for 3 consecutive frames that do not contain transient.

4. The method of claim 3 , wherein when the band comprises 4 coefficients, the first value is 3 and the second value is 6.

5. A computer program product comprising a non-transitory computer readable medium storing a computer program for frame loss concealment, the computer program comprising computer readable code which when run on a processor causes the processor to perform the method of claim 1 .

6. An apparatus for frame loss concealment, comprising:

a memory; and

a processor coupled to the memory, wherein the apparatus is configured to:

analyze sign changes of transform coefficients in received audio frames by determining a number of sign changes between corresponding transform coefficients of bands, each comprising a plurality of coefficients, of consecutive frames that do not contain transient;

accumulate the number of sign changes in corresponding bands of consecutive frames; and

reconstruct a lost frame by copying the transform coefficients from a previous frame, but with reversed signs of transform coefficients in bands having an accumulated number of sign changes that exceeds a selected threshold, wherein

the apparatus is configured to select the threshold based on a number of consecutive frames that do not contain transient.

7. The apparatus of claim 6 , wherein the apparatus is configured to randomize signs of copied transform coefficients if any of two previous frames contains transient.

8. The apparatus of claim 6 , wherein the apparatus is configured to select a first threshold for 2 consecutive frames that do not contain transient and is configured to select a second threshold for 3 consecutive frames that do not contain transient.

9. The apparatus of claim 8 , wherein the first threshold has a value of 3 and the second threshold has a value of 6.

10. An audio decoder comprising the apparatus of claim 6 .

11. A mobile communication device comprising the apparatus of claim 6 .

12. An audio frame loss concealment method for improving audio quality, the method being performed by a decoder and comprising:

receiving a first audio frame, the first audio frame comprising a plurality of bands, each of the plurality of bands of the first audio frame comprising a plurality of transform coefficients;

for each one of the plurality of bands of the first audio frame, storing a sign switch value that is associated with the band and that indicates a number of transform coefficient sign switches;

determining that a next audio frame immediately following the first audio frame is lost; and

as a result of determining that the next audio frame is lost, reconstructing the lost audio frame such that:

the reconstructed lost audio frame comprises a plurality of bands, wherein each one of the plurality of bands of the reconstructed lost audio frame corresponds to one of the plurality of bands of the first audio frame and wherein each band of the reconstructed lost audio frame comprises a plurality of transform coefficients,

for each transform coefficient included in the reconstructed lost audio frame, the absolute value of the transform coefficient is identical to the absolute value of a corresponding transform coefficient included in the first audio frame, and

for each band of the reconstructed lost audio frame, the sign of each transform coefficient included in the band is determined based on the sign switch value that is associated with the band of the first audio frame to which the band of the reconstructed lost audio frame corresponds.

13. An audio frame loss concealment method for improving audio quality, the method being performed by a decoder and comprising:

receiving a first audio frame, the first audio frame comprising a plurality of bands comprising a first band and a second band, each of the plurality of bands of the first audio frame comprising a plurality of transform coefficients;

storing a first sign switch value that is associated with the first band of the first audio frame and that indicates a first number of transform coefficient sign switches;

storing a second sign switch value that is associated with the second band of the first audio frame and that indicates a second number of transform coefficient sign switches;

determining that a next audio frame immediately following the first audio frame is lost; and

as a result of determining that the next audio frame is lost, constructing a replacement audio frame such that:

the replacement audio frame comprises a plurality of bands comprising a first band and a second band, wherein the first band of the replacement audio frame corresponds to the first band of the first audio frame and the second band of the replacement audio frame corresponds to the second band of the first audio frame,

the first band of the replacement audio frame comprises a plurality of transform coefficients, each of which corresponds to one of the transform coefficients included in the first band of the first audio frame,

for each transform coefficient included in the first band of the replacement audio frame, the absolute value of the transform coefficient is identical to the absolute value of its corresponding transform coefficient included in the first band of the first audio frame,

for each transform coefficient included in the first band of the replacement audio frame, the sign of the transform coefficient is determined based on the first sign switch value,

the second band of the replacement audio frame comprises a plurality of transform coefficients, each of which corresponds to one of the transform coefficients included in the second band of the first audio frame,

for each transform coefficient included in the second band of the replacement audio frame, the absolute value of the transform coefficient is identical to the absolute value of its corresponding transform coefficient included in the second band of the first audio frame, and

for each transform coefficient included in the second band of the replacement audio frame, the sign of the transform coefficient is determined based on the second sign switch value.

14. The method of claim 13 , further comprising calculating the first sign switch value, wherein calculating the first sign switch value comprises calculating S n (1)+S n-1 (1), wherein S n (1) is a state value associated with the first band of the first audio frame and S n-1 (1) is a state value associated with a first band of an audio frame that preceded the first audio frame.

15. The method of claim 13 , further comprising calculating the first sign switch value, wherein calculating the first sign switch value comprises:

determining whether x n (1)*x n-1 (1)<0; and

calculating (S n (1)=S n (1)+1) as a result of determining that x n (1)*x n-1 (1)<0, wherein

the first band of the first audio frame comprises a first transform coefficient,

x n (1) is the value of the first transform coefficient of the first band of the first audio frame,

x n-1 (1) is the value of a first transform coefficient included in a first band of an audio frame that preceded the first audio frame,

S n (1) is the first sign switch value.