Method and apparatus for performing packet loss or frame erasure concealment
A method for performing packet loss or Frame Erasure Concealment (FEC) for a speech coder receives encoded frames of compressed speech information transmitted from an encoder. The method determines whether an encoded frame has been lost, corrupted in transmission, or erased, synthesizes properly received frames, and decides on an overlap-add window to use in combining a portion of the synthesized speech signal with a subsequent speech signal resulting from a received and decoded packet, where the size of the overlap-add window is based on the unavailability of packets. If it is determined that an encoded frame has been lost, corrupted in transmission, or erased, the method performed an overlap-add operation on the portion of the synthesized speech signal and the subsequent speech signal, using the decided-on overlap-add window.
1. A method for creating an output signal in response to receiving one or more unexpected packets, where an unexpected packet includes a packet that is lost, corrupted, erased or delayed, and where an expected packet includes a packet that is not lost, corrupted, erased or delayed, the method comprising:
generating one or more synthesized audio signal segments, R j-1 , R j-2 , . . . R j-k , when a corresponding one or more packets are unexpected;
decoding an expected packet to form an audio signal segment, R j ;
performing an overlap-add operation on the audio signal segment, R j , to modify a portion of said the audio signal segment R j-1 with a portion of the audio signal segment R j , to form a modified audio signal segment, R′ j , where a duration of the portions of both the audio signal segment R j-1 and the audio signal segment R j is a function of k; and
outputting the modified audio signal segment R′ j as the output signal.
2. The method of claim 1 where the duration is
⌈
1
4
P
+
4
(
k
-
1
)
⌉
10
msec
msec
for k>1, where P is a pitch period of the audio signal segment R j-1 , and ┌•┐ 10msec designates a 10 msec ceiling.
3. The method of claim 1 where, when a duration of the one or more packets that are unexpected exceeds 10 msec, the audio signal segment R j-1 is attenuated before performing the overlap-add operation.