Patent CA-2140364-A1 (CA)
Title: Rejected frame concealment
Inventor: WIGREN KARL T
IPC: G10L25/78
Filed: 1994-06-02
Abstract: CLAIMS 1. A method in a frame based radio communication system, for concealing rejected frames in a speech decoder of a receiver, which speech decoder is of the source-filter type and is controlled by received parameters representing audio signals transmitted over a communication channel on a frame by frame basis, said method being characterized by: (a) accepting or rejecting received frames depending on whether the parameters contained therein are considered to represent proper audio signals; (b) detecting whether accepted frames represent primarily speech or background sounds; (c) concealing rejected frames by updating the parameters contained therein in accordance with a first concealing algorithm if the last accepted frame primarily represented speech and in accordance-with a second concealing algorithm if the last accepted frame contained primarily background sounds. 2. The method of claim 1, characterized in that said second concealing algorithm comprises locking filter and gain parameters to their values in the last accepted frame. 3. The method of claim 2, characterized by said second concealing algorithm decreasing the output level of the audio signal from said speech decoder when a first predetermined number of consecutive frames have been rejected. 4. The method of claim 3, characterized by said second concealing algorithm repeatedly decreasing the output level of the audio signal from said speech decoder each time said first predetermined number of further consecutive frames have been rejected. 21 5. The method of any of the preceding claims, characterized by said first concealing algorithm interpolating predetermined received parameters from the first rejected frame with corresponding parameters from the last accepted frame. 6. The method of claim 5, characterized by said first concealing algorithm interpolating predetermined received parameters from the current rejected frame with corresponding parameters from the previous rejected frame and decreasing the output level of the audio signal from said speech decoder if at least two consecutive frames have been rejected. 7. The method of claim 6, characterized by said first concealing algorithm muting the output signal from said speech decoder if a second predetermined number of further consecutive frames have been rejected. 8. The method of any of the preceeding claims, characterized in that transmitted parameters are encoded in accordance with a first encoding algorithm if they represent speech and in accordance with a second encoding algorithm if they represent background sounds. 9. The method of any of the preceeding claims, characterized in that received parameters are decoded in accordance with a first decoding algorithm if they represent speech and in accordance with a second decoding algorithm if they represent background sounds. 10. An apparatus in a frame based radio communication system, for concealing rejected frames in a speech decoder of a receiver, which speech decoder is of the source-filter type and is controlled by received parameters representing audio signals transmitted over a communication channel on a frame by frame basis, said apparatus being characterized by: (a) means (20, 28) for accepting or rejecting received 22 frames depending on whether the parameters contained therein are considered to represent proper audio signals; (b) means (34) for detecting whether accepted frames repre sent primarily speech or background sounds; (c) means (32) for concealing rejected frames by updating the parameters contained therein in accordance with a first concealing algorithm (LOOP S) if the last accepted frame primarily represented speech and in accordance with a second concealing algorithm (LOOP B) if the last accepted frame contained primarily background sounds. 11. The apparatus of claim 10, characterized by means (32) for performing said second concealing algorithm by locking filter and gain parameters to their values in the last accepted frame. 12. The apparatus of claim 11, characterized by said means for performing said second concealing algorithm decreasing the output level of the audio signal from said speech decoder (40) when a first predetermined number of consecutive frames have been rejected. 13. The apparatus of claim 12, characterized by said means for performing said second concealing algorithm repeatedly decreasing the output level of the audio signal from said speech decoder each time said first predetermined number of further consecutive frames have been rejected. 14. The apparatus of any of the preceding claims 10-13, characterized by means (32) for performing said first concealing algorithm by interpolating predetermined received parameters in the first rejected frame with corresponding parameters from the last accepted frame. 15. The apparatus of claim 14, character