Speech transcoding in GSM networks

1. A method of transcoding an Enhance Full Rate (EFR) 12.2 Kbps encoded frame into an Adaptive Multi-Rate (AMR) 12.2 Kbps encoded frame for use by a first gateway, the method comprising:

receiving the EFR 12.2 Kbps encoded frame from a first codec;

determining if the EFR 12.2 Kbps encoded frame is a Silence Insertion Descriptor (SID) frame;

if the EFR 12.2 Kbps encoded frame is determined to be the SID frame, the method further comprising:

calculating and quantizing an average log energy for the frame;

updating gain predictor values for the frame;

calculating index of quantized average Line Spectral Pair (LSP) by split three VQ and index of lowest prediction residual energy; and

setting frame type to indicate an AMR SID frame.

2. The method of claim 1 , wherein prior to the determining if the EFR 12.2 Kbps encoded frame is the SID frame, the method further comprising:

saving the LSP of fourth subframe; and

using post-filtered synthesis of the frame to calculate log energy based on the frame energy.

3. The method of claim 1 further comprising:

determining if the EFR 12.2 Kbps encoded frame is a No Data (NT) frame;

if the EFR 12.2 Kbps encoded frame is determined to be the NT frame, the method further comprising:

setting a frame type to 15; and

setting a frame quality indicator to 1.

4. The method of claim 1 further comprising:

determining if the EFR 12.2 Kbps encoded frame is a transition frame from SID or No Data (NT) to speech;

if the EFR 12.2 Kbps encoded frame is determined to be the transition frame, the method further comprising:

calculating fixed codebook gain values using save gain predictor values; and

updating EFR parameter list with the fixed codebook gain values.

5. The method of claim 4 further comprising:

determining if the EFR 12.2 Kbps encoded frame is a speech frame;

if the EFR 12.2 Kbps encoded frame is determined to be the speech frame, the method further comprising: transmitting the speech frame unaltered.

6. A transcoder for transcoding an Enhance Full Rate (EFR) 12.2 Kbps encoded frame into an Adaptive Multi-Rate (AMR) 12.2 Kbps encoded frame, the transcoder comprising:

a receiver configured to receive the EFR 13.2 Kbps encoded frame from a first codec;

wherein the transcoder is configured to determine if the EFR 12.2 Kbps encoded frame is a Silence Insertion Descriptor (SID) frame, and wherein if the EFR 12.2 Kbps encoded frame is determined to be the SID frame, the transcoder is further configured to calculate and quantize an average log energy for the frame, update gain predictor values for the frame, calculate index of quantized average Line Spectral Pair (LSP) by split three VQ and index of levied lowest prediction residual energy, and set frame type to indicate an AMR SID frame.

7. The transcoder of claim 6 , wherein prior to the determining if the EFR 12.2 Kbps encoded frame is the SID frame, the transcoder is further configured to save the LSP of fourth subframe, and use post-filtered synthesis of the frame to calculate log energy based on the frame energy.

8. The transcoder of claim 6 , wherein the transcoder is further configured to determine if the EFR 12.2 Kbps encoded frame is a No Data (NT) frame, and if the EFR 12.2 Kbps encoded frame is determined to be the NT frame, the transcoder is further configured to set a frame type to 15, and set a frame quality indicator to 1.

9. The transcoder of claim 6 , wherein the transcoder is further configured to determine if the EFR 12.2 Kbps encoded frame is a transition frame from SID or No Data (NT) to speech, and if the EFR 12.2 Kbps encoded frame is determined to be the transition frame, the transcoder is further configured to calculate fixed codebook gain values using save gain predictor values, and update EFR parameter list with the fixed codebook gain values.

10. The transcoder of claim 9 , wherein the transcoder is further configured to determine if the EFR 12.2 Kbps encoded frame is a speech frame, and if the EFR 12.2 Kbps encoded frame is determined to be the speech frame, the transcoder is further configured to transmit the speech frame unaltered.

11. A method of transcoding an Adaptive Multi-Rate (AMR) 12.2 Kbps encoded frame into an Enhance Full Rate (EFR) 12.2 Kbps encoded frame for use by a first gateway, the method comprising:

receiving the AMR 12.2 Kbps encoded frame from a first codec;

determining if the AMR 12.2 Kbps encoded frame is a Silence Insertion Descriptor (SID) frame;

if the AMR 12.2 Kbps encoded frame is determined to be the SID frame, the method further comprising:

calculating average of Line Spectral Frequency (LSF) of the frame, quantizing and splitting by five (5) matrix quantization;

calculating unquantized fixed codebook gain of the frame based on energy of Linear Prediction (LP) residual signal; and

setting a frame type to indicate an EFR SID.

12. The method of claim 11 further comprising:

determining if the AMR 12.2 Kbps encoded frame is a speech frame;

if the AMR 12.2 Kbps encoded frame is determined to be the speech frame, the method further comprising:

calculating reference Line Spectral Frequency (LSF) vector by averaging a history of quantized LSF vectors; and

updating fixed codebook gain history with fixed codebook gains for the speech frame.

13. A transcoder of transcoding an Adaptive Multi-Rate (AMR) 12.2 Kbps encoded frame into an Enhance Full Rate (EFR) 12.2 Kbps encoded frame, the transcoder comprising:

a receiver configured to receive the AMR 12.2 Kbps encoded frame from a first codec;

wherein the transcoder is configured to determine if the AMR 12.2 Kbps encoded frame is a Silence Insertion Descriptor (SID) frame, and if the AMR 12.2 Kbps encoded frame is determined to be the SID frame, the transcoder is further configured to calculate average of Line Spectral Frequency (LSF) of the frame, quantizing and splitting by five (5) matrix quantization, calculate unquantized fixed codebook gain of the frame based on energy of Linear Prediction (LP) residual signal, and set a frame type to indicate an EFR SID.

14. The transcoder of claim 13 , wherein the transcoder is configured to determine if the AMR 12.2 Kbps encoded frame is a speech frame, and if the AMR 12.2 Kbps encoded frame is determined to be the speech frame, the transcoder is further configured to calculate reference Line Spectral Frequency (LSF) vector by averaging a history of quantized LSF vectors, and update fixed codebook gain history with fixed codebook gains for the speech frame.