This thesis gives the description of transcoder for Tandemless Operation between AMR and 13kbps Qcelp which are the standard speech coder of IMT-2000 services.
A simple solution for the compatibility problem between two different coding standards is obtained by the Tandem Operation (TO). The tandem operation generates a speech signal by using a decoder from one standard and then encodes again the signal by the other standard. Therefore the speech signal undergoes encoding decoding process twice and these results in quality loss and extra delay.
To avoid these degradations, Tandem Free Operation (TFO) is used in some mobile networks. TFO passes speech-encoded packet from one Mobile station (MS) to Another MS without generating intermediate PCM through a gateway. But, TFO properly works only when two MS has a same speech coder.
These problems can be reduced by the Transcoder for Tandemless Operation (TLO). Transcoder for TLO directly converts the speech packet from one to the other coder. Whereas the packet conversions are performed in parameter domain, we can get less degraded speech with TLO than speech with TO.
The proposed transcoding algorithm for TLO is composed of two main parts: (1) speech parameter conversions of LSP, Pitch, Codebook, (2) rate adaptation from one to another coder. The proposed algorithm reduces total delay and loss of quality in end-to-end mobile communication. Objective quality evaluation confirmed that the proposed algorithm introduces less degraded speech compared to TO.
