A new method for hiding digital data in the bitstream of an ACELP speech codec is proposed in this paper. The key element of our method is an alternative search strategy for the ACELP codebook which allows for joint data hiding and speech coding. The concept has been examplarily applied to the AMR speech codec (12.2 kbit/s mode) and it is shown that steganographic data can be reliably transmitted at a rate of up to 2 kbit/s both with a negligible effect on the subjective quality of the coded speech and with reasonable computational complexity. Apart from data hiding, it is further pointed out that our method can also be exploited to reduce the codec bit rate.
We consider the problem of transmitting a wideband speech signal with a cut-off frequency of fc = 7 kHz over a standardized narrowband (fc = 3.4 kHz) communication link in a backwards compatible manner. In a previous contribution [1] we have shown that backwards compatibility can be achieved by using digital watermarking: We embedded compact side information about the missing high frequency band (3.4 − 7 kHz) into the narrowband speech signal. Here, we present a related system which is especially tailored to state-ofthe-art narrowband speech communication networks such as GSM or UMTS. Therefore, we propose an improved low bit rate bandwidth extension algorithm along with a robust watermarking scheme for CELP-type speech codecs. The practical relevance of our system is shown by speech quality evaluations and by link-level simulations for the "Enhanced Full Rate Traffic Channel" (TCH/EFS) of the GSM cellular communication system.
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