Watermarking is a technique used to label digital media, to protect copyright ownership, by hiding information into the signal. Watermarks must be imperceptible and robust against attacks. Besides, watermark robustness against several signal processing techniques can be ensured by exploiting a technique related to spread spectrum communications. In this paper, we propose a procedure based on frequential substitution using a BPSK modulation with an adaptive carrier frequency, the choice of which depends on the original signal features. A secret key, generated during the watermarking process, is used for retrieving hidden information.