Modification of Pitch Parameters in Speech Coding for Information Hiding

The article presents a method of using F0 parameter in speech coding to transmit hidden information. It is an improved approach, which uses interpolation of pitch parameters instead of transmitting exact original values. Using an example of the Speex codec, we describe six variants of this method, named originally as HideF0, and we compare them by analyzing the capacity of the hidden channels, their detectability and the decrease in quality introduced by pitch manipulation. In particular, we perform listening tests using 20 participants to verify how perceptible the pitch manipulations are. The results are presented and discussed. We prove that minor modifications of pitch parameters are hardly perceptible, what can be used to create hidden transmission channels. One of the best proposed variants, called HideF0-FM, is shown to enable hidden transmission at the bitrate of over 120 bps at no speech quality degradation at all. Higher bitrates are also possible, only with minor quality degradation and limited detectability.