Recovering asynchronouswatermark tones from speech

A new, low complexity method facilitates low burden embedding and recovery of tonal watermarks in speech. A watermark composed of a periodically extended sequence of sub-audible DTMF tones is added to speech asynchronously, without regard to momentary speech characteristics. It is detected through a combination of a bit manipulation enhancement and a data-directed correlation, ideal for simple hardware implementations. Three methods of bit manipulation enhancement were auditioned and the best selected for further investigation. It showed an average 26 dB processing gain vs. correlation alone, sufficient to detect the asynchronous sub-audible tones by a comfortable margin.