A watermark encoding system encodes an audio signal with both a strong and
a weak watermark. The strong watermark identifies the content producer
and is designed to survive all typical kinds of processing and malicious
attacks. The weak watermark identifies the content as an original and is
designed to be significantly removed as a result of most normal signal
processing (other than A/D and D/A). The watermark encoding system has a
converter to convert an audio signal into frequency and phase components
and a mask processor to determine a hearing threshold for corresponding
frequency components. The watermark encoding system also has a pattern
generator to generate both the strong and weak watermarks and a watermark
insertion unit to selectively insert either the strong or weak watermark
into the audio signal. The watermark insertion unit adds the strong
watermark to the audio signal when the signal exceeds the hearing
threshold by a buffer value (e.g., 1-8 dB) and adds the weak watermark
insertion unit when the signal falls below the hearing threshold by the
buffer value. When the signal falls within the buffer area about the
hearing threshold, the insertion unit takes no action. A watermark
detecting system is equipped with a watermark detector that determines
which block interval of the watermarked audio signal contains a watermark
pattern and if the strong or weak watermark is present in that block
interval of the signal.
