An Adaptive Encoder for Audio Watermarking B. GUNSEL, S. SENER,Y. YASLAN Department of Electrical and Electronics Engineering Istanbul Technical University 34469 Maslak, Istanbul TURKEY Abstract: - This paper introduces an adaptive technique for audio watermarking. Almost all of the published audio watermarking methods report a tradeoff between audibility and detectibility, and performs well in digital environment. However, in most of the applications audio should be transmitted through the channels (in analog form) in which the detection performance relies on the strength of embedded watermark signal. This work proposes a new frequency-domain technique which allows to increase watermark strength in a data adaptive way while preserving inaudibility. The method is compatible to MPEG Layer 1 and robust to stereo-to-mono conversions. Key-Words: Physcoacoustic masking, data hiding, watermark decoding. 1 Introduction In the past few years, digital technologies made easy to create and copy multimedia content and to distribute this information over Internet at very low cost. However, these enabling technologies also made it easy to illegal copy, modify and redistribute multimedia data without paying anything. Digital watermarking is a method to embed sideband data, such as author copyright information, into host data thus is seen a solution to the problem of protecting digital media. This paper deals with the protection of audio content. Many techniques have been proposed for audio watermarking, both in time [1] as well as in frequency domains [2]. Although, most of these techniques perform well in theoretical domain, their performance falls down radically over real communication channels. This work proposes an adaptive frequency-domain audio watermarking technique which controls the decoding accuracy at watermark encoding stage. Thus allows to keep watermark strength at adequate level in a data adaptive way, while preserving inaudibility . The method is compatible to MPEG Layer 1 and robust to stereo-to-mono conversions. Section 2 presents the proposed audio watermarking technique. Section 3 describes our decoding scheme for watermarked audio. Performance of the proposed adaptive technique as well as the non-adaptive techniques are reported in Section 4. Section 5 summarizes cnclusions. 2 Adaptive Audio Watermarking Let s t i (n) refers to the nth original audio samples of ith audio frame at state t. In our work, audio data is sampled at 44100 Hz and for each frame the number of samples is N=512. Thus each audio frame is 11 msec in length. Since our adaptive watermarking method requires iterative insertion of one watermark bit, j w , into each audio frame, t is the counter of these insertions thus refers to the state of insertions. The initial value of t is equal to one. Watermark bits can be either 1 or -1, j can be any integer from 1 to L where L is the watermark length . Figure 1 illustrates a general block diagram of a watermarking system. The audio data is processed frame by frame. At each instant, the encoder takes an original audio frame, s t i (n), n=0,1,..,N-1, as its input and transmits the corresponding watermarked frame, s t i WM (n), over the communication channel. At the receiver side, the received watermarked signal, s t i R (n), is decoded by a watermark decoder and the watermark bit j w ) is estimated. Note that the WM decoder only needs secret key, k, to be able to extract watermark data. Figure 2 presents a block diagram of the proposed adaptive watermark encoding scheme. Our watermark embedding technique embeds a modulated secret key, k m , into each audio frame while inserting one watermark bit per frame. Physcoacoustic frequency masking module generates masking thresholds in a data adaptive way. The secret key is modulated in frequency domain by using masking thresholds. Data embedding as performed in