Military Communications and Information Technology: A Trusted ...
Military Communications and Information Technology: A Trusted ... Military Communications and Information Technology: A Trusted ...
498 Military Communications and Information Technology... shift against the start of data decoding. In the described steganographic system data are modulated with the use of differential coding of binary patterns in the host signal spectrum. II. Description of algorithm A. Watermark embedder – principle of operation The schematic of the watermark embedder is provided in Fig. 1. Figure 1. The watermark embedder Embedding the watermark in the background of the host audio signal takes place on the level of frequency. The binary symbols (0 or 1) generated in the Binary Signature (BS) unit are assigned specific patterns, dividing the host signal spectrum into 5 subspectra. Depending on the specific pattern, the spectral line values of the host signal in particular subspectra increase or drop. In terms of the robustness of steganographic signal against blurring it is desirable for the spectrum amplitude modification to be as high as possible. On the other hand, in order to ensure the transparency of the watermark signal in the background of the host signal, the modification of spectral line values cannot take place indiscriminately. This problem has been solved by correcting the host signal spectrum amplitude to the level of Just Noticeable Difference (JND). In psychoacoustics the JND level is defined as the distortion level noticed in audio tests by 50% of listeners with normal hearing. In the presented algorithm, the JND level is determined with the use of the Human Auditory System (HAS) model by establishing the minimum masking threshold for the host signal LT min , using the MPEG psychoacoustic algorithm. Establishing the masking threshold for the host signal was carried out on the basis of an 8-stage signal processing procedure, compliant with ISO CD 11172-3 (MPEG–1) standard, and a single-stage correction process, compliant with the description in [10]. Detailed descriptions of the stages of establishing the masking threshold are provided in table 1.
Chapter 4: Information Assurance & Cyber Defence 499 Table I. Signal processing stages in MPEG psychoacoustic algorithm Step I Step II Step III Step IV Step V Step VI Step VII Step VIII Calculation of the FFT for time to frequency conversion Determination of the sound pressure level for each subband Determination of the threshold in quiet (absolute threshold) Finding of the tonal and non-tonal components of the audio signal Decimation of the maskers, to obtain only the relevant maskers Calculation of the individual masking thresholds Determination of the global masking threshold Determination of the minimum masking threshold in each subband Fig. 2 presents the method of correction of spectral lines of the host signal for hiding bit ‘0’. Figure 2. Watermark embedder – principle of operation Clearly visible is the binary pattern corresponding to bit ‘0’. In this case, it is the alternating sequence of the {1 0 1 0 1} pattern, where {1} stands for the increase of the spectral line amplitude and {0} stands for the reduction. For the sequence of the ‘0’ pattern and in the case of the spectral line value being lower than LT min level, the host signal is clearly damped. As previously mentioned, the correction of spectral lines is determined by the value of the LT min level. However, such manner of correction would not include all spectral lines in the analyzed subspectra. In the proposed algorithm the values of spectral line amplitudes that cannot be corrected against the LT min level are increased/reduced by the experimentally established value of 0.4 dB. The signal generated in the Orthogonal Frequency Division Multiplexing (OFDM) is a composite of 14 harmonics. A single harmonic is generated in accordance with:
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498 <strong>Military</strong> <strong>Communications</strong> <strong>and</strong> <strong>Information</strong> <strong>Technology</strong>...<br />
shift against the start of data decoding. In the described steganographic system<br />
data are modulated with the use of differential coding of binary patterns in the host<br />
signal spectrum.<br />
II. Description of algorithm<br />
A. Watermark embedder – principle of operation<br />
The schematic of the watermark embedder is provided in Fig. 1.<br />
Figure 1. The watermark embedder<br />
Embedding the watermark in the background of the host audio signal<br />
takes place on the level of frequency. The binary symbols (0 or 1) generated<br />
in the Binary Signature (BS) unit are assigned specific patterns, dividing<br />
the host signal spectrum into 5 subspectra. Depending on the specific pattern,<br />
the spectral line values of the host signal in particular subspectra increase or<br />
drop. In terms of the robustness of steganographic signal against blurring it is<br />
desirable for the spectrum amplitude modification to be as high as possible. On<br />
the other h<strong>and</strong>, in order to ensure the transparency of the watermark signal<br />
in the background of the host signal, the modification of spectral line values<br />
cannot take place indiscriminately. This problem has been solved by correcting<br />
the host signal spectrum amplitude to the level of Just Noticeable Difference (JND).<br />
In psychoacoustics the JND level is defined as the distortion level noticed in audio<br />
tests by 50% of listeners with normal hearing. In the presented algorithm,<br />
the JND level is determined with the use of the Human Auditory System (HAS)<br />
model by establishing the minimum masking threshold for the host signal LT min ,<br />
using the MPEG psychoacoustic algorithm. Establishing the masking threshold<br />
for the host signal was carried out on the basis of an 8-stage signal processing procedure,<br />
compliant with ISO CD 11172-3 (MPEG–1) st<strong>and</strong>ard, <strong>and</strong> a single-stage<br />
correction process, compliant with the description in [10]. Detailed descriptions<br />
of the stages of establishing the masking threshold are provided in table 1.
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