# Synchronization of Acoustic Signals for Steganographic Transmission
## 4 Technique Development and Implementation
### Pattern Insertion Detection  
The synchronization method consists of inserting a synchronizing marker into the speech signal preceding the steganographic transmission. This method was developed for the needs of a steganographic system, which was designed to embed and extract data in a VoIP stream.  
The principle of operation is based on the analysis of the signal that will be the information carrier. The start of steganographic transmission is determined by the detection of the speech signal. The presence of speech in the signal is detected on the basis of the analysis of the values of two parameters [58]:  
Signal power  
\[E_{i}=\frac{1}{N}\sum_{n=0}^{N-1}{(x_{i}(n)w(n))}^{2}, \tag{8}\]  
ZCR (Zero Crossing Rate)  
\[ZCR_{i}=\frac{1}{N}\sum_{n=-\frac{N}{2}}^{\sum\limits_{n=-\frac{N}{2}}{|sign[x _{i}(n)]-sign[x_{i}(n-1)]|}}, \tag{9}\]  
where:  
\(x_{i}(n)\)\(-\)\(i\)-th frame of signal,  
\(w(n)=0.54-0.46\cos\bigl{(}\frac{2\pi m}{N-1}\bigr{)}\)  
\(N\)--number of samples in the signal frame.  
The duration of a single frame was set to 24 ms (192 samples). The implementation assumes that the presence of a speech signal is determined when the power value exceeds \(-\)50 dB and the number of zero crossings coefficient is less than 0.5. If three consecutive  
Figure 12: An exemplary course of the value of the cross-correlation function between the received signal and the DSS signal.  
signal frames meet the above conditions, then these frames are corrected according to the attenuation pattern, the characteristics of which are shown in Figure 13. The characteristics of the attenuation pattern have been empirically established based on the preliminary research of the method. Document [59] states that the permissible IP (Internet Protocol) packet loss during the conversation should not exceed 3%. This value additionally depends on the speech signal codec used during communication. In addition, it assumes the use of the Packet Loss Concealment mechanism (PLC). The adopted attenuation pattern shape reduces the power of the speech signal in the 11 ms window, which is a value similar to the typical frame lengths in speech codecs used in VoIP. In three consecutive signal frames (72 ms) the mentioned reduction of the signal power occurs twice, see Figure 14.  
The next stage, after performing the signal correction procedure (inserting a synchronizing marker into the signal), consists of embedding a portion of steganographic data in the speech signal, according to the algorithm described in the Section 3. The data portion size was set to 16 bits. The algorithm then restarts from scratch detecting the speech signal again.  
In the receiving part, the synchronization procedure consists of continuously checking whether the currently analyzed signal fragment includes a synchronizing marker in its  
Figure 14: Sample waveform of the original signal and the signal corrected according to the attenuation pattern.  
Figure 13: Signal attenuation pattern.  
structure. This process is based on the analysis of the signal energy value and the number of zero crossings according to the Formulas (8) and (9).