# Synchronization of Acoustic Signals for Steganographic Transmission
## 2 Related Work  
There are many published papers dealing with acoustic steganography. For the purposes of this paper, many solutions have been analyzed. The most popular and characteristic methods will be presented, which will sufficiently indicate how complex the problem is to hide information in an acoustic signal. Depending on the place of embedding and extraction of payload in the speech signal in the telecommunications chain, the acoustic steganography algorithms can be divided into three groups [4]. The first variant consists of a certain modification of the operation of the selected speech signal codec. Such a mechanism was used, for example, in [5], where the G.729 codec code book was modified, which allowed for a hidden data rate of 2 kbit/s. In [6], it is proposed to hide information by changing the values of the linear prediction coefficients. The authors presented the results of experiments involving coding of the original signal with various codecs (G.721, GSM, G.728, G.729). In [7] the iSAC codec (internet Speech Audio Codec) was analyzed, hiding 12 bits per frame, which corresponds to 400 bit/s. The second variant presents a situation in which hiding information is performed by modifying the data stream obtained at the output of the speech signal codec. The way of embedding information here is usually done by modifying the appropriate parameter or individual bits in the data stream. These types of solutions are relatively easy to implement and provide a high speed of payload transmission. Here we find a whole range of Least Significant Bits (LSB) methods from the simplest implementations [8; 9; 10; 11] to the more complex [12; 13; 14]. The last, third variant involves embedding the information in the speech signal, just downstream of the analogue to digital converter, operating only on samples of the signal. The embedding of payload takes place with the use of various digital signal processing methods. One way is to hide information by coding or manipulating the phase of the original signal [15; 16; 17; 18; 19]. Information hiding algorithms based on phase modification are characterized by high resistance to signal degrading factors and, depending on the carrier signal and the size of the data block being analyzed, by the hidden transmission rate from single bits to even kilobits per second [19; 20]. A slightly different approach was proposed in [21]. Namely, instead of embedding the information in the phase of the original signal, the authors proposed to embed an OFDM signal in the original signal and encode the payload by changing the phase angle of these additional harmonics. This method is characterized by a data rate of about 40 bit/s and is resistant to degradation factors occurring in real VHF radio links or during speech signal transmission in GSM cellular lines. In the works [17; 22] a procedure of signal synchronization was presented, consisting in shifting, with a certain step, the receiving window in relation to the received signal and an attempt to extract the embedded bits. If periodically repeated maxima indicative of bit detection were obtained, the synchronization was considered to be achieved. In [20; 21; 23] the synchronization mechanism is described, the principle of which is based on the phase analysis of selected harmonics of the signal, in which the steganographic data is embedded. Apart from the methods that modify the signal phase, there are methods that allow you to hide information by changing the amplitude spectrum of the signal [23; 24; 25; 26; 27; 28]. Thesemethods are characterized by a hidden data rate ranging from several bits per second to even several hundred bits per second. They are resistant to lossy compression, filtering, and changing the sampling frequency or analogue to digital conversion. Apart from Fourier transform, often used in acoustic steganography, a number of publications are devoted to other transformations. In [29; 30; 31] the data hiding mechanism was presented, consisting in the quantization of the wavelet transform coefficients.