# Synchronization of Acoustic Signals for Steganographic Transmission
## 5 Results  
### Signal Quality Assessment  
The methods of assessing the quality of audio signals can be divided into two main groups subjective and objective methods. The objective assessment of the signal quality was carried out based on the ITU-T P.862 PESQ (Perceptual Evaluation of Speech Quality) [62]. Measurements were carried out using a dedicated MultiDSLA tester [63; 64]. Documents ITU-T P.862 PESQ [62] and ITU-T P.862.3 [65] describe a number of requirements related to the selection of signals constituting the research material. The ITU-recommended test signals are contained in Annex B to ITU-T P.501 [66]. Samples for the Polish language were used in the research. The MultiDSLA tester assesses the quality of the signals and returns the results in the form of several values:  
* \"raw\" data (PESQ raw score or PESQ score);
* PESQ LQ (Listening Quality);
* P.862.1 (MOS--LQO, Mean Opinion Score Listening Quality Objective);
* P.862.2 (PESQ--WB). Raw data changes range is from \(-0.5\) to \(4.5\). These data are then transformed into the remaining results, the values of which range from \(1\) to \(4.5\) and correspond to the values on the MOS scale [67; 68; 69]. The subjective evaluation of the quality of the signals was based on the recommendations of ITU-R BS.1116-3 [70]. As with the objective tests, the document ITU-R BS.1116-3 describes a number of considerations on how to conduct the test. The study was conducted on a group of 20 students. A set of 16 test signals was used [71]. The test procedure consisted in listening by the listener of the original (undistorted) signal, marked as signal A, and two copies of this recording, marked as signals B and C, where, randomly, one of these signals is reference signal A and the other signal is distorted signal subject to evaluation. These types of tests are often referred to as \"ABC (ABX) tests\" or \"forced choice tests\". The result of the assessment is the value in the SDG (Subjective Degradation Grades) scale, the range of possible scores is from \(-4\) to \(4\). A positive value means that the listener has incorrectly determined which of the two signals he/she listens to is distorted. Figures 15 and 16 show the results of the objective evaluation of signal quality carried out on the basis of ITU-T P.862 PESQ recommendations [62]. The line plotted in the drawings sets the reference level and represents the quality rating obtained by embedding steganographic information in the original signal. All of the developed methods, except for the MES method, reduce the evaluation of the signal quality. However, this is a slight reduction, amounting to about 6% for both analyzed values of the coefficient \(K_{min}\). Summing up, it is worth adding that in all studies, the mean value of the MOS scale greater than 3 was obtained each time, which in the case of special applications, especially military, is a highly satisfactory result [72]. Figures 17 and 18 show the results of the conducted listening tests. Again, the straight line defines the reference threshold and represents the SDG score obtained from tests that compared the original signal and a signal where only the steganographic information was embedded. This result is in line with the SDG assessment for the MES method. In the case of the value of the \(K_{min}=500\) coefficient, individual methods of synchronization cause a deterioration of the SDG assessment in the range from 34% to 90% compared to the assessment for the MES method. However, it is worth noting that the determined average values do not take values lower than \(-1\), which should be perceived as an inaudible signal distortion. For \(K_{min}=1250\) we can observe a much smaller, relative decrease in the SDG rating ranging from 15% to 40%. However, in this scenario, the benchmark is an SDG score of around \(-0.8\). As a result, the SDG score for the DSS method slightly exceeds the value of \(-1\). Figure 16: MOS evaluation for various synchronization methods, \(K_{min}=1250\).