## 4 Validation of Proposed Method
### Setting of Embedding Strength  
The embedding strength level \(L_{\text{all}}\) in the proposed method is determined to minimize the PESQ, LSD, and BER simultaneously. As shown in Equation (6), the embedding strength is calculated from the power level of the host signal, the watermarked signal, and the embedding-strength level \(L_{\text{SSL}}\).  
Bit detection and sound quality tests were carried out to determine the optimal \(L_{\text{SSL}}\). In the sound quality test, the PESQ International Telecommunication Union Telecommunication Standardization Sector ( ITU-T) P.862 and LSD were used as objective measures, as in our previous study [15]. In speech watermarking, it is generally helpful if the security code can be detected with a bit rate of 6 bps and a BER of less than or equal to 10%.  
We carried out the sound quality tests (PESQ and LSD) to determine how well the watermarked signal was perceived. PESQ is the perceived sound quality of the watermarked signal relative to the original signal, expressed as mean opinion scores (MOS). The MOS varies on a scale of 1 (bad), 2 (poor), 3 (fair), 4 (good), and 5 (excellent). Typically, the PESQ threshold for speech watermarking is 3 (fair or slightly annoying). LSD is defined as  
\[\text{LSD}(X,Y)=\sqrt{\frac{1}{Q}\sum_{q=1}^{Q}\biggl{(}10\log_{10}\frac{|X( \omega,q)|^{2}}{|Y(\omega,q)|^{2}}\biggr{)}^{2}}, \tag{16}\]  
where \(X(\omega,q)\) and \(Y(\omega,q)\) are short-term Fourier transforms of the original signal and watermark signal, \(x(t)\) and \(y(t)\), in the \(q\)-th frame. Typically, the LSD criterion for speech watermarking is less than or equal to 1 dB.  
Figure 9 shows the results of PESQ, LSD, and BER from the blind detection process with frame synchronization of the front-side form, where the horizontal axis represents embedding-strength level \(L_{\text{SSL}}\) and the vertical axis represents (a) PESQ, (b) LSD, and (c) BER. As we can see, the BER decreases as \(L_{\text{SSL}}\) increases, and distortions increase as \(L_{\text{SSL}}\) increases. The \(L_{\text{SSL}}\) was determined to be \(-15\) dB when the bit rate was 4 and 8 bps. At this time, the mean \(L_{\text{all}}\) was \(-43.7\) dB. When the bit rate was 16 bps, there was no optimal \(L_{\text{all}}\). Thus, the \(L_{\text{SSL}}\) was determined to be \(-10\) dB, in which BER \(<10\%\), LSD \(<2\) dB, and PESQ 2 MOSs, at this bit rate. Hence, the mean \(L_{\text{all}}\) was \(-38.7\) dB.  
Figure 10 shows the results of PESQ, LSD, and BER from the blind detection process with frame synchronization of the back-side form, where the horizontal axis represents embedding-strength level \(L_{\text{SSL}}\) and the vertical axis represents (a) PESQ, (b) LSD, and (c) BER. We can see here that the BER decreases as \(L_{\text{SSL}}\) increases, and distortions increase  
Figure 8: BER of proposed blind detection with frame synchronization regarding bit rate: (**a**) front-side and (**b**) back-side forms. Each line shows the results for a different embedding-strength level \(L_{\text{SSL}}\).  
as \(L_{\text{SSL}}\) increases. The \(L_{\text{SSL}}\) was determined to be \(-15\) dB when the bit rate was 4 and 8 bps. At this time, the mean \(L_{\text{all}}\) was \(-43.7\) dB. When the bit rate was 16 bps, there was no optimal \(L_{\text{all}}\). Thus, the \(L_{\text{SSL}}\) was determined to be \(-10\) dB, in which BER \(<10\%\), LSD \(<2\) dB, and PESQ 2 MOSs, at this bit rate. Hence, the mean \(L_{\text{all}}\) was \(-38.7\) dB.  
After applying the two embedding processes for the blind detection process with the frame synchronization process, the optimal embedding-strength level \(L_{\text{SSL}}\) was determined to be \(-15\) dB.  
Figure 10: Relationship between embedding-strength level \(L_{\text{SSL}}\) of back-side form: (**a**) LSD, (**b**) PESQ, and (**c**) BER.  
Figure 9: Relationship between embedding-strength level \(L_{\text{SSL}}\) of front-side form: (**a**) LSD, (**b**) PESQ, and (**c**) BER.