In many studies on steganography, I see a lot of studies using PSNR 30dB as a reference when evaluating imperceptibility, but I can't find an exact reference. Everyone "generally says..." but I'm curious about the exact reference. If the PSNR is more than 30dB, it cannot be distinguished by the human visual system. Is there a result of someone doing a blind test? Of course, I know that there are some discussions about using SSIM or others because PSNR doesn't reflect HVS properly. Please teach me.