In echo data hiding, the secret message is embedded into which element as an echo?

• A. Phase spectrum of a digital signal
• B. Pseudo-random signal
• C. Pseudo-spectrum signal
• D. Cover audio signal

Answer: (D)

In echo data hiding, the secret message is carried by modifying the original audio itself. You create a delayed, attenuated copy of the cover audio and add it back to the original signal. This superimposed echo contains the encoded bits, so the hidden data resides in the cover audio signal. The other options refer to spectral components or separate signals, but the technique uses the actual audio waveform as the carrier, with the echo serving as the data-carrying feature. Detecting the message involves analyzing the echo pattern—its delay and amplitude—within the cover audio.