  The widespread application of audio communication technologies has speeded up
audio data flowing across the Internet, which made it a popular carrier for
covert communication. In this paper, we present a cross-modal steganography
method for hiding image content into audio carriers while preserving the
perceptual fidelity of the cover audio. In our framework, two multi-stage
networks are designed: the first network encodes the decreasing multilevel
residual errors inside different audio subsequences with the corresponding
stage sub-networks, while the second network decodes the residual errors from
the modified carrier with the corresponding stage sub-networks to produce the
final revealed results. The multi-stage design of proposed framework not only
make the controlling of payload capacity more flexible, but also make hiding
easier because of the gradual sparse characteristic of residual errors.
Qualitative experiments suggest that modifications to the carrier are
unnoticeable by human listeners and that the decoded images are highly
intelligible.
