A message is communicated between devices by passing a host signal, such as an audio signal, with the message written into the spectrogram of the host signal. For host audio signals, the message is authored, converted to an image format (if not already in suitable image form) and written into the spectrogram of the audio signal. The resulting audio signal can be communicated in a variety of ways, but in one particular embodiment, the audio signal is played from a speaker (e.g., of a sending mobile device) and captured through the microphone of a receiving device. The spectrogram of the received audio signal is computed and then rendered to a display screen (e.g., the display of a receiving mobile device).