  In this paper, we propose a novel semantic digital analog converter (sDAC)
for the compatibility of semantic communications and digital communications.
Most of the current semantic communication systems are based on the analog
modulations, ignoring their incorporation with digital communication systems,
which are more common in practice. In fact, quantization methods in traditional
communication systems are not appropriate for use in the era of semantic
communication as these methods do not consider the semantic information inside
symbols. In this case, any bit flip caused by channel noise can lead to a great
performance drop. To address this challenge, sDAC is proposed. It is a simple
yet efficient and generative module used to realize digital and analog
bi-directional conversion. On the transmitter side, continuous values from the
encoder are converted to binary bits and then can be modulated by any existing
methods. After transmitting through the noisy channel, these bits get
demodulated by paired methods and converted back to continuous values for
further semantic decoding. The whole progress does not depend on any specific
semantic model, modulation methods, or channel conditions. In the experiment
section, the performance of sDAC is tested across different semantic models,
semantic tasks, modulation methods, channel conditions and quantization orders.
Test results show that the proposed sDAC has great generative properties and
channel robustness.
