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Both old AC97 and new HDA specifications supports not only sound cards, but also software modems. HDA is more flexible and allows to connect any arbitrary device within its about 50Mbit/s bandwidth. As far as I know, Linux supports AC97 and HDA software modems through the same ALSA API as sound cards. In that case sound goes to user-level for processing via the same paths, while some additional interfaces used for dialing and so on. All modem emulation there implemented by user-level daemon. I don't know much about AC97 to say why in this specific case modem was detected as sound device, but for HDA and on Free/PCBSD that should not happen. I've seen many HDA software modem codecs (separate and integrated with audio) and snd_hda driver properly detected them and doesn't try to attach to modem part. From perspective of completeness it could be interesting to implement modem support, but it is obvious that their time has passed years ago.
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