General Description
Often, the term "Winmodem" or "softmodem" is used in a derogatory manner, as opposed to hardware or "real" modems. The argument is that a softmodem isn't a real modem at all, but rather a simple electrical interface between computer and phone line, limiting itself to very basic functions such as voltage/current adaptation and functioning essentially as a DAC/ADC, much like a sound card which handles pure PCM and analog signals from and to the telephone line, while the host's CPU does the actual job of synthesizing or analyzing all necessary waveforms (carrier, dialing tones) and applying all necessary DSP techniques (FSK, QAM, PSK etc.) to a "virtual" signal, in order to encode and decode inbound or outbound data.
This means that at least the simplest softmodem is nothing more than a special purpose sound card with mono DAC/ADC's and a telephone line interface, while all actual signal encoding/decoding (as well as compression/decompression, error correction etc.) is done by the host machine, hence the terms HAM (Host Assisted Modulation) or HSP (Host Signal Processing). Many of the latest softmodem chipsets, e.g., the Intel Ambient, are even built around a standard AC'97 audio codec interface.
Read more about this topic: Softmodem
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