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(詳細はanswering machine applications. Similar to the Hayes command set used for data modems, in which the host PC commands the modem via a series of commands known as AT commands, there exists a well-defined set of common voice AT commands that are somewhat consistent throughout the industry. ==Implementation problems== Because voice mode is not the typical use for a modem, many modems on the market have poor or buggy support for their voice modes. Characteristics of a good voice modem depend greatly upon the intended application, and include: * Reliable operation. Many modems simply "lock up" or crash the host PC - though this is more common with Winmodems. Others have flow control bugs and other implementation bugs, possibly causing calls to hang, audio to skip, or audio to keep playing after an attempted abort. * Good audio characteristics. Some modems have an uncorrectably low signal volume or produce audio noise. Some modems are unable to recognize all but the best DTMF signals. Some modems do a poor job of recording, or detecting and reporting silence or the end-of-call voltage reversal, which some applications need. * Support for Caller ID, if needed. "Type-1 caller ID" as used in North America is missing from the vast majority of modems. Nearly all modem chipsets support caller ID, but because the typical dial-up Internet user doesn't need caller ID, the extra components needed to support caller ID are often omitted for cost reasons. * Support for multiple instances. The drivers for many internal modems (typically Winmodems) cannot tolerate more than one of the same device inside a single computer. Symptoms of incompatibility include crashes, blue screens of death, or simple inoperability of all but a single modem. External RS232-based (serial) modems do not have this limitation because each modem contains its own microprocessor and is unaware of other modems on the same host. USB modems may or may not have this problem, because some USB modems are simply serial modems with a "USB-to-serial" converter chipset (in which case there should be no problem), and other USB modems are "host-controlled" and are essentially externally attached Winmodems (in which case the problem may persist). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Voice modem command set」の詳細全文を読む スポンサード リンク
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