Voice Modems

voice modems 2026-04-26 If you've done much with modern cellphones, you've probably noticed just how odd the architecture can be around audio. Specifically, I mean call audio: modern smartphones have made call audio less of a special case (mostly by just becoming more complicated in general), but in older phones you would often find arrangements where the cellular modem 1 had direct analog audio to the microphone and speaker, perhaps via some switching to share amplifiers. That design meant that the cellular modem functioned basically as a completely independent device, a fully-capable "cellular phone" with the ability to make and receive voice calls. The role of the rest of the smartphone, and its operating system, was just to provide control messages for starting and ending calls. In modern phones the audio path to and from the modem is digital and it's more integrated into the operating system audio service, but still not fully. You might have noticed, for example, that it is excessively difficult to record call audio on most phones. Regulatory and liability pressures are one reason for this, but another is that it's actually kind of difficult: there may not be any physical way for software running on the main processor to receive audio from the cellular modem. The designer has to put in explicit effort to make that work, effort that only became common more recently to facilitate automatic transcription—and VoLTE, a whole complication that I will simply ignore for the sake of a cleaner historical narrative. You come here to read about old phones, not new ones. You've probably read enough of my writing to know where this is going: the design of cellular radios, which assume call audio to be part of Their Exclusive Domain, is a legacy of an age-old architectural decision traceable to the original Hayes Smartmodem. It relates to a feature of modems that was widely available, but sparsely used, for much of the PC revolution. The details are odd! First, for context, let's recede into our mind palaces and travel back to the 1980s. AT&T-designed modems like the Bell 103 had created a standardized family of protocols for data over voice lines, and a company called Hayes introduced a Bell 103-like implementation called the Smartmodem. The Smartmodem was quite successful on its own, but it was more significant for having introduced a common control interface between the modem and the computer. Previous modems had acted as transparent devices that expected Something Else to perform call setup tasks, while the Hayes Smartmodem could pick up the line and dial all on its own. That required that the computer send commands to the modem to configure and start a call. Hayes designed a simple scheme for sending commands to the modem and switching it in and out of transparent data mode, and that protocol was then widely copied by other modem manufacturers. You could call it the "Hayes command set," and older documents often do, but these days it's more commonly known by the two characters that prefix most commands: the AT protocol. From its origin in 1981, AT has shown remarkable staying power. Virtually all computer-connected modems, to this very day, continue to use AT commands for basic configuration. Likewise, the basic architecture of the Smartmodem persists: the Smartmodem connected to the host computer using a single RS-232 link that switched between carrying control messages and data. The very latest 5G modems still work the same way,…

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