voltage to a fixed offset. The capacitor would slowly recharge and be reset by the next "one' voltage. These improvements meant the modem was able to follow changes in the reference tone no matter what the source of that drifting was, local or remote. It also eliminated the need for a local tunable oscillator, reducing the price of the system. Commercialization In 1976, Felsenstein was visiting "the junk man", Marty Spergel of M&R Electronics. Felsenstein had been talking about building a design for a data terminal he called the Tom Swift Terminal, but the design wasn't ready for development. Felsenstein then asked if Spergel would be interested in a related project, a modem that he had previously designed. Spergel was able to cobble together a version of the modem for a price of $109. The two then sent a copy of the schematics to Les Solomon at Popular Electronics magazine, where it was featured on the cover of its March 1976 issue. M&R offered the Pennywhistle in kit form for $129.95, or fully assembled for $225. It was offered for sale for several years. For comparison, the Novation CAT was listed at $179 in 1981 after several years of falling electronics prices. Description Like other acoustic coupled modems, the Pennywhistle design was dominated by the two large rubber cups on the top of the device that were used to hold the handset of a standard Western Electric phone. The Pennywhistle had a "step" cut into the front of the case where the various control switches were located, and the RS-232 port projected directly out of the front of the case. References Notes Bibliography Kip Crosby, "From Vacuum Tube Flip-Flops to the Singing Altair, an Interview with Lee Felsenstein", The Analytical Engine, Volume 3 Number 1, (November 1995) Modems