Pennywhistle modem
Encyclopedia
The Pennywhistle was an early acoustic coupler
modem
originally designed and built by Lee Felsenstein
in 1973, and later commercialized and offered for sale in 1976. It was one of the earliest modems available for hobbyist computer users. Like most acoustic coupler modems, the Pennywhistle was replaced by the Hayes Smartmodem and similar models from the early 1980s.
bulletin board system
, Lee Felsenstein had found an Omnitech modem ("or something like that"). Designed to operate at rates as high as 300 bits per second (bps), the modem was able to change its speed to match conditions or differences in the modems at either end. In general it was good for only 100 bps, the speed that was used for much of its operational life. The modem was attached to a Teletype
Model 33 ASR machine at Leopold's Records in Berkeley, California
and connected to the SDS 940
mainframe computer
in San Francisco.
Felsenstein was unimpressed by the Omnitech design, especially its price of $300, and convinced himself he could design a better version. He found that one half of the design problem was easy; generating the proper tones for transmission was simple. The other half of the problem was much harder; listening to the incoming signal and processing the tones back out. The traditional solution was to generate a local reference tone and compare it to the incoming signal, but this was subject to many problems, from noise
or distortion
on the line, to drifting of the local tone due to events as mundane as temperature changes.
Felsenstein found two key improvements that led to a dramatically less expensive and more reliable design. The first was to ignore the idea of a variable baud rate, which was useful in some contexts, but not when used purely for data communications over known-good lines. The other improvement, which would prove key to the design, was to use the incoming signal itself as the reference tone. While working at Ampex
, Felsenstein had learned that the signal would always return to a "one" tone between sending bits of data; his new design looked for these signals and used them to resynchronize a local phase locked loop (PLL). The system generated the "zero" tone as a fixed difference from the reference signal coming from the PLL.
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. Felsenstein built several copies of the design as Community Memory added more terminals.
The two then sent a copy of the schematic
s 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.
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.
Acoustic coupler
In telecommunications, the term acoustic coupler has the following meanings:# An interface device for coupling electrical signals by acoustical means—usually into and out of a telephone instrument....
modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
originally designed and built by Lee Felsenstein
Lee Felsenstein
Lee Felsenstein is an American computer engineer who played a central role in the development of the personal computer...
in 1973, and later commercialized and offered for sale in 1976. It was one of the earliest modems available for hobbyist computer users. Like most acoustic coupler modems, the Pennywhistle was replaced by the Hayes Smartmodem and similar models from the early 1980s.
Origins
As part of the effort that would lead to the Community MemoryCommunity Memory
Community Memory was the first public computerized bulletin board system. Established in 1973 in Berkeley, California, it used an SDS 940 timesharing system in San Francisco connected via a 110 baud link to a teleprinter at a record store in Berkeley to let users enter and retrieve messages...
bulletin board system
Bulletin board system
A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...
, Lee Felsenstein had found an Omnitech modem ("or something like that"). Designed to operate at rates as high as 300 bits per second (bps), the modem was able to change its speed to match conditions or differences in the modems at either end. In general it was good for only 100 bps, the speed that was used for much of its operational life. The modem was attached to a Teletype
Teletype Corporation
The Teletype Corporation, a part of American Telephone and Telegraph Company's Western Electric manufacturing arm since 1930, came into being in 1928 when the Morkrum-Kleinschmidt Company changed its name to the name of its trademark equipment...
Model 33 ASR machine at Leopold's Records in Berkeley, California
Berkeley, California
Berkeley is a city on the east shore of the San Francisco Bay in Northern California, United States. Its neighbors to the south are the cities of Oakland and Emeryville. To the north is the city of Albany and the unincorporated community of Kensington...
and connected to the SDS 940
SDS 940
The SDS 940 was Scientific Data Systems' first machine designed to support time sharing directly, and was based on the SDS 930's 24-bit CPU built primarily of integrated circuits. It was announced in February 1966 and shipped in April, becoming a major part of Tymshare's expansion during the 1960s...
mainframe computer
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
in San Francisco.
Felsenstein was unimpressed by the Omnitech design, especially its price of $300, and convinced himself he could design a better version. He found that one half of the design problem was easy; generating the proper tones for transmission was simple. The other half of the problem was much harder; listening to the incoming signal and processing the tones back out. The traditional solution was to generate a local reference tone and compare it to the incoming signal, but this was subject to many problems, from noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
or distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
on the line, to drifting of the local tone due to events as mundane as temperature changes.
Felsenstein found two key improvements that led to a dramatically less expensive and more reliable design. The first was to ignore the idea of a variable baud rate, which was useful in some contexts, but not when used purely for data communications over known-good lines. The other improvement, which would prove key to the design, was to use the incoming signal itself as the reference tone. While working at Ampex
Ampex
Ampex is an American electronics company founded in 1944 by Alexander M. Poniatoff. The name AMPEX is an acronym, created by its founder, which stands for Alexander M. Poniatoff Excellence...
, Felsenstein had learned that the signal would always return to a "one" tone between sending bits of data; his new design looked for these signals and used them to resynchronize a local phase locked loop (PLL). The system generated the "zero" tone as a fixed difference from the reference signal coming from the PLL.
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. Felsenstein built several copies of the design as Community Memory added more terminals.
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 schematic
Schematic
A schematic diagram represents the elements of a system using abstract, graphic symbols rather than realistic pictures. A schematic usually omits all details that are not relevant to the information the schematic is intended to convey, and may add unrealistic elements that aid comprehension...
s to Les Solomon at Popular Electronics
Popular Electronics
Popular Electronics was an American magazine started by Ziff-Davis Publishing in October 1954 for electronics hobbyists and experimenters. It soon became the "World's Largest-Selling Electronics Magazine". The circulation was 240,151 in April 1957 and 400,000 by 1963. Ziff-Davis published Popular...
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
Novation CAT
Novation was an early modem manufacturer whose CAT series were popular in the early home computer market in the late 1970s and early 1980s, notably on the Apple II...
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 ElectricWestern Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...
phone. The Pennywhistle had a "step" cut into the front of the case where the various control switches were located, and the RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
port projected directly out of the front of the case.