IEEE Solid-State Circuits Magazine - Summer 2015 - 23

electronic systems. In order for PCM
to be practical, regenerative repeaters had to be placed periodically
along the transmission lines. Vacuum
tube repeaters had been successfully
designed and used in the telegraph
and voice network for a number of
years prior to the development of
the transistor but suffered from obvious reliability problems. However,
the solid-state regenerative repeater
designed by L.R. Wrathall in 1956
was a breakthrough that allowed the
realization of practical PCM systems
[37]. This repeater was demonstrated
on an experimental cable system
using repeater spacings of 2.3 mi on
19-gauge AWG cable and of 0.56 mi
on 32-gauge AWG cable.
The Wrathall repeater used germanium transistors designed by
Bell Labs and built by Western
Electric. The silicon transistor was
invented in 1954 by Gordon Teal at
Texas Instruments and gained wide
commercial acceptance because of
improved temperature performance
and reliability. Finally, the invention
of the IC by Jack Kilby and Robert
Noyce [38], [39] in 1958, followed
by the planar process invented by
Jean Hoerni in 1959 [40], [41] set the
stage for future PCM developments.
With the development of the
Wrathall repeater, it was clear in
1956 that PCM could be effectively
used to increase the number of voice
channels available on existing copper cable pairs. This was especially
attractive in metropolitan areas,
where many cable conduits were
filled to capacity. Many of these pairs
were equipped with loading coils at
a spacing of 1.8 km to improve their
response in the voice band. It was
natural to consider replacing the
loading coils with solid-state repeaters and to extend the capacity from
1 to 24 channels by using PCM.
For these reasons, a decision
was made at Bell Labs to develop a
PCM carrier system, and a prototype
24-channel system was designed
and tested during 1958 and 1959 on
a link between Summit, New Jersey,
and South Orange, New Jersey. This

1111
1110
1101
1100
1011
1010
1001
Shadow Mask 1 0 0 0
0111
0110
0101
0100
0011
0010
0001
0000
MSB
LSB

1000
1001
1011
1010
1110
1111
1101
Shadow Mask 1 1 0 0
0100
0101
0111
0110
0010
0011
0001
0000
MSB
LSB

(a)

(b)

Figure 7: Electron-beam coder shadow masks for binary and Gray code : (a) 4-bit binary
code and (b) 4-bit reflected-binary code (Gray code).

system, called the T-1 carrier system,
transmitted 24 voice channels using
a 1.544 MHz pulse train in a bipolar
code. The system used 7-bit logarithmic encoding with 26 dB of companding and was later expanded to 8-bit
encoding. The solid-state repeaters
were spaced at 1.8-km intervals, corresponding to the placement of the
existing loading coils. The first T-1
operating link went into service in
1962, and by 1984 there were more
than 200 million circuit-kilometers
of T-1 carrier in the United States.

Commercial Data Converters
of the 1950s
Up until the mid-1950s, data converters were primarily developed
and used within specialized applications, such as the Bell System work
on PCM and message encryption
systems of World War II. Because of
vacuum tube technology, the converters were very expensive and
bulky, and they dissipated lots of
power. There was practically no
commercial usage of these devices.
The digital computer was a significant early driving force behind commercial ADC development. The ENIAC
computer development project was
started in 1942 and was revealed to
the general public in February 1946.
The ENIAC led to the development of
the first commercially available digital computer, the UNIVAC, by Eckert

and Mauchly. The first UNIVAC was
delivered to the United States Census
Bureau in June 1951.
Military applications, such as ballistic trajectory computation, were
early driving forces behind the digital computer, but as time went on,
the possibilities of other applications in the area of data analysis and
industrial process control created
more general interest in digital processing, and so the need for data converters. In 1953 Bernard M. Gordon,
a pioneer in the field of data conversion, founded a company called
Epsco Engineering in his basement
in Concord, Massachusetts. Gordon,
who had previously worked on the
UNIVAC computer, saw the need for
commercial data converters. In 1954
Epsco introduced an 11-bit, 50 kS/s
vacuum-tube based ADC. This converter is believed to be the first commercial offering of such a device.
The Epsco Datrac converter dissipated 500 W, was designed for rack
mounting (19 in × 15 in × 26 in), and
sold for US$8,000-9,000 [42]. A photograph of the instrument is shown
in Figure 8. The Datrac was the first
commercially offered ADC to utilize
the shift-programmable successive
approximation architecture, and
Gordon was granted a patent on the
logic required to perform the conversion algorithm [43]. Because it had a
sample-and-hold function, the Epsco

IEEE SOLID-STATE CIRCUITS MAGAZINE

su m m E r 2 0 15

Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Summer 2015