United States Patent 3810154

Apparatus for increasing substantially the amount of information that can transferred by teletype communication systems in a given amount of time without additional increase in bandwidth, transmission facilities, or teletype system equipment. At a transmitting station, parallel, fixed-place digital-coded message characters are translated to serial, variable-place, digital-coded message characters. After reception at a remote station, the received characters are retranslated into output characters which are equivalent to the parallel, fixed-place characters transmitted. The conversion apparatus essentially comprises a switching network which produces in a novel manner digital representations of characters which are equivalent to the corresponding character to be translated.

Application Number:
Publication Date:
Filing Date:
The United States of America as represented by the Secretary of the Navy (Washington, DC)
Primary Class:
Other Classes:
341/91, 341/101
International Classes:
H03M7/40; (IPC1-7): H03K13/00; H04L3/00
Field of Search:
178/26,26A,36 340
View Patent Images:
US Patent References:
3028581Switching device1962-04-03Thorpe
2864008Relay selecting circuit1958-12-09Moore
2716156Code converter1955-08-23Harris
2604538Record card controlled code converter1952-07-22Halvorsen

Primary Examiner:
Robinson, Thomas A.
Attorney, Agent or Firm:
Sciascia, Rubens Mclaren R. S. G. J. J. W.
1. Apparatus for converting Teletype message characters represented by fixed-length, parallel, digital codes into equivalent characters represented by variable-length, serial, digital codes and comprising:

2. The apparatus of claim 1 wherein said input means comprises a parallel

3. The apparatus of claim 1 wherein said means for generating said binary

4. Apparatus for converting Baudot-coded messages wherein five parallel digits define an input character, into Huffman-coded messages wherein a variable number of serial digits define an output character equivalent to said input character and comprising:

5. The apparatus of claim 4 wherein said switch means comprises 31, double-pole-double-throw switches connected in a dichotomy network configuration with one switch at the input and sixteen switches at the

6. The apparatus of claim 4 wherein said parallel signal paths are connected to said output terminal pairs by means of blocking diodes to provide isolation between said switch network means and said output means.

7. In Teletype communication systems, apparatus for converting Baudot-coded messages into Huffman-coded messages for transmission thereof and for converting said Huffman-coded messages back into Baudot-coded messages after reception thereof and comprising;


Existing Teletype transmission systems generally employ a digital code that is fixed in the number of digits required for operation. Because the length of the code is fixed, the apparatus cannot take advantage of statistical characteristics of the information being transferred. Furthermore, machine instructions must also be transmitted within the digits thereby drastically reducing system efficiency to about two-thirds of its potential value. Since electromechanical design and construction features of the prior art preclude the feasibility of upgrading these machines, code improvement is thus the only possible upgrading method available. The present invention comprises a code improvement which enables a transfer of a substantially greater amount of information in a much shorter length of time without additional increase in Teletype equipment.


In Teletype communication systems, apparatus are disclosed for converting Baudot-coded messages into Huffman-coded messages for transmission and reconverting same into Baudot for recording upon reception. Baudot messages are read by parallel-input apparatus to produce a digital signal which is unique for each different character read. A pair of complementary, digital signals are coupled through a switching network to one of 32 output terminal pairs in response to the digital signal. The output is coupled simultaneously to selectively predetermined ones of 10 parallel signal paths, and the resulting outputs are then sequentially recorded and transmitted. Upon reception, the received messages are read by series-input apparatus to produce a unique digital signal for each different character read. A digital signal is coupled through a switching network to one of 32 output terminals in response to the analog signal. The signal is then coupled simultaneously to selectively predetermined ones of five parallel signal paths, and the resultant outputs are then recorded simultaneously wherein the parallel, five-place digital word recorded represents a Baudot-coded output character equivalent to a corresponding input character read before transmission thereof.


It is the primary object of the present invention to provide apparatus for Teletype transmission that will allow the transfer of a substantially greater amount of information in a much shorter length of time without any additional increase in circuit bandwidth, transmission facilities, or Teletype system equipment.

It is another object to provide apparatus for converting fixed-length digital-codes into variable-length digital-codes and vice versa.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.


FIG. 1 represents a simplified block diagram of apparatus embodying the present inventive concept for achieving a code translation from a fixed-length digital code to a variable-length digital code.

FIGS. 2(a) and 2(b) represent simplified electrical schematic diagram of the circuit of FIG. 1.

FIG. 3 illustrates a simplified block diagram of code translation apparatus embodying the present inventive concept for translating from a variable-length code to a fixed-length code.

FIGS. 4(a) and 4(b) are electrical schematic diagrams of the apparatus of FIG. 3.


The inventive concept of the present invention is illustrated generally in the block diagrams of FIG. 1 and FIG. 3. The Teletype apparatus shown in FIG. 1 can translate a message character from a fixed-length code such as the 32 character Baudot code to a variable-length digital code such as the Huffman code. A message originator Teletype system 10 provides a Baudot-coded message on paper tape to an input circuit 12. The paper tape has punched thereon five-place, digital characters which comprise the message to be transmitted.

The input circuit 12 is triggered by the tape input and produces a digital output which is fed to a switching network 14. The switching network also receives a complementary digital pair, i.e., "1" and "0," from the tone generator 16. The digital pair is coupled through the switching network 14 by the output of the input circuit 12.

The output of the network is coupled to a conversion circuit 18 to convert the fixed-place code to a variable-length code in a manner to be described hereinafter. The conversion network essentially comprises a prewired matrix having diode isolation as shown in detail in FIG. 2(b).

The converted output is fed into an output circuit 20 which can comprise a paper tape punch or a magnetic tape machine. The resulting output can be coupled to a storage device 22 or to a transmission system 24.

As shown in FIG. 2(a), the input circuit 12 comprises five, parallel sensors, each of which is responsive to a different one of the five, parallel digits represented by the five-place paper tape from the Teletype system 10. The tape travels through the circuit 12 in the direction shown by the arrow. Each sensor, 12a, 12b, 12c, 12d, and 12e is operatively connected to the switching network 14, which is also shown in detail in FIG. 2(a).

The sensors are responsive to the parallel input representing a particular character to produce a digital output only when, for example, a hole, ("1") in the paper tape is sensed by the appropriate sensor. An unpunched place ("0") in the paper tape serves as an insulator whereby the corresponding sensor is not energized and hence it produces no output.

The sensors preferably, but not necessarily, comprise electromagnetic devices such as coils on relays, control coils on magnetic amplifiers, or emitter-ground connections on transistors.

The switching network 14 comprises 31, substantially identical, double-pole, double-throw (DPDT) switches which are electrically connected to each other in a symmetrical dichotomy configuration in which the output of the single switch 14a is connected to the pair of switches 14b1 and 14b2. The outputs of 14b1 and 14b2 are in turn connected to the four switches 14c1, 14c2, etc., and the outputs of 14c1, 14c2, etc., are connected to the eight switches 14d1, 14d2, etc., and on to the 16, output switches 14e1, 14e2, etc.

The switches designated a, b, c, d, or e, are responsive to the output digital signal from the corresponding sensors 12a, 12b, 12c, 12d, and 12e, respectively, to switch from the rest position shown by the solid arrows in the detailed drawing of switch 14a to the energized position shown by the dashed arrows therein. The other 31 DPDT switches are shown. in simplified form to simplify FIG. 2(a); however it should be understood that the representation thereof would be identical to that of 14a.

The operation of the input device 12 and the network 14 will now be illustrated by means of, for example, the character for space () which has the Baudot code 00100. As paper tape having SPACE thereon is sensed by the input device 12, only the sensor 12c is energized, since only it senses a "1" (hole) in the corresponding digit space. Thus only the four switches, 14c1, 14c2, etc., are switched from the rest position to the energized position.

Consequently since the sensor 12a is not energized, the digital pair (0, 1) from the tone generator 14 is applied to the "rest" terminals of the switch 14a.

Likewise, since the sensor 12b is not energized, the two switches 14b1 and 14b2 remain in the rest position, and the "1" and "0" are coupled thereto from the switch 14a.

The sensor 12c is energized; thus the three switches 14c1, 14c2, etc., are switched to the energized position indicated by the dashed arrows. Since neither sensors 12d nor 12e are energized, the corresponding switches remain at rest, and the "1" and "0" are thus outputted at the energized output terminals of the switch 14e3 of FIG. 2(a). It can be seen from FIG. 2(b) that the output terminals of the switch 14e3 correspond by design to the symbol for the SPACE input character.

The output of the switch assembly 14 is coupled to the conversion circuit 18 wherein the following action occurs. As shown in FIG. 2(b), the circuit comprises a prewired matrix with diode isolation and consists of ten parallel lines or signal paths, 18a, 18b, etc. The output terminal pairs of each of the DPDT switches 14e1, 14e2, etc., are connected directly to selectively predetermined ones of the paths whereby, for example, the switch 14e3 corresponding to the symbol for SPACE couples its "1" and "0" output simultaneously to the top three paths 18a, 18b, and 18c, in a selectively predetermined order.

A blocking diode is connected between each switch output and any connection therefrom to any one of the ten lines to provide isolation therebetween. Other conventional diode isolation circuits can be used advantageously, and the diode circuit shown herein is merely typical of conventional isolation circuits.

Consequently the output of the top three lines, as read sequentially, is equal to 101. In can be appreciated that the 101 represents a variable-length, Huffman code SPACE character equivalent to the input fixed-length, Baudot code SPACE character. It can also be seen that each of the 32 input pairs from the sixteen switches 14e1, 14e2, etc., corresponds to a different one of 32 characters equivalent to the 32 characters normally associated with the Baudot-code, and that the apparatus of FIGS. 1 and 2(a) and 2(b) function to translate the characters from one code to another as described.

The sequentially read Huffman-code characters are recorded by the output device 20 which can comprise a magnetic tape device having 10 recording heads 20a, 20b, etc., as shown in FIG. 2(b), and in which the tape travels as shown by the arrow. The recorded output can be stored in storage means 22 or transmitted by conventional means 24.

The tone generator produces a "1" and a "0" to absolutely define the number of digits in the Huffman code since if only a single tone was employed, all Huffman code segments would have ten digits, and the identity of the code segments could not exist. Thus, in FIG. 1 and FIGS. 2(a) and 2(b) "1"'s and "0"'s represent positive indicators, and a blank cannot be substituted for a zero as is done in the Baudot code. The tone generator can comprise an audio generator and amplifier circuit.

When each Huffman-code segment is recorded on magnetic tape at the output circuit 20, the tape mechanism is stepped forward an amount that places the last recorded digit one position beyond the head designated 20a, to position clean tape under the tape heads. After the output is stepped forward to the clean tape position, the input tape is then stepped forward one position, thereby initiating a new cycle.

The generation of a Huffman code involves two primary considerations. The first of the finite list of defined characters to be coded which as discussed herein comprises the 32 functions generally associated with the conventional Teletype keyboard. The second consideration is the frequency of occurrence for each character when operating in a closed system involving the total character population. This value is conventionally known as probability.

The Huffman code characters shown at the input of the conversion circuit 18 have a frequency of occurrence (normalized) as shown by the following table and as derived from statistical information: ---------------------------------------------------------------------------

FREQUENCY TELETYPE OF OCCURRENCE HUFFMAN CHARACTER (NORMALIZED CODE SEGMENTS __________________________________________________________________________ SPACE 0.116 101 E 0.088 0001 LETTERS 0.070 0011 FIGURES 0.070 0101 T 0.063 0111 R 0.058 0110 I 0.053 1001 N 0.052 1111 O 0.051 1110 A 0.050 1101 S 0.040 00101 CR 0.032 01001 D 0.028 10001 L 0.025 11001 C 0.023 11000 H 0.023 000001 F 0.021 000011 U 0.021 000010 P 0.019 001001 M 0.017 001000 LF 0.016 01001 Y 0.015 100001 G 0.013 100000 W 0.010 0000001 V 0.009 0100001 B 0.007 00000001 X 0.003 01000001 K 0.002 000000001 Q 0.002 010000001 J 0.001 010000000 Z 0.001 0000000001 BLANK 0.001 0000000000 __________________________________________________________________________

the transmitted information is received at a remote station by apparatus as shown in FIGS. 3 and 4(a) and 4(b) and which reconverts the message from a variable-place code to a fixed-place code in the following manner. A receiving system 26 receives the Huffman-coded information and its paper or magnetic tape output is fed to the input circuit 28 which is responsive to a Huffman code character in a manner to be described hereinafter. The tape travels through the circuit 28 in the direction shown by the arrow in FIG. 4(a).

The input unit 28 is triggered by the tape input and produces an analog digital output which is fed to the switching network 30 which also receives a "1" input from the tone generator 32. The output of the switching network is coupled to the conversion circuit 34 which in turn feeds its output to the output circuit 36. The output circuit is connected to either storage apparatus 38 or to printout apparatus 40.

As shown in FIG. 4(a), the input circuit 28 comprises ten, serial sensors, 28a, 28b, etc., each of which is capable of identifying a "1," a "0," or a blank. If a digital "1," for example, is read by any one of the ten sensors, the digital "1" portion of the sensor will be energized and will produce an digital output. The digital output energizes all of the switches in the network 30 which are located in a vertical column below the portion. If a "0" is read, the "0" control device for that sensor will be energized thereby closing the vertical column of switches below that particular device. Obviously, no action results when a blank is identified. The input function is a parallel input when considered with regard to the switching action.

At the initiation of the cycle in which the individual Huffman code segments are read and converted to Baudot, the first "1" or "0" is traversed to the left to the first sensor 28a. When this position is reached all 10 sensors will have a "1" or a "0" to identify, and one of the three control devices will be energized. Even if all 10 sensors are energized to produce either a "1" or a "0" from the control devices, only one of the 32 horizontal lines of switches will be completely closed to thereby allow a signal from the tone generator 32 to pass through. This selectivity is based upon the fact that each of the 32 characters comprises a unique combination and number of "1" and "0" digits.

Each of the 32 horizontal lines in the conversion circuit 34 of FIG. 4(b) functions as a storage cell for a different and unique code segment and the simultaneous summation of horizontal lines comprises the switching function which produces a character equivalent to the input character.

Each of the horizontal lines from the switch network 30 (except the bottom one which is all "0" responsive) is connected to one or more of the vertical lines 34a, 34b, etc., which lead to the recording heads 36a, 36b, 36c, 36d, and 36e. This vertical network of signal paths performs the conversion function which translates the Huffman segments to Baudot segments. Again, diodes provide the necessary isolation.

In the conversion back to Baudot only a digital "1" tone generator 32 is required because in the constant five-place code, a blank and a "0" are the same.

When each Baudot segment has been recorded on the output device 36, which can comprise a tape perforator, the tape mechanism is also stepped forward by an output from any of the 32 switches.

Thus, it can be appreciated that a novel concept has been disclosed for improving the efficiency of Teletype communication systems, by providing approximately 65 percent greater information transfer rate or capacity with respect to existing techniques.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

It can be appreciated that the components shown in the figures are merely exemplary and that depending on the requirements and resources of a user, electromechanical relays, vacuum tubes, solid state devices, saturable reactors, electrical optical switches, or pneumatic switching devices can be used to practice the inventive concept herein disclosed.