Telepen

Telepen is a highly secure, full-ASCII barcode symbology developed in the UK starting in 1971, encoding data as binary sequences of wide and narrow bars with a fixed 3:1 ratio, and remains widely used in library systems today.

The original motivation to develop a barcode system dates back to 1971. Computer technology had advanced rapidly, but the means to enter data into computer systems had not kept pace. This gap was particularly apparent in a very successful system for automating hospital biochemical laboratories, where fast, accurate input of test request data and correct identification of patients and samples were essential.

This application, together with the potential for others such as point of sale, provided sufficient motivation to set up a company in 1972 to develop a hand-held black-and-white barcode system.

Although a British and an American company were also working along similar lines, this was already known, so it was necessary to devise a distinct symbology.

The perceived requirements were as follows:

• Representation of the full ASCII character set.
• Space required should not exceed twice that of human-readable printing.
• Must be possible to produce on conventional printers.
• Very high security.
• All characters should take up the same amount of space.

It was considered that any general-purpose data entry device should be capable of handling the full range of characters. The information content of any symbology is clearly of crucial significance.

For widespread acceptance of the technology, it was considered essential that the code be generatable with conventional printer mechanisms, especially since the fundamental concept is machine-readable data on ordinary paper.

Since a major benefit of barcoding is accuracy, negligible misreading was considered absolutely vital.

The length of the code for a given number of characters clearly should not vary with the data. Adjusting the widths of the bars within a character to achieve this is also not acceptable.

A wide-to-narrow bar ratio of 3:1 was adopted, since an integral relationship is necessary to fulfil the conventional printing requirement, and 2:1 does not allow sufficient margin for print tolerances.

There are four possible combinations of wide and narrow bars and spaces. Ideally, these would represent binary data in a way that simplifies decoding.

If the first pattern is defined as representing binary 1, there is, at first sight, no way to represent binary 0 in the same space. It is, however, possible to represent binary sequences.

We need to be able to represent binary 00, 010, 0110, 01110, and so on.

The second pattern, twice the width of the first, may be defined as representing binary 00.

The third pattern, three times the width of the first, may be defined as representing binary 010.

The fourth pattern may be defined as representing binary 01; sequences such as 01110 could easily be represented by inserting the first pattern appropriately in between. There is, of course, no possibility of a fifth pattern, so at first the scheme would appear to fail. Fortunately, it is not necessary to repeat the fourth pattern, since binary 01 01 would be encoded as 010 1. It is therefore possible for it to represent 01 and 10 alternately.

As an example, 01110 is encoded as follows:

A binary string can now be encoded extremely efficiently — exactly twice as efficiently as, for example, Plessey code. Although multi-width symbologies such as EAN and Code 128 appear to have good packing densities, they require much tighter tolerances, so for a given printing method they are not nearly as efficient. Reading is also, of course, more difficult.

The benefit of a fixed wide-to-narrow ratio should also not be overlooked. Although tolerances may vary with the ratio, the decoder must allow for the specified range. The ability to detect a transition is the only criterion in defining tolerances. The scanner spot diameter may also be larger relative to the module width if precise measurement is not necessary.

There are two further levels involved in defining a complete Telepen symbology.

First, with the procedure described above, 8-bit even-parity characters are encoded least-significant-bit first, as with serial data transmission.

Second, the data characters, together with a start code, stop code, and block check character, are formatted into the complete symbol.

Although the symbology allows for intercharacter gaps, since the width of the last space is implied, readers normally check for continuity, so the standard is defined without gaps.

The front (start) code is binary 01011111 (ASCII "_").

The rear (stop) code is binary 11111010 (ASCII "z").

When reading forward, a wide black bar is encountered first following a series of narrow bars, which the reader may use for synchronization if required. When reading in reverse, a wide space is encountered first after the same series of narrow bars.

The block check character is modulo 127.

The symbol is highly secure: in addition to the powerful block check character, a valid end code is required, as well as even-parity binary characters and consistent ratios.

An interesting additional attribute of the check character is that it is impossible for two erroneous marks or two missing marks to compensate for each other.

There is no mirror-image problem.

Encoding or decoding may, of course, be accomplished by reference to a table of character patterns, as with other symbologies. The ability to decode directly into binary potentially enables real-time decoding with high-speed laser scanners. Telepen was originally decoded by TTL logic, since microprocessors were not yet available.

For numeric-only applications, twice the packing density can be achieved by packing two digits into one character. Of the remaining 28 characters, 10 pack 0X–9X, and the rest remain control characters. Note that this is simply a procedure for packing data and is not strictly part of the symbology itself.

When generated at 80 dots per inch (a nominal element width of 0.0125 inch), the actual packing density is 5 alphabetic characters per inch, or 10 digits per inch.

At the maximum recommended density, over 16 digits per inch is achievable.

Any future extended character sets can, of course, be easily accommodated by increasing the number of bits per character.

Telepen systems have been implemented in many countries and very widely in the UK. Most universities and other academic libraries use Telepen, as do many public libraries.

Other users include the motor industry, the Ministry of Defence, and numerous other well-known organizations for a wide range of applications.

Telepen is the only symbology to meet all of the essential requirements listed above.

The ultimate test is, of course, that Telepen has been used very effectively for over 25 years without problems.

For more information visit .

Legacy links (no longer active) (1)

• www.telepen.demon.co.uk