Punched tape: a historic data-storage medium

Construction and encoding

Typical punched tape was made from paper, thin card, or later from plastic or metal for greater durability. A tape contains a number of parallel tracks across its width; each track corresponds to one bit position of a code. Early teleprinter networks commonly used fixed-width codes such as Baudot (five bits) while some computer and control applications used seven or eight bit schemes. Feed holes or sprockets kept the tape aligned through punches and readers.

• Materials: paper, card, Mylar/plastic, metal strips.
• Mechanisms: punch units, hand punches, reader sensors.
• Properties: sequential access, relatively low density, tolerant of simple mechanical damage if redundancy present.

History and development

The idea of encoding patterns on a strip to control machines goes back to automated looms of the 18th and early 19th centuries, notably devices that evolved into the Jacquard loom system. Such punched media were later adapted for musical instruments such as barrel organs and player piano rolls. In the late 19th century, equipment for large-scale data tabulation adopted perforated media: Herman Hollerith implemented punched cards and related punched-tape principles for the 1890 census.

Uses and examples

Punched tape saw broad use in telecommunications and computing in the 20th century. Teleprinters and teletypewriters used tape for message buffering and retransmission. Early computers used tape as a program and data medium, and numerical control systems used punched tape to feed machining instructions. Other examples include textile pattern control (modern tartan looms sometimes use rigid punched media for repeated motifs) and mechanical music devices.

• Teleprinters: message storage and feed.
• Computers and CNC: program input for early systems.
• Weaving and musical instruments: pattern and score control (compare tartan weaving and organ rolls).

Comparison and legacy

Punched tape shares principles with the punched card but differs mainly in continuity and variable length: cards are fixed pages, tape is a continuous stream. Compared with magnetic tape and random-access media that followed, punched tape is slower and lower in density but is simple to read and robust against some failure modes. Its use declined as magnetic and solid-state storage became cheaper and more compact, though punched-tape formats influenced later serial-data storage conventions.

Notable technical facts

Punched tape also intersected with cryptography. The telecommunication engineer Gilbert Vernam designed a cipher system that could combine tape-encoded text with a key tape to produce ciphertext; combining the ciphertext with the same key again recovers the original text. This operation is an application of the logical exclusive OR and is conceptually related to the one-time pad. The Vernam method and related patents have been influential in the history of secure communications.