A report on Manchester code and Group coded recording

An example of Manchester encoding showing both conventions for representation of data

Manchester code was widely used for magnetic recording on 1600 bpi computer tapes before the introduction of 6250 bpi tapes which used the more efficient group-coded recording.

- Manchester code

Prior to 6,250 bpi tapes, 1,600 bpi tapes satisfied these constraints using a technique called phase encoding (PE), which was only 50% efficient.

- Group coded recording
An example of Manchester encoding showing both conventions for representation of data

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Overall
The binary signal is encoded using rectangular pulse-amplitude modulation with polar NRZ(L), or polar non-return-to-zero-level code

Non-return-to-zero

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Binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negative voltage, with no other neutral or rest condition.

Binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negative voltage, with no other neutral or rest condition.

The binary signal is encoded using rectangular pulse-amplitude modulation with polar NRZ(L), or polar non-return-to-zero-level code
Unipolar NRZ(L), or unipolar non-return-to-zero level
Non-return-to-zero space
Encoder for NRZS, toggle on zero
An example of the NRZI encoding, transition on 1
The opposite convention, transition on 0
Encoder for NRZI, toggle on one

For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the baseband bandwidth required by the Manchester code (the passband bandwidth is the same).

Synchronized NRZI (NRZI-S, SNRZI) and group-coded recording (GCR) are modified forms of NRZI.