## Duo-binary signalling |
In-depth |

#### In a duo-binary system, the input data sequence is combined with a 1-bit delayed version of the same sequence (the controlled intersymbol interference) and then passed through the pulse-shaping filter. A binary input with voltages +V and -V thus produces a three-level output with voltages +2V, 0 and -2V. |

| #### For simplicity, if we make the pulse shaping filter an ideal brick-wall filter with cut-off at 0.5 x 1/Tb, then the impulse response and frequency response of the combined duo-binary encoder and pulse-shaping filter are as shown (see in-depth). As expected, energy from one input bit is seen to be spread over two bit periods in the transmitted signal. The composite frequency response, however, is the key to the success of duo-binary signalling. A filter with this response is relatively easy to synthesize and can be used to replace the duo-binary encoder and unrealizable brick-wall filter and realistically achieve a data throughput of 2 bits/second/Hz.