SDIF is an older standard that is still popular in Japan. It transmits two serial data streams in unbalanced form over two coaxial cables terminated in BNC connectors. When used for PCM, each data stream contains up to 24-bit data for one channel, messages (to identify the data), status flags and synchronisation pulses (which can be used to extract the word clock). When used for DSD, the two cables carry DSD data only. The ‘Channel 1’ cable carries left channel data, the ‘Channel 2’ cable carries right-channel data. SDIF data streams are different to both AES3 and S/PDIF, they are not compatible. For reliable operation, the output stage impedance, the characteristic impedance of the cable and the load impedance at the receiver should all be 75 ohms. This interface is DC-coupled and carries a DC offset of typically +1.25V – it will not work if coupling capacitors or transformers are inserted in the cable. To avoid interference from other appliances, the screen wires must be securely connected to chassis at both ends. There are 3 common forms of SDIF: SDIF-2 PCM, SDIF-2 DSD and SDIF-3 which is for DSD only. These variations make it difficult to extract a word clock reliably, so it is common practice to use a third cable to carry word clock – dCS equipment requires this. A balanced version of SDIF is sometimes used in multi-channel systems, a multi-pin connector is used.