Digital Signal Processor: Tone Detection for CCITT No. 5 Transceiver

The application of a recently developed LSI digital signal processor, the DSP,1 to tone detection in a proposed digital CCITT NO. 5 signaling unit is described. This unit is to be part of an echo canceler terminal for No. 4 ESS international switching centers. At present, an analog configuration, which includes an analog transceiver, terminates trunks with C C I T T NO. 5 signaling. Line signaling information in the CCITT N O . 5 signaling system is transmitted via 2400- and 2600-Hz tones used either separately or in combination. The block diagram of Fig. 1 shows the section of the proposed digital receiver to be implemented using the DSP. The bandpass filters (BPFS) detect energy at one of the two signaling frequencies. The band elimination filter (BEF) serves both as a guard filter, to detect energy other than at the signaling frequencies, and as an attenuator of any signaling energy present in the input signal. The detector compares the outputs of the three filters and determines if either, or both, of the tones are present. This information is then used by the time validation circuit that follows. The circuit, called control and output logic in Fig. 1, determines if the tone(s) is(are) present for 1687