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4.2 BELL SYSTEM MULTIFREQUENCY SIGNALING 85Figure 4.2-4. Protection against talkoff: (a) forward transmission path during conversation and(b) tone detector (TD).In the first place, a tone detector (TD) does not simply detect the presence of the2600-Hz tone. As shown in Fig. 4.2-4(b), the signal received from the trunk is fedinto band-pass filter BPF that passes a narrow band of frequencies around2600 Hz only, and to band-elimination filter (BEF) that passes all voiceband frequencies,except the frequencies passed by BPF. Comparator (COMP) comparesthe signal strengths at the outputs of the filters.When signaling tone is being received, the output of BPF is stronger than theoutput of BEF, even when speech is present at the same time. TD thus indicatestone-on (on-hook) to the exchange processor. When only speech is received, theoutput of BEF is stronger than the output of BPF, because speech power ismainly concentrated in the 300–1000 Hz range, and only a small part of it liesaround signaling frequency. In this condition, TD indicates off-hook.As a second safety measure, exchange processors do not recognize receivedchanges to on-hook or off-hook as valid signals until the new state has persistedduring a recognition time of a certain length. With the exception of winks, the recognitiontime for most signals is 30 ms. However, as an extra protection againsttalk-off, the recognition time for clear-forward signals is 300 ms.