Cadenced tone detection

With cadenced tones (also known as "interrupted" tones), detection is a little more difficult. It is controlled by the following parameters in the Signaling Type parameter group:

Cadence Energy Maximum (dBm) (Cadence Energy Maximum (dBm))
Cadence Energy Minimum (dBm) (Cadence Energy Minimum (dBm))
Cadence Off Time Maximum (ms) (Cadence Off Time Maximum (ms))
Cadence Off Time Minimum (ms) (Cadence Off Time Minimum (ms))
Cadence On Time Maximum (ms) (Cadence On Time Maximum (ms))
Cadence On Time Minimum (ms) (Cadence On Time Minimum (ms))
Cadence Silence Maximum (dBm) (Cadence Silence Maximum (dBm))
Hang Up Detection (Hang Up Detection)

The Hangup Detection parameter must be set to Cadence Energy Detection.

Cadenced means that the hangup signal is intermittent. It consists of a series of signals (known as on phases) separated by silence (known as off phases). Blueworx Voice Response uses a cadenced tone detector to monitor the energy levels of the on and off phases and how long each phase lasts.

The cadenced tone detector runs constantly except when the channel is dialing, and when DTMF is being received. The start of dialing or the arrival of a DTMF tone causes the cadenced tone detector to reset.

Blueworx Voice Response waits for two on phases and two off phases (on-off-on-off) to pass before confirming that the signal is indicating hangup.

Energy Levels

The energy level of the on pulses must be within a range of values known as the high band and the off pulses must be within a range of values known as the low band. These bands are controlled by the parameters Cadence Energy Maximum, Cadence Energy Minimum and Cadence Silence Maximum as shown in Figure 1. The energy levels of the electrical signals are measured in dBm. The frequency (pitch) of the tone is not checked.

Figure 1. Cadenced hangup tone detection: use of dBm level parameters

A graph of energy levels in dBm against time showing four bands of energy levels: low, medium, high and too-high. The low band consists of all energy levels below the Cadence Silence Maximum. The medium band consists of the energy levels between the Cadence Silence Maximum and the Cadence Energy Minimum. The high band consists of the energy levels between the Cadence Energy Minimum and the Cadence Energy Maximum. The too-high band consists of all energy levels higher than the Cadence Energy Maximum.

The detector checks that the dBm level of all the on pulses are between the Cadence Energy Minimum and Cadence Energy Maximum values. The on pulse is allowed to settle or rise to a level within the configured band of allowable dBm range until it stabilizes; after that, it must remain relatively constant (±1dBm) until the pulse ends.

Timing

The detector also monitors the length of time taken for each on pulse and the length of time taken for each off pulse. The time taken for an on pulse must always be within a range of values controlled by the parameters Cadence On Time Minimum and Cadence On Time Maximum. Similarly, the time taken for an off pulse must always be within a range of values controlled by the parameters Cadence Off Time Minimum and Cadence Off Time Maximum. This is shown in Figure 2.

Figure 2. Cadenced hangup tone detection: use of On and Off Time parameters

A graph of energy levels in dBm against time showing three on pulses and three off pulses. The on pulses are all in the high energy band and last for longer than the Cadence On Time Minimum but for less time than the Cadence On Time Maximum. The off pulses are all in the low energy band and last for longer than the Cadence Off Time Minimum but for less time than the Cadence Off Time Maximum.

The detector reports the hangup when it detects the rising edge of the third on-pulse.

Settle time

Figure 3 shows a signal that starts at a low dBm level, and rises to a high dBm level not smoothly, but in steps. Part of this stepping is caused by the electrical properties of the line and hardware used to generate the pulse, and part is caused because energy levels are being reported every 20 ms.

Figure 3. Cadenced hangup tone detection: settle time

A graph of energy levels in dBm against time showing one on pulse. The pulse initially rises in steps before settling to a constant energy level. It reaches a constant energy level before the end of the allowed settle time (60 ms).

The signal must settle down to a steady (or nearly steady) energy level before the settle time expires. After the settle time has expired, the dBm level of the on-pulse is checked every 20 ms to ensure it differs from the settled dBm level by no more than 1 dBm. Failure to do this causes the detector to wait for the next valid off period, then start searching for the next on-period again. This exceptions to this behavior are when glitches are detected (see Glitches).

The settle time is defined as 60 ms constant.

During the settle time, the dBm level is not monitored except that, if it leaves the High Band, the detector resets and waits for the next valid off time before restarting.

During the off period in the cadenced sequence, the dBm level is monitored only to ensure that it does not rise above the threshold specified by Cadence Silence Maximum. If it does, the detector resets and starts looking for a valid silence before restarting.

If the energy level ever increases beyond the level specified by Cadence Energy Maximum, although that increase might be inside 1 dBm of the settled dBm level, the detector resets and waits for the next valid off period before restarting.

This settle time applies to all rising edges; that is, at the start of all on-phases of the cadenced sequence.

Glitch-detection is not active during the settle time.

Glitches

A glitch (also known as a spike) is caused by a brief variation in the energy level of the sample, during either the on-phase or off-phase of the cadence sequence. Such glitches are generally ignored by the detector, so two consecutive on-phases and off-phases, followed by a third rising edge, can still be detected, although the line might be noisy.

The detector considers any deviation from the settled on-time dBm level to be a glitch, providing it lasts for less than 40 ms. If the dBm level does not return to the settled level for more than 40 ms, it is considered to be a nonvalid on-signal, and the detector resets and starts again.

The detector can ignore a small number of glitches in any single on-state or off-state. More than this in any single state causes the detector to restart.

Glitches are not detectable during the settle time.