Analog Telephony (Part 2)

Welcome back. Let’s talk specifically about POTS integration in Q-SYS.

QSC supports POTS telephony integration directly on the Q-SYS Core 110f processor (with a single line)

and the CTEL4 I/O card in configurable Q-SYS devices.

In Q-SYS Designer Software, there’s a POTS out block that represents the audio signal to the far end.

There’s a POTS in block that represents the audio signal received from the far end.

And of course, there’s the DTMF control section that allows user call control.

The first thing to do is to check the POTS line’s country of use in the Core’s properties,

particularly if you are not using it in the US, which is the default for Q-SYS.

The country setting tells Q-SYS the ring voltage,

the cadence and the other country-specific signals that are used in the call control.

You can also enable a second output of the POTS Receive block.

All receive audio will go through the original receive node,

while all received DTMF and call progress tones will be routed to the second one.

The POTS controller block is, of course, the same keypad we use on most phones and is relatively self-explanatory.

Next to the Dialer tab, there are two important tabs for configuration and troubleshooting.

The controller tab has several configuration options.

The buttons at the top determine how the system handles a far-end hang-up.

There are two options: Loop drop, which is the traditional method,

and call progress, which is used by some PBX systems.

By the way, there’s no harm in having both of these options enabled at the same time.

Next we have ‘hook flash time’.

If the hook flash is being used for any special features in an installation,

the timing may need to be adjusted here depending on the needs of the system interpreting it.

Dial tone gain simply sets the level of the dial tone on the room if the interface is taken off hook before dialing.

DTMF controls in the next section are not yet user-definable.

To avoid any hiccups caused by DTMF and call progress tones being sent back to the far end,

Q-SYS does NOT play these tones in the room.

In the next section, audio files can be specified to play back into the room when the respective call progress

or DTMF tones are present on the line.

Be warned that if a given call progress tone file is not enabled, specified or at the sufficient volume,

the user will not hear anything when that tone is heard by the interface.

This sometimes gets some users pretty angry

because they expect to hear those tone as feedback from their button pushes.

We try to explain it like this:

Let’s say I’m entering a conference code and the first number is ‘1’.

I hit the ‘1’ key, and the DTMF tone plays that overhead in the room.

The microphones hear that DTMF tone and then transmit it to the far end as well.

The conferencing bridge then hears two ‘1’s in the DTMF,

making it impossible to properly enter the conference code.

We want to avoid this frustration by playing a ‘neutral’ sound

that provides end user feedback but doesn’t confuse the conference bridge.

On the status tab we find troubleshooting and the state information of the interface.

The top three pieces of information are among the most important:

line voltage, line current and line fault.

Here’s a good one: If you see 47 volts and zero amps, what does that tell me?

The answer is: I know the interface is on-hook, because there is no loop current.

And there’s adequate voltage on the line and there’s no reported fault condition,

so I can assume that the line is good.

I also see the ‘line ready’ indicator lit below, which is also a good sign.

Below the status indicator LED,

you’ll find a readout to show whether any DTMF tones have been received by the interface.

Finally, Event Logging allows you to enable POTS logging in the Q-SYS eventlog for troubleshooting.

To outline the most common states on the telephone interface,

we’ll compare what we see in the status tab and the dialer at the same time.

When you have an on-hook with a ‘good’ line, you'll see line voltage is between 40-50v,

keeping in mind that polarity doesn’t matter and the line current should always be 0A.

You’ll see a lit line ready LED and the call progress in the dialer should read ‘Idle’ in this state.

When the interface goes off hook, line voltage drops as we start to see current measured on the line.

It is typical to see the voltage drop to between 6 and 10VDC. The current will typically be around 25mA.

When dial tone is present, the corresponding led will light up there, too.

Often times, you’ll see different transitional or progress states in the dialer call progress

depending on how fast the service provider reacts to the sensing of the line current.

Telephone lines use a feature called ‘line echo cancellation’

to make sure you only hear the far end audio when in call.

“Calibrating” means that this cancellation is being calibrated on the line.

‘Waiting for dialtone’ means just that – the calibration is finished and we’re waiting for the service provider to send the dialtone.

And finally the ‘ready’ status, which means we’re ready to dial.

The transition of these states to ‘Ready’ typically happens very quickly,

so there’s not a long wait for the user.

When the number is entered by the user and the process begins, you’ll see the ‘line in use’ stay lit.

The ‘dial in progress’ LED stays lit until the ringback tone is detected,

which means that you’ve gotten through to the far end.

The ‘ring(back) tone’ led will light for the duration of the ringback tone.

The dialer call progress field will go through the states we’ve already outlined.

It will display ‘Dialing’ the number we’re calling,

and then move to ‘Call Ringing’ and then finally to ‘Connected to’ when the far end answers.

Note that the ‘off hook’ LED on the dialer will light when the interface is taken off hook.

The ‘ringing’ LED, however, only lights up when there’s an incoming call.

The interface does not need to be taken off hook before the user enters the number.

The user can enter the number first and simply hit ‘connect’. The interface will go through the same states,

only a little faster than the manual dialing process.

Okeydokey, we’ve made it through the first few stages of connection.

When we start back up, we’ll start with what happens when we’re actually in a call, hangs up and incoming calls.