Quiz/Question

[floppybootstomp]

OK, just for fun, here's a problem for all you electronics buffs.

This is a real life situation.

A 100 volt line loudspeaker network broadcasts three different signals:

a) Fire Alarm Tone, a continuous 400Hz square wave.
b) Class change 'pips' a pulsed (on-off) 1Khz square wave.
c) Speech from a microphone.

The office staff don't want to hear the microphone annoucements or the class change pips, but obviously for health and safety reasons they must hear the fire alarm signal. The office staff work in two rooms, each with it's own 100 volt line loudspeaker.

The question is 'How would you block the unwanted signals being broadcast from those loudspeakers yet allow the fire alarm signal to be heard?'

There are 88 other loudspeakers on the school site powered by 2 x 100 watt amplifiers and the three different audio signals are prioritised through a logic circuit, with the Fire Alarm having greatest priority.

Circuit diagram please

I actually solved this problem 18 years ago and had it patented, now it's your turn

 

[ChristopherP]

Well thats easy.

Obviously the Fire Alarm tone will be the loudest so you give all the kids in the school ear defenders. They wont hear the mic announcements/pips but they sure will hear the fire tone.

 

[floppybootstomp]

Young man, it's a serious question, hehe...

Now then, go and make your 1000th post somewhere else...

 

[ChristopherP]

ok then

 

[floppybootstomp]

You bugger!

And captured for posterity:

 

[ChristopherP]

haha thats great!!

at least i assume it is...well i dont know what posterity is

i like the screenshot hehehehehe

 

[newbuilder]

i don't know anything about this but what about:

Change the analogue mic signal to digital and use some sort of filter to block all frequencies lower than the fire alarm?

Or have a siren box in the office for the alarm and use a speaker system for the pip and microphone.

I am probably being to simple at my thinking,

 

[KGB-911]

Easy way:

Buy a special fire alarm! (you know the red circular bell things)

 

[ChristopherP]

What about signal cancelling speaker wire? stuff that only lets certain stuff through?

Place a little box in the circit that detects frequencies and cancells as nescacary?

 

[floppybootstomp]

Change the analogue mic signal to digital

A loudspeaker is an analogue device

use some sort of filter to block all frequencies lower than the fire alarm?

The fire alarm is the lowest frequency, although the Mic signal will sometimes go that low as well. Therefore, blocking frequencies below 400Hz would be ineffective.

Or have a siren box in the office for the alarm and use a speaker system for the pip and microphone.

That would actually work. The Fire Alarm initiation signal could be used to switch a relay to supply 24V DC to a couple of sounders.

However, the cost of cabling on a large (school) site would not make this cost effective. Unless, of course, the audio rack was in close proximity to the offices.

Furthermore, Health and Safety regulations state that a Fire Alarm warning signal should be uniform across a whole site, mixed warning tones are not recommended.

In answer to that, I suppose a local oscillator/loudspeakers could be set up, but again, not cost effective.

 

[newbuilder]

A loudspeaker is an analogue device

duh!!! (me not thinking)

Saturday noght is not a good night to think straight!!

 

[floppybootstomp]

What about signal cancelling speaker wire? stuff that only lets certain stuff through?

Eh? Tell me about this stuff, I want some

Place a little box in the circit that detects frequencies and cancells as nescacary?

You're getting warm Although it's a little simpler than that...

 

[newbuilder]

miss out the office speakers in the wiring loop for the pip and mic?

 

[ChristopherP]

i guess some speaker wire has a tendancy to do that... though its not meant to

http://www.audiokarma.org/forums/showthread.php?threadid=1486&s=

 

[floppybootstomp]

miss out the office speakers in the wiring loop for the pip and mic?

Nice try, but there is only one loudspeaker wiring circuit, and all three audio signals share it (see first post about prioritisation of signals)

 

[ChristopherP]

A little box that creates an antinoise (the opposite of the noise that is going through the wire) therefore effectivly cancelling the noise at selected frequencies (as used on the Dornier 328 regional transport aircraft) and making it inaudiable

 

[floppybootstomp]

A little box that creates an antinoise (the opposite of the noise that is going through the wire) therefore effectivly cancelling the noise at selected frequencies (as used on the Dornier 328 regional transport aircraft) and making it inaudiable

Nope

I'll post a few clues, but I'd like a few other people to see this thing first.

 

[Spooky Mouse]

If the fire alarm is for a school....
well i would gather that the frequencey and tone eould be quite important to stop the little Pumpkins burning to death so the best thing to do is....
Drag all the schoolchildren out beforehand and Shoot them dead!!
This way you could also use the frequeney for somthing more usefull....


all the best.. sorry im going mad again

 

[Matt Jason H]

I have your answer sah'.

All you need is a basic op-amp 'band-pass' filter, tuned to block out signals around the 1Khz range.

You'll need 1 op amp, 3 resistors and 2 capacitors. If you need any help calculating and designing I'll cook you up a diagram later.

 

[floppybootstomp]

I have your answer sah'.

All you need is a basic op-amp 'band-pass' filter, tuned to block out signals around the 1Khz range.

You'll need 1 op amp, 3 resistors and 2 capacitors. If you need any help calculating and designing I'll cook you up a diagram later.

A band pass filter would certainly block the class change pips but not the microphone signals. Different people use the mic and although the frequency range is limited by the 100 volt line system, a typical range for a microphone would be 200Hz to 12Khz.

A 100 volt line system works by modulating the audio signal with 100 volts AC through a transformer. The 100 volts enables the signal to travel large distances without loss. The 100 volts is taken off locally via a transformer at each loudspeaker location.

Clues:

100 volt line system. Time.