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Dead Air Detector

copyright ©2000 Paul Blitz



Someone on the "Hospital Radio Mailing List" asked if anyone had a design for a "Dead-Air Detector"... What they wanted was a device that listened to the station output, and if something went wrong, so that the station started to transmit silence, alarms would ring, a standby service would be transmitted etc.  

No such device seemed to exist, so the "Tuesday Evening Coffee Club" (aka Paul and Nigel at Winchester hospital radio) were asked for help.  

Thus I got around to designing a Dead-Air Detector. As of Sept 99 the design has not even been prototyped, just designed... however, we are willing to help anyone who would like to build one.  

The schematics are available for download and online viewing. It is assumed that you have an ability to build circuits from schematics, and that we don't need to tell you where to attach power to the logic chips .  

What follows is a description of the circuit, and should be read whilst you view the circuit diagram.  

Block Diagram

Block Diagram Description

The Input Buffer take the stereo input, buffers it, and makes it into mono. It also allows us to adjust the input sensitivity. The Rectifier then gets rid of the negative bits of the signal, leaving just a load of pulses (of course, if there is silence, the pulses will be very small, or completely absent!).

The Detect Low compares the input signal with a relatively small voltage, so that a bit of low noise isn't detected as valid input. This gives out pulses for as long as there is audio. These keep re-triggering the Timer output , so that it stays "on". However, if the pulses disappear, then after a length of time (settable by component change between 5 seconds and 60 seconds) the output will go "off".

The on-to-off transition causes Pulse to output a pulse, which will "set" the Latch to indicate that silence has been detected.

Back to the top again: the Detect High bit causes pulses when the input is "relatively high" (and certainly higher than the "relatively small" signal used in the Detect Low). If the input is not high enough, then the output will stay "high", but when there is high enough audio present there will be low pulses.

The idea is that if silence is detected, but an audio signal re-appears, then the unit will automatically reset itself. However, these pulses then pass through a jumper (not shown on the block diagram): if the jumper is not fitted, then the unit does NOT automaticall reset.

The signal then passes through the + combiner, to reset the Latch.

The Reset input, if low, will also pass through the + combiner to reset the Latch. If the jumper above is not fitted, then momentarily grounding this input will reset the unit (however, see note below).

In addition, if this input is HELD low (by a switch, rather than by a push-button) then the silence detector is disabled.

The output of the Latch, along with a repeat of the Reset input signal are then used to drive whatever indicators etc are needed (see below for more details)

Let me download the circuit details!

Low-res Circuit in GIF format (11k)

High-res Circuit in GIF format (36k)

Detailed Circuit Description

The first section is the input buffer: this reduces the load on the external circuit being monitored, and allows you to set the gain to match your setup (ie if you have a low output level, then you'll need more gain, and if you have a high output level, then you will need a lower gain). The gain is set by the feedback resistor (actualkly a preset pot).

The next stage is a peak rectification circuit: this converts the normal sound waveform into positive-going pulses (effectively removing the negative peaks) of variable height... the height depending on the instantaneous input level.

Next are a pair of comparators: these each compare the DC level to different preset levels.

The "LOW" comparator sets the level which defines "silence"... normally, with incoming audio this will be producing regular positive pulses, caused by the input exceeding the "low reference" level. If the audio input goes below that level (= dead air!), then there will be no more pulses to keep re-triggering the monostable (see below).

The "HIGH" comparator is used to reset the circuit after dead-air has been detected. The "high reference" level is higher than the "low reference", to make sure we get some noticible audio so we can be happy that "the audio is back again". Again, this prduces pulses, but these are negative pulses, and when the input disappears, the input stays high.

This signal, via a jumper, is combined with an active low "reset" or "disable" input, and is used to (constantly) reset the "dead air detected" latch (see below). If the link (lk1) is fitted, then the detector will auto-reset when audio returns, and the detector may be disabled by keeping the "reset" input low (thus the "reset" input is also a "disable" input). On the other hand, if the link is omitted, then the detector must be reset manually, by momentarily pulsing the "reset" input low. Again, keeping the "reset" input low will disable the detector. (you may want to do this if you know that there will be a planned silence... eg on Armistace Day)

[At this point, I must make an important note - in keeping the design simple, there is a minor "limitation": if you use the "reset" to clear the "silence detected" state, then a further period of silence will not be detected until SOME audio input (enough to reach the "low" level) has occurred. I don't see this to be a big issue, as if someone has manually reset the system, they will then be "in control of the situation", and will be reinstating the audio in any case!]

The next bit of the circuit is a "monostable" (U3a): whenever the "LOW" level is pulsing, then the monostable will be constantly triggered, and the output of the monostable will remain high. If the pulses stop, then the output will go low after a several seconds (typically this would be something between 10 and 60 seconds): if that happens, then we have detected "dead air".

The "dead air length" is determined by R14, R15 and C5. R14 and C5 are fixed, but the length can be adjusted by R15: 0 ohms (ie a wire link) sets about 5 seconds, 150k gives about 20 seconds, and 390k gives about a minute. If you need a larger time, then you can increase the value of R15 further, to 1 meg max. You could also increase the size of C5. [Beware of the capacitor leakage current if you use a particularly large value for R15]

The second monostable takes this low-going output, and generates a short (several milliseconds) pulse, which sets the latch.

The "dead air detected" latch (U4A) is the bit that holds the current "Dead Air" state: normally this is low ("ok"), but is set high ("alarm" state) by the monostable circuit above.

It gets reset as described above, by either the audio returning or by the reset input being held low.

There are 2 outputs from the latch, one is HIGH when silence ("silence det"), the other is LOW when silence ("/silence det"). These 2 ouputs are used to drive one (or more) output circuits.

It may be that you would also like an output to indicate when the unit is disabled (eg to light a big warning light!): one is high, the other low, when silence detection is disabled.

What do I do with the outputs?

If you go & look at Article 7 you'll learn all about the Remote Start circuit: to work, that circuit needs a low signal to start, and a high signal to stop. Its input may therefore be directly connected to the silence det output (pin 5) of the latch. This will allow you to start something when dead air is detected, and stop it when normal audio returns.

However, you may well want to have one of the following outputs instead, or as well, on the latch output or the disabled output:

  1. an LED
  2. an open collector to 0v output
  3. a +5v signal output
  4. relay contact outputs
In that case, go take a look at Article 8 which details the (very simple) circuits needed. Most need to be driven from the silence det output (pin 5) of the latch (or the disabled output), with the single exception of the "+5v signal" which needs to be driven by the /silence det or /disabled outputs (ie the active LOW outputs).

Power Supply

The circuit requires a +5v supply for all the logic, and a dual supply for the audio input stage. The dual supply would normally be a regulated +/- 12 volt supply (quite sensible if the silence detector is part of some other piece of equipment, such as a station output switch). However, a +/- 5 volt regulated supply would be ok.... you could actually use the logic supply for the +5v.

Whilst the current used from the dual supply is minimal (< 10 mA per rail), the current requirement for the +5v supply is larger, and depends greatly on how many LEDs and relays you use. As a good guesstimate, assume that the electronics uses 50 to 100 ma, then add on the current used by the leds (typ 5mA each) and relays (a 500 ohm relay uses 10 mA when on, whilst a 200 ohm unit uses 25 mA).

Whilst it is probably OK to use a switch-mode PSU, it is probably best avoided, not only to prevent PSU noise from affecting other audio equipment, but to prevent noise causing incorrect operation of the audio levels. If you are having trouble finding a suitable PSU, then refer to Article #3 in this series, which deals with Power Supplies.

And Finally....

Finally, if you need any help with the circuit in any way, then feel free to email me with your questions.  


(plb, rev 2.1, Jan 2000)  


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