Threshold vs Volume

[sborowko]

What does the threshold control do that distinguishes it from the volume control? Is it somehow changing the way the return signal is sampled?

 

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[jmc24]

Are you asking about the threshold control on the current TDI SL. There is no volume control on the TDI SL. You can use headphones with an inline volume control. So, at least on the TDI SL, the threshold tone is a background sound that is the same tone that generates the target tone as far as I can tell. It is important for ground balancing, for setting the gain and for setting the pulse delay on noisy ground and EMI. On the TDI SL it is a really steady tone and if set at a barely audible level and with smaller coils, it can give easy to hear responses on very small gold or micro jewelry. Depending on whether you are using the High/Low/All conductivity switch, it will also slightly null or break up a bit on discriminated targets. It is also possible to set it too low and really small gold nuggets, micro jewelry or tiny trash-ground noise will not be heard. It does not make an individual target sound louder if you turn up the threshold tone. It just makes the constant background tone louder. If you are hunting for bigger stuff like coins, bigger jewelry or relics you can turn it down to the point where you can't hear the background tone but you will still hear a target response if you pass the coil over a decent sized target. As far as return signal sampling...............no clue!

Jeff

 

[sborowko]

Hi Jeff, thanks for the excellent info on setting the threshold. I’m looking at the TDI Beach Hunter, which has both threshold and volume controls.

I am trying to get a better understanding of how these two controls affect the received waveform. My guess is that the volume control amplifies the threshold, noise and target signals equally, in which case the s/n ratio is unaffected by the volume control. If that is true, then this control is used to set personal loudness preference, or to adjust for quiet or wind noise, etc. Does that sound right?

The theory behind the threshold control seems a bit more nuanced. If I understand it correctly, it sets the threshold level either: 1) independent of target signal levels, or 2) affects both, but not equally, with an emphasis on changing the threshold level.

If viewed on an oscilloscope, would the threshold control be affecting the amplitude of threshold signal, independent of the target signal, or is it doing something else?

 

[jmc24]

I am not an audio engineer and have absolutely zero interest in becoming one so I cannot answer your questions in that direction. I am a detector user and I will answer your questions as they relate to what actually happens when I operate the TDI SL as an end user.

First, the TDI Beach Hunter has a threshold control and a separate volume control for one simple reason. Waterproofing. It has hardwired headphones which cannot be disconnected easily and which have no adjustable inline or ear cup volume control. The TDI BH also does not have an external speaker like the TDI SL does. The TDI BH also has a hardwired, non-removable search coil too. The TDI SL can use a multitude of different screw on connector coils.

So again, I can only describe what happens on the TDI SL like I tried to do in my first post. I don't have a Beach Hunter. It is my understanding that they have exactly the same motherboard however. Using headphones with an inline volume control, if I turn up or turn down the volume level, the threshold tone and target tone volume levels will increase or decrease simultaneously equally. If I use the same target and I turn down the intensity of the threshold tone to where it is barely audible or even silent, the target tone volume level stays the same unless I change the headphone inline volume level. If I use the same target, do not adjust the inline volume level and turn up the threshold tone really high, the volume of the target tone stays the same.

Anytime a target is detected while there is an audible threshold tone, there is a momentary silencing of the threshold tone followed by a modulated target response, followed by a momentary silencing of the threshold tone after which the threshold tone becomes steady again at its former volume level. So, some aspect of the audio circuitry of the threshold tone generates the target tone when a target is detected. On some detectors made today that is not the case. The threshold tone on those detectors is only a reference tone and is used for signaling ground phase and ground mineralization changes along with nulling on discriminated targets. The actual target tones are on a completely separate circuit.

Jeff

 

[sborowko]

Thanks Jeff, that’s a very helpful explanation!