Audio tone/pitch control help - Page 1
Forum section: electronics / beginners

author: downhalo
author_rank: Contributor
timestamp: August 06, 2014, 11:36:05 am
content: Hi all, Newbie here looking for some info/help..Here's the 'problem', I have a minelab PI (pulse induction) metal detector, it has a fixed Audio threshold tone, no volume control or tone control.. quite annoying as the default tone is ear rape.Volume isn't actually a problem.. I can deal with that easily, what I'm after is a way to modify the tone or 'pitch' of the audio threshold (constant background tone), As i said its currently a fixed 'high' tone. I'd like it to be lower as i mentioned its quite ear rapey..Unfortunately minelab have a policy of painting the PCB/components white, so I can't really provide much info..The audio tone outputs from the detector to a small powered Class D Amplifier i put together... No idea if the audio tone can be modified after the fact? by some kind of filter?? Sorry can't be of much help, pretty new to all the electronics stuff.Anyway If you've got any ideas I'd love to hear them..Appreciate your time..-Dh.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 06, 2014, 12:09:21 pm
content: Being a beginner who knows more about synthesizers than electronics here's what came into my head...I don't now how to modify the detector to change the tone internally but if it's a square wave (if it's not you could square it up first) you could divide it down to 0.5 or 0.25 the original frequency using plain old flip-flops or a Johnson Counter.http://en.wikipedia.org/wiki/Frequency_dividerIf the square wave is too annoying you could probably use an integrator to convert it back into something more pleasant like a triangle wave before it goes into your amp.http://www.electronics-tutorials.ws/opamp/opamp_6.html

author: downhalo
author_rank: Contributor
timestamp: August 06, 2014, 12:30:03 pm
content: The opamp Is worth a try, I'd like to do it without modifying the actual machine itself, (though it can and has been done) a lot of the people doing these modifications are not so open source with the info on how to lol..I'm sure its not overly complicated to modify the detector itself, but I dare not for fear of killing a $2200 machine..

author: dentaku
author_rank: Frequent Contributor
timestamp: August 06, 2014, 10:11:52 pm
content: By the way, what kind of tone is it?Sine, Triangle, Square? What frequency is it?

author: peps1
author_rank: Regular Contributor
timestamp: August 06, 2014, 10:18:49 pm
content: Could you not just solder in a passive tone control in between the board and speaker? (being passive there will be some volume drop, but you can wire this whole tone-control onto one pot)

author: downhalo
author_rank: Contributor
timestamp: August 06, 2014, 10:34:38 pm
content: Its a Sine, between 700-800 Hz (well thats what Mr Siglent scope said..

author: w2aew
author_rank: Super Contributor
timestamp: August 07, 2014, 01:26:49 am
content: A tone-control circuit will not alter the frequency of the signal going into it - it will just alter the gain for various frequencies. So, if you have a single frequency tone coming out of the detector, a tone control will just alter the volume. If the signal has any harmonic content, then a tone control will change the way it sounds, but won't alter the fundamental tone. But, since the base frequency is 700-800Hz, the harmonic content will be above that, so it won't make it sound lower than the 700-800Hz.

author: downhalo
author_rank: Contributor
timestamp: August 07, 2014, 01:42:40 am
content: Yeah, i figured it wouldn't, has to be internal.. Guess I'll wait for someone to post the mod on Geotech Thanks all..

author: dentaku
author_rank: Frequent Contributor
timestamp: August 07, 2014, 02:50:58 am
content: Quote from: downhalo on August 07, 2014, 01:42:40 amI still think it would be an interesting experiment to convert that 750Hz sine wave audio signal to a square (with a comparator or a 40106 Schmitt Trigger inverter) then divide it so it's an octave lower (with a JK Flip-Flophttp://www.electronics-tutorials.ws/counter/count_1.html) then lowpass filter it again so it's not so annoying

author: dentaku
author_rank: Frequent Contributor
timestamp: August 09, 2014, 02:27:41 am
content: I know it's probably not practical for the original purpose of this post but if someone WOULD want to pitch down an audio signal which is a simple single frequency sine wave by an octave, would this be a ridiculous thing to build or not?A comparator to convert it to pulses, a D-Type flip-flop to divide the frequency by 2, some amplification and integrating/filtering to make it less annoying (tirangles are less annoying than squares trough headphones).I know an LTSpice simulation and the real world are different but it looks like an interesting experiment.Here's 2 .wave files. One of the input and one of the output.https://app.box.com/s/ta67a7tm2jq6unols62ghttps://app.box.com/s/6rsvkzc9bwd99bqw1wrb

author: downhalo
author_rank: Contributor
timestamp: August 09, 2014, 08:28:17 am
content: It would probably do the job.. but not practical.

author: rs20
author_rank: Super Contributor
timestamp: August 09, 2014, 11:03:37 am
content: Quote from: dentaku on August 09, 2014, 02:27:41 amI think this is actually pretty practical, especially if the circuit is simplified a bit -- just two chips and some passives:-- The first TL072 isn't doing much, is it? Just gain? Get rid of it, do all the gain you need at the same time as integrating.-- [ Bonus: Make the the low-pass filtersecond-orderfor improved non-spikiness ]-- Now you've got a spare half-TL072, use it instead of the LM339. At ultra-low frequencies like this, it's going to work fine, and saves you one pull-up resistor too.-- Don't re-generate 2.5V 3 different times, no need to keep them clean and isolated in this circuit. Especially since the right-hand side doesn't need 2.5V anymore.Simplified circuit attached. One "trap for young players"; LTSpice's model for the D flip flop outputs 1V, which is different to a real flip flop which outputs supply voltage. No big deal, just different volume at the output.[ All this aside, wouldn't it be simpler to just detect the presence of the input signal and output a tone of whatever frequency we choose? That'd be an even simpler single-chip solution with a 556 dual-timer. Or is the frequency changing so that it's important to maintain a relationship with the original frequency? ]

author: dentaku
author_rank: Frequent Contributor
timestamp: August 09, 2014, 04:49:50 pm
content: Quote from: rs20 on August 09, 2014, 11:03:37 amBeing quite clueless about what I was doing I'm amazed my simulation worked at allThis morning I was curious if it would be better and simpler to skip the dedicated comparator and just use an opamp. I was also wondering if the flip flop would like the mushy rounded edge square wave you sometimes get by not using a real comparatorI was also wondering how to use the filter to adjust the gain, so thanks for that info.I was quite puzzled why the flip-flop was only outputting 1V. That added to the complication.I'm going to try your circuit in LTspice right now. I was worried about how the output of every stage was offset up into the positive voltages but I guess it doesn't really matter as long as the final output is correctly AC coupled.

author: KJDS
author_rank: Super Contributor
timestamp: August 09, 2014, 05:32:33 pm
content: Find the oscillator circuit and add some more capacitance until the frequency is at an acceptable level.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 09, 2014, 06:16:01 pm
content: Quote from: KJDS on August 09, 2014, 05:32:33 pmI think the original poster doesn't want to modify the expensive detector in any way.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 09, 2014, 06:24:01 pm
content: Quote from: rs20 on August 09, 2014, 11:03:37 amI did a simulation using your circuit with an LM324 (it didn't work with my TL072 model for some reason) and it seems to work fine.input:https://app.box.com/s/gh00qvmjes7dgbse68eisquare:https://app.box.com/s/z8ewwwkz9iyzdqvw2sshoutput:https://app.box.com/s/gm9l78jaj3cg6dooyetrI guess I'll have to build one in real life now

author: dentaku
author_rank: Frequent Contributor
timestamp: August 10, 2014, 11:35:01 pm
content: well, so far I've got the 1Vpp sine to 4V square part working well and the D-Flipflop divides that square by half and gives me a nice 5V square output.I'm having trouble with the filter though. It's putting out a strange looking signal. In fact it's all messed up even at the output of the flip-flop before it even reaches the filter but fine when I removed the filter. It's just a chunky square-ish looking thing I didn't take a photo of it before removing that part of the circuit.All of the inputs on the unused side of the 4013B are connected to ground but I'm not sure what to do with the two opamps that aren't being used.I might just build an integrator and try to get a triangle wave instead of filtering it back to something more sine-like.I think I'm going to try with different opamps too because I was just experimenting with a crusty old LM324.

author: dentaku
author_rank: Frequent Contributor
timestamp: August 11, 2014, 01:30:59 am
content: I just used caps and resistors to make a simple lowpass filter that works fine at 800HzI'm still going to get a filter working with an opamp but this works well.

author: IanB
author_rank: Super Contributor
timestamp: August 11, 2014, 02:22:19 am
content: Quote from: downhalo on August 06, 2014, 11:36:05 amPresumably the tone goes down in the presence of metal near the detector?So depending on the proximity/quantity of metal, the tone could decrease all the way down to something inaudible?Therefore if you lower the frequency of the base tone, you will presumably make some things undetectable. Is that what you want?

author: rs20
author_rank: Super Contributor
timestamp: August 11, 2014, 07:52:14 am
content: I have to take back my advice that op amps can be pressed into service as comparators, or at least attach a few "if's" and "but's" to it:http://www.analog.com/en/all-operational-amplifiers-op-amps/operational-amplifiers-op-amps/products/raq_jbryant_comparators_opamps_may_issue11/resources/faq.html?display=popup

author: downhalo
author_rank: Contributor
timestamp: August 11, 2014, 08:12:02 am
content: Basically, the detector gives a (Low-High) tone for high or larger conductivity targets, and (High-Low) on small or lower conductive targets..So the threshold is running all the time at 800-850 Hz (maybe 900 scope is a bit all over the shop.. anyway constant tone with wobble (due to RF)Wouldn't mind modifying the detector itself, However the bastards at Minelab painted the entire PCB white, and filed off all the part numbers..all the guys 'in the know' aren't willing to share the info, when they can charge between $580-$1250 to modify them for you (which is fair) everyone wants to make a buck..Anyway I have the technical ability to do it, but not the electronics knowledge... Oh well.. I'll just have to deal with the nasty ear rape threshold..

author: downhalo
author_rank: Contributor
timestamp: August 11, 2014, 10:18:57 am
content: This is the Audio "section"? of the PCB.. as you can see the frigging white pain covers all the goodies.I grounded an earbud and probed all the resistors and pins on the IC's my thinking was, in other models they used variable resistors to adjust the tone.. so i figured, probe behind some of them, maybe pick up the un altered tone?Remember < noob lol..Help me become a turbo geek!

author: dentaku
author_rank: Frequent Contributor
timestamp: August 11, 2014, 06:13:22 pm
content: If the sound is being created by a simple analog circuit then there's a capacitor and a resistor somewhere that set the frequency.If the resistor or capacitor is made larger the tone will get lower.If it's purely digital I'm not sure.By the way, I learned some new stuff by kinda hijacking your postI now have a circuit that inputs a tone and outputs a roughly sine-ish tone (or square if I omit the filter) one octave lower. All it needs is an opamp, a D-type flipflop some resistors and capacitors.

author: downhalo
author_rank: Contributor
timestamp: August 12, 2014, 01:13:17 am
content: I don't mind the hijacking! long as someone's learning something... I believe it is an analog circuit, I've gotten hints from people that changing a cap and adding a pot is the way to go.. just a shame none of the jackasses want to share more info.

author: downhalo
author_rank: Contributor
timestamp: August 13, 2014, 09:58:03 am
content: Quote from: dentaku on August 11, 2014, 06:13:22 pmWell It might be something i put together at some point since i can't seem to get any info elsewhere.. Not even Minelab can help.. apparently the "information is archived" LOL!?Yep, the pricks filed off the part ID's..