Scope audio - this should work shouldn't it?

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I'm tempted to make and post a YouTube video showing my method in this but before I lose all my marbles can I make a quick check with reality? 

 

So if you have a sound source (probably a synth but to narrow it down I ended up trying this with my mobile). If I plug in my Sony headphones, that have a 3.5mm TRS jack at either end of the cable, and play some music on the phone (it was actually Transvision Vamp but could have been anything) then, sure, I hear Wendy's dulcet tones in the headphones. So the signal is routing correctly and the cable is working. 

 

If I now unplug at the headphone end, clip the ground clip of a scope onto the Shield of TRS and then touch/clip the probe of the scope to Tip or Ring and press "Auto" on the scope I should just see "music" which stops and starts as I start/stop the track on the phone displayed on the DSO shouldn't I? Only thing is, I don't. There's a noise flaw but I was expecting a wildly varying signal. But there's virtually nothing. 

 

Can I assume there's nothing more to this - like the signal has to actually play into a load to be monitored or something like that is there? If the pins are just open to the world I should still see the signal (even an FFT if I enable that) on the scope? 

 

Scope is Hantek DS02D15 by the way.

 

I just don't know what I'm missing or what I'm doing wrong or failing to do. 

 

Any ideas? 

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IIRC the phone has some smarts in it which uses the connected load to determine just what type of thing is connected. 

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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Maybe try putting a few hundred, or even 1k load on the output wires---maybe that will "wake up" the output.  This seems odd that you might need it, since if going to an amp instead, that would generally be a very high impedance.

 

and press "Auto" on the scope I should just see "music" which stops and starts as I start/stop the track on the phone displayed on the DSO shouldn't I? 

Make sure the music is playing BEFORE auto (set)...automatic adjustments only works when a signal is there so it can tune itself to show, say, 3 cycles of the wave.

Auto mode is a triggering mode, that just forces sweeps, rather than being hung up forever waiting for a sweep trigger to trip.  Yes, in auto mode you should see something, assuming you turned all the knobs properly or used autoset.  Sometimes autoset can be fooled & you must turn the knobs yourself!

 

I have a couple of scope probes who's witches hat's look like they clip to the wire fine, but inside the hat's bottom it isn't connecting to the probe proper, unless the hat is seated firmly down on the assembly.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Aug 3, 2022 - 08:43 PM
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The thing is I was only doing this with my phone as a reality check as the audio (headphone) output of a synth I was trying to monitor behaved the same. I guess it's possible its headphone output has the same "smarts" as the phone to try and balance the signal based on load?

 

Will try the resistor load suggestion and see what transpires. In fact I have a ton of 10K pots so might just try that so I can "twiddle" the value. 

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I've done exactly what you've done but with a PC instead of a phone. I played an impulse response to obtain the pretty sinc function from the digital filter in the DAC.

I've no scope at home so can't reproduce your test.

 

clawson wrote:
and press "Auto" on the scope

I find that only works if you have a repetitive signal. Music is all over the place.

 

E.g.

 

 

Auto may turn the timebase up to MAX to try to "find" a waveform.

 

I'd turn the timebase right down into "Roll Mode". You'll always see some sort of signal there even if it's just a big bar of fuzz. Then adjust the timebase faster from there.

 

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Phones have a 4 way connector usually, the extra one is for the mic, for the sleeve is have the size of a stereo connector.

 

Unless you have the correct connector you may be shorting out something.

 

There seem to be different wiring for the 4 way connector, the picture below shows the ground can be the 1st or 2nd connection.

 

 

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

Last Edited: Wed. Aug 3, 2022 - 10:19 PM
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I think Cliff did exactly what I did and that's unplug the jack at the headphones end, leaving the cable connected to the phone//PC. I.e. the 4-way TRRS jack stays in the phone.

 

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An expected audio signal into headphones/earphones will be on the order of a volt or two. Professional audio expects to see a 600 ohm load and will give around a volt peak to peak (775mV for a zero level sine wave) and 0.5-1ms per division is a good scan speed - if there's something there, you'll see it. If it's driving into an open circuit, it'll just be a taller signal. Though 'auto' with audio tends to set the scan speed way too high, which can be confusing if you're not expecting it.

 

But as indicated earlier, phone and computer audio is weird and wonderful stuff, with complex switching algorithms that can't tell the difference between a microphone and the short circuit of a tip-ring-sleeve jack... it could well have turned your audio off for reasons of its own.

 

Neil

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Why dont you start by using an (old) headphone and measure parallel to the speaker ? A perfect debugger which at least will tell you where to search for the problem.

Besides the other things mentioned I was also thinking of on of these causes :

* Signal too low for scope to show

* Scope's ground -> Is you phone also connected to ground ?

 

Last Edited: Thu. Aug 4, 2022 - 06:03 AM
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I did a job years ago squirting DTMF into iPhones and Androids. Needed a 2k resistor across the mic input before the phone would see the mic input. I don't remember if that also wakes up the headphone output, or if there's a separate resistor needed - the device I was building that used the output was powering the device from a phone generated sine wave, so already fairly low impedance.

Do note the earlier post from js re connections. The mic and ground are sometimes swapped.

Four legs good, two legs bad, three legs stable.

Last Edited: Thu. Aug 4, 2022 - 08:10 AM
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* Scope's ground -> Is you phone also connected to ground ?

Make sure your phone is on battery (or the scope on battery) for this testing ...you don't want some crazy mis-ground swap shorting.   It could even be that neither output wire is at gnd (like a differential output) and pulling either to the phone gnd causes a "short".

 

Long ago we had some volt meter LCD modules that used a 9V battery.  We got the "great" idea that instead of using a battery (and needing to unscrew an industrial panel & replace) , we'd just tie to gnd & a 9V source...big mistake-- the meter signal - and meter power -  must float apart, as they will (must!) have a difference during proper module operation.   The 9V - would actually ride (float) part way between the input signal +/- terminals.  so back to te battery it was!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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John_A_Brown wrote:
I did a job years ago squirting DTMF into iPhones
smiley I did a job that accepted DTMF tones FROM iPad earphone outputs into a MT8870 DTMF decoder chip. The load on the earphone output was of the order of 100Kohms. Made 50 units for a client in Sweden.

Ross McKenzie, Melbourne Australia

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N.Winterbottom wrote:

I think Cliff did exactly what I did and that's unplug the jack at the headphones end, leaving the cable connected to the phone//PC. I.e. the 4-way TRRS jack stays in the phone.

This.

 

I know that PCs and phones can also accept a "headset" with a microphone (TRRS) as well as just headphones (TRS) but this works because in TRS the shield is longer while in TRRS (for the mic) the Mic connection is the lower part of the shield (nearest the body of the jack). So the fact that I use a three pin TRS (as pretty much everyone plugging headphones in might) should not make a difference.

 

BTW the headphones are Sony WH-700N which are principally Bluetooth headphones and this is why, at the headphone end, it is not just simply hardwired (as most wired only headphones are) but it has a jack connection at that end of the cable too as usually you would use the headphones without any cable. That's why I was able to "break in" and put the probes on the wire that was working when it plugged into the headphones.

 

So yeah, I will try manually setting timebase and volts/div in case Auto has put me in the wrong place.,

Paddy72 wrote:
Why dont you start by using an (old) headphone and measure parallel to the speaker ?

Simply because that involves "breaking in" to a cable. I do have any number of 3.5mm TRS so I may "butcher" one to break into it. Actually I have a number of these:

 

So if I put two back to back to back with 3 connecting wires I should be able to break in at the screw terminals.

 

Paddy72 wrote:
* Scope's ground -> Is you phone also connected to ground ?
Nope. It's a "mobile" phone ;-) I naively assumed that if I connected the scope's ground clip to the ground in the TRS/audio jack that it would make a common ground - is that not the case ?

 

Thanks folks - I think I have a few leads (oh dear!) to go on here.

 

(again it's not really about the phone - that was just to make a test to see if I could see any kind of audio - it was because I couldn't see audio from a synth that I then tried the phone experiment - but hopefully if I can resolve this for phone source the same will work for synth. I was simply trying to "see" how operator feedback in an FM synth morphs the wave between triangle/sine/square in fact! I was also keen to see the FFT in the scope in operation to see the various overtones (obviously none in case of sine))

 

 

Last Edited: Thu. Aug 4, 2022 - 10:00 AM
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barnacle wrote:

But as indicated earlier, phone and computer audio is weird and wonderful stuff, with complex switching algorithms...

 

I've had problems with laptops that, when connected directly to an audio mixer, would 'blend' the stereo a little thinking that there was a pair of headphones connected. By 'blend' I mean mix a bit of L into R and vice versa.

 

That's fine for stereo audio but not for a track holding mono audio on one channel and SMPTE timecode on the other.

 

And to complicate tracking down the problem it depended on whether the audio system was connected before or after booting.

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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Too long as a pro broadcast engineer: audio happens on PO B-gauge jack; tip is signal or left; ring is -signal or -right, sleeve is ground. Anything else was playground stuff. It's all probably gone digital since I left broadcast :D

 

Neil

Last Edited: Thu. Aug 4, 2022 - 07:17 PM
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Cliff the breakout connector you show is a 3 way tip-ring-sleeve which is NO GOOD. You need a 4 way connector with Tip-Ring-Ring-Sleeve. See the images in #6.

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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OK just to report that on 100mV/div and 1ms/div I got a signal that is something like +/- 200mV that clearly flat lined when I stopped playback. I was expecting 1Vpp but it's enough to see what's going on. 

 

That was out of phone. I just need to retry with Sonicware Liven XFM. 

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Cliff, did you have your probe set to X10 perhaps?

Ross McKenzie, Melbourne Australia

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Yeah probe (and scope!) were set to x10

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Yeah probe (and scope!) were set to x10

I remember some guy counting gridlines in the lab, thought that probe would make the signal 10 times bigger.  So all his readings were off by a factor of 100! 

 

Also, remember always use DC coupling (unless you effectively can't).

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Just for completeness I made a video about something else (shiny new digital recorder) but you can see the scope working in the background of this...

 

https://youtu.be/Hco4SCH05TM

 

Oh and this one shows one of the basic oscillators of the synth producing Sine, saw, square (kind of!) and noise... 

 

https://youtu.be/9cDQj_MKats

 

In the end I can only get signals like this using the line level output of the synth. If I plug the same 3.5mm TRS into the adjacent headphone jack I can't get anything visible. Not sure why - but I have a solution and that's all that really matters.