Got given a strange issue, by an acquaintance with the problem, and thought maybe someone here already knew all about this sort of hardware issue. (If this is indeed the right place to ask about it.)
A tablet device playing a particular video media file via its internal speaker behaves as expected. Speech from the narrator/on-screen speaker with a background of atmospheric music.
With headphones (earphones, actually, and only ones immediately at hand) inserted, the audio one can hear is pretty much all the atmospheric music with a hint of electronic noise that would seems to be remnants of the speech element. As of this time, I have had no opportunity to test vs other media, nor other audio plug-ins.
Three ideas as to why this happens, occured to me:
1) The internal sound-source is something like 5.1 (certainly more than stereo) and the media player/firmware/hardware is only sending the 'secondary' channels to the headphone port, and the media file is channelled so that speech is restricted to the 'front and centre(right/left)' channels that aren't being shoved into the stereo 3.5mm socket, apart from a small amount of 'leaked' vocal-channels signal; meanwhile the background music is on the more auxilliary channels (or all of them, mixed) so being sent through the connector to be heard,
2) Very similar to this, the 3.5mm socket (which must already have facility to take mic-in signal from suitable peripheral devices) supports multi-coaxed barrels beyond the current plug's three contact areas (left, right, common-ground) and somehow the plug isn't seating correctly and so its L, R, G barrels are picking up only the (overwhelmingly) non-speechy signals,
3) The headphones are internally damaged. A common problem… the reason I don't have any others to test with right this moment is that my last lot of my own have gotten touchy with internal cross-talk/shorting and I had meant to go buy another £5 set (to replace this less-cheap but seemingly more fragile £10 set that were the cheapest I could buy when I last went for some) today if I hadn't been asked to deal with what turned out to be this particular problem. Anyway, is it possible that the damage in this particular set of audio cabling is acting like a high/low-pass filter on the audio that attenuates a majority of the speech frequencies without distorting the music? Long shot, this one, except that what sound there is can be modulated by manipulating the headphone wire just outside the moulded-on flexisheath that acts to crimp the cable into the plug, so already we know that he is imminently in need of a new buds, like I was.
I've suggested they get some newer earphones anyway, and maybe they'll work. Meanwhile I'm going to look in my various toolkits for something small and thin to delicately prod within the headphone socket to see if (as I've seen before) there's fluffy detritus in the bottom of the socket that prevents the plug from plunging all the way in, properly (thus mismatching against the >3 internal contact bands, perhaps), plus if I can get the problem media off the device in a form that I can shove through ffplay/ffmoeg I can identify how many audio channels it has (and visually see which are doing what during speech-filled parts) or at least try it on other hardware. (Also, I'll perhaps create a media file with each channel 'manually' merged together with my own material, to identify which of the up-to-5.1 channels go where, if anywhere.)
At a push, I can get someone else who is mlre comfortable with opening the tablet device to check the metal tracks/soldering leading up to the socket for breaks/dry-jointing. If it's firmware or something gone wrong in the OS audio driver/handler in software then that might never get resolved short of complete reinstall, or not even then.
But I don't know if I'll get all of my ideas tried out before Monday, anyway. And you lot may have seen this sort of thing already. Which might be useful even if I 'solve' the problem by juat taking things apart and putting them back together again (as is not unknown!), without even properly discovering the original cause. Comments and suggestions about ideas I've otherwise overlooked may thus still be welcome for the forseeable future.
The magic smoke.
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