So it appears the basic functionality I need for balanced signal generation and media player/recorder is easily do-able on the stripped out Iphone
No 3rd party apps required , no ET phone home in the middle of the night ,no wifi ,bluetooth ,phone function or sim ,Apple health permanently off ,almost every green toggle in the entire iOS is set to off , All the many bundled apps and reminders are gone, uninstalled or otherwise out lunch ,
I think I might actually be able to live with the thing
Combined with a good Fluke meter that reads in dbu/dbv ,youre test tones on the handheld and a few patch cables , you already have a very powerful on the go test rig ,
Re-jigging the microphone input on the 3.5mm TRRS as an aux input to a scope type waveform display on the phone is the obvious icing on the cake . Of course theres many many ossiilloscope apps both free and very cheap , the kind of thing IT college kids do as projects mainly , there are a few nicer ones with multimeter and db scale readouts , and your still down the bargain basement at less than a 5 euros for the licence . Theres even a few that mimic the phosforesence with persistance curve , very nice indeed , pleasing on the eyes .
An important thing I was able to do was go into audio settings and set it so the default file format is WAV , now I can easily find a set of high quality WAV test tones at any sampling frequency /bit depth, any fundamental frequency or level . That also ensures any so called lossless data compression /format conversion never happens in the archiving system , assuming its only wavs I'll put on to it of course.
There was one other nice gadget that caught my eye , its an audio adapter cable for the Iphone which can function as either stereo line in or out on a single TRS jack . That way if you have an older phone with the built in headphone jack you also have the use of the line in via the Lightening socket . I know there might be better DAC's and DA's but the miniscule size of these audio interface are what makes them super handy in a mobile test rig situation .Battery supply is also easy to arrange.