iPod Touch, SpectrumView and external mic input

A few days ago I mentioned SpectrumView as being a very useful package for the iPhone and iPod Touch 4g for audio measurements and spectrum analysis. Well, I've now got a 4 pin jack plug and wired it up (according to the best published data I can find) as an external mic input. To my great disappointment nothing happened and the internal mic was still being selected. I am wondering if something has to be asserted to select the external mic input? Anyone know please? What did I do wrong? The ring nearest the jack body is the mic input and the next ring down is supposed to be the ground. I checked the plug was correctly wired. Help!