The lavalier microphone from Walimex kommit with a RD. 120cm long cable and can be plugged inconspicuously to clothing thanks to clip and its small dimensions. The recording quality is significantly better than the direct use of the integrated smart phone microphone, overall I was surprised at the quality. However, one should be sure to use an app that can regulate the level, otherwise there will if necessary clipping.
Anyone wishing to use the microphone, which has a 4-pin connector to other devices has to if necessary an adapter to 3-pin plug get, but the there for a few EUR.
