Portable multi-purpose audience measurement systems, apparatus and methods are disclosed. An example method to monitor a media presentation device disclosed herein comprises sensing a media content signal output by the media presentation device using a portable device separate from the media presentation device, the portable device supporting media metering functionality and native functionality, attempting to detect a code encoded in the sensed media content signal using the portable device, and generating a signature from the sensed media content signal using the portable device when the portable device is electrically coupled to an external power supply or the portable device has at least a threshold amount of local battery power, but not generating the signature when the portable device is not electrically coupled to the external power supply and the portable device does not have at least the threshold amount of local battery power.