CCTV systems may operate continuously or only as required to monitor a particular event. A more advanced form of CCTV, utilizing digital video recorders DVRs, provides recording for possibly many years, with a variety of quality and performance options and extra features such as motion detection and email alerts. More recently, decentralized IP cameras, perhaps equipped with megapixel sensors, support recording directly to network attached storage devices, or internal flash for completely stand alone operation. The earliest video surveillance systems involved constant monitoring because there was no way to record and store information. The development of reel to reel media enabled the recording of surveillance footage. These systems required magnetic tapes to be changed manually, which was a time consuming, expensive and unreliable process, with the operator having to manually thread the tape from the tape reel through the recorder onto an empty take up reel.