period of two hours. The original requirement was for a CVR to record for 30 minutes, but this has been found to be insufficient in many cases because significant parts of the audio data needed for a subsequent investigation occurred more than 30 minutes before the end of the recording. The earliest CVRs used analog wire recording, later replaced by analog magnetic tape. Some of the tape units used two reels, with the tape automatically reversing at each end. The original was the ARL Flight Memory Unit produced in 1957 by Australian David Warren and instrument maker Tych Mirfield. Other units used a single reel, with the tape spliced into a continuous loop, much as in an 8-track cartridge. The tape would circulate and old audio information would be overwritten every 30 minutes. Recovery of sound from magnetic tape often proves difficult if the recorder is recovered from water and its housing has been breached. Thus, the latest designs employ solid-state memory and use digital recording techniques, making them much more resistant to shock, vibration and moisture. With the reduced power requirements of solid-state recorders, it is now practical to incorporate a battery in the units, so that recording can continue until flight termination, even if the aircraft electrical system fails. Like the FDR, the CVR is typically mounted in the rear of the airplane fuselage to maximize the likelihood of its survival in a crash. Combined units With the advent of digital recorders, the FDR and CVR can be manufactured in one fireproof, shock proof, and waterproof container as a combined digital cockpit voice and data recorder (CVDR). Currently, CVDRs are manufactured by L3Harris Technologies and Hensoldt among others. Solid state recorders became commercially practical in 1990, having the advantage of not requiring scheduled maintenance and making the data easier to retrieve. This was extended to the two-hour voice recording in 1995. Additional equipment Since the 1970s, most large civil jet transports have been additionally equipped with a "quick access recorder" (QAR). This records data on a removable storage medium. Access to the FDR and CVR is necessarily difficult because they must be fitted where they are most likely to survive an accident; they also require specialized equipment to read the recording. The QAR recording medium is readily removable and is designed to be read by equipment attached to a standard desktop computer. In many airlines, the quick access