In the mid-2000s, networks cried foul, blaming Nielsen for inaccurate rating measurements. This public attention was just the beginning, as Nielsen implemented its automated Local People Meter (LPM) technology. The LPM marked the shift from active, diary based local measurement to more passive, meter-monitored measurement of local markets. Technologically, the LPM is very similar to the original Nielsen People Meter. The key advancement was that the LPM provided accurate measurements of particular local markets. The LPM system has also allowed the industry to measure year-round, rather than the quarterly "sweeps" periods. Researchers believed that the LPM more accurately reported the full range of programming viewers watched, including that while channel-surfing. Arbitron's Portable People Meter uses a microphone to pick up and record subaudible tones embedded in broadcasts by an encoder at each station or network. It has even been used to track in-store radio.