PPMs are often implemented as 'bargraph' incremental displays using solid-state illuminated segments in a vertical or horizontal array. For these, IEC 60268-10 requires a minimum of 100 segments and a resolution better than 0.5 dB at the higher levels. Many operators prefer the moving-coil meter type of display, in which a needle moves in an arc, because they feel the angular movement is easier for the human eye to monitor than the linear movement of a bar graph. PPMs can also be implemented in software—in a general-purpose computer or by a dedicated device that inserts a PPM image into a picture signal for display on a picture monitor. Level definitions A variety of terms such as 'line-up level' and 'operating level' exist, and their meaning may vary from place to place. In an attempt bring clarity to level definitions in the context of programme transmission from one country to another, where different technical practices may apply, ITU-R Rec. BS.645 defined three reference levels: Measurement Level (ML), Alignment Level (AL) and Permitted Maximum Level (PML). This document shows the reading corresponding to these levels for several types of meter. Alignment Level is the level of a steady sine-wave alignment tone. Permitted Maximum Level refers to the permitted maximum meter indication that operators should aim for on speech, music etc., not tone. Scales and scale marks PPMs often use white-on-black displays, to minimise eyestrain especially with extended periods of use. PPMs are usually calibrated in one of these ways: In decibels relative to Alignment Level (e.g., Nordic, EBU) In decibels relative to Permitted Maximum Level (e.g., DIN, ABC, SABC) In decibels relative to 0 dBu (e.g., CBC) In decibels relative to 0 dBFS (e.g., IEC 60268-18) In simple numerical marks that can be correlated with any of the above (e.g., British) Whichever scheme is used, usually there is a scale mark corresponding to Alignment Level. Most PPMs have an approximately logarithmic scale, i.e., roughly linear in decibels, to provide useful indications over a wide dynamic range. Integration time Quasi-PPMs use a short integration time so they can register peaks longer than a few milliseconds in duration. In the original context of AM radio broadcasting in the 1930s, overloads due to shorter peaks were considered unimportant on the grounds that the human ear could not detect distortion due to momentary clipping. Ignoring momentary clipping made it possible to increase average modulation levels. In