modulation. They created a prototype meter with an integration time of about 1 ms. They found that the ear tolerates distortion of only a few ms, and that a 'registration time' of 4 ms is sufficient. They made the return time a compromise between a rapid return, which was tiring to the eye—and a slow return, which made control difficult. Engineers decided that the meter should take between 2s and 3s to drop back 26 dB. The BBC PPM became the subject of several formal standards: BS 4297:1968 (superseded); BS5428:Part 9:1981 (superseded) and then BS 6840-10:1991. The text of the latter is identical to the Type IIa PPM in IEC 60268-10:1991. Alignment level (0 dBu) and Permitted Maximum Level (+8 dBu) correspond to scale marks '4' and '6' respectively. The BBC PPM was adopted by commercial broadcasters in the UK. Other organisations around the world, including the EBU, CBC and ABC used the same dynamics but with slightly different scales. Modern British PPMs have a 4 dB spacing between the scale marks. Older designs had a 6 dB spacing between '1' and '2'. This discrepancy can sometimes also be found at the equivalent position on the derived CBC and ABC scales. From its inception in 1939 until 2009, the PPM display was available in the form of an electro-mechanical, moving-coil meter movement with a demanding ballistic specification. For many years these were manufactured by Ernest Turner and Company, and in later years by Sifam, based in Torquay. In 2009, Sifam announced it was ending production of the Type 74 dual-needle meter movement. In 2010, Sifam ended all PPM meter movement manufacturing. Three major users—Bryant Unlimited, Canford Audio, and TSL—placed final orders with Sifam for large stocks of the meters to supply manufacturing and maintenance activities for several years. Stereo British PPMs In the UK, twin-needle PPMs are sometimes used for stereo. Red and green needles are used for left and right. This colour convention comes from port and starboard navigation lights, in accordance with an ancient BBC in-joke about Broadcasting House resembling a battleship. White and yellow needles are used for sum and difference (M and S). A more recent variation is to use a black needle with a dayglo orange tip for S instead of yellow. The sensitivity of the S indication can be increased on some meter installations by 20 dB; this is to aid line-up procedures, e.g.,