Other important microphones introduced by Neumann during the immediate postwar period included the M 49 and M 50, both based on designs researched and engineered at the NWDR in Germany. The M 49 used the M 7 capsule in a configuration whose directional pattern was remotely controlled, the first microphone to offer such a feature. The M 50 featured a small, diffuse-field equalized pressure transducer embedded in the surface of a 40 mm hard plastic sphere, which gave it increasing directionality above the mid-range frequencies. The company also produced equipment for electroacoustic measurement, including calibrated measurement microphones and chart recorders. During the period from 1953 to 1956 Neumann introduced a series of small condenser microphones (KM 53, 54 and 56) especially for use in television broadcast studios. In 1957 they introduced the SM 2 microphone, which was essentially a pair of KM 56 microphones in a single body, arranged so that their directional patterns could be controlled remotely. The SM 2 was the world's first stereo microphone. End of the 1950s At the end of the 1950s, the Telefunken VF 14 vacuum tube on which the circuitry of the U 47 and U 48 had been based, was discontinued, so Neumann came under pressure to develop a successor. They decided to offer all three of those two models' directional patterns in a single microphone. In the meantime, the rock 'n' roll era had begun and some engineers were recording loud vocals with singers singing directly into microphones at very close range; when the U 47 or U 48 were used in this way, the result was considered by many engineers at the time to sound unacceptably harsh. (This could be considered ironic, since the U 47 and U 48 have a cult following today specifically for use in close-up vocals, with some engineers seeming to fancy that they are re-creating a "vintage" sound—whereas in fact, they are creating a sound quality that was specifically abhorred by many of the "golden ears" of the era—notable exceptions being Beatles producer George Martin and engineers Norman Smith and Geoff Emerick.) The result was the U 67, a microphone with less emphasis in its upper midrange response, giving it less of a "forward" tone color. The U 67 uses a new capsule, the K 67. Unlike the K 47, the K 67 utilizes a two-piece backplate, allowing the diaphragms to be tuned separately and