magnetic cartridge on the market capable of playing it. After the release of other demonstration discs and the respective libraries from which they were culled, the other spur to the popularity of stereo discs was the reduction in price of a stereo cartridge, for playing the discs–from $250 to $29.95 in June 1958. The first four mass-produced stereophonic discs available to the buying public were released in March 1958 Johnny Puleo and his Harmonica Gang Volume 1 (AFSD 5830), Railroad – Sounds of a Vanishing Era (AFSD 5843), Lionel – Lionel Hampton and his Orchestra (AFSD 5849) and Marching Along with the Dukes of Dixieland Volume 3 (AFSD 5851). By the end of March, the company had four more stereo LPs available, interspersed with several Bel Canto releases. Although both monaural as well as stereo LP records were manufactured for the first ten years of stereo on disc, the major record labels issued their last monaural albums in 1968, relegating the format to 45 RPM singles, flexidiscs and radio promotional materials which continued until 1975.RCA Victor Red Seal Labelography (1950–1967). Broadcasting Radio Early experimentation The earliest approach to stereo (then commonly called "binaural") radio used two separate transmissions to individually send the left and right audio channels, which required listeners to operate two receivers in order to hear the stereo effect. In 1924 Franklin M. Doolittle was issued U.S. patent 1,513,973 for the use of dual radio transmissions to create stereo reception. That same year Doolittle began a year-long series of test transmissions, using his mediumwave broadcasting station, WPAJ in New Haven, Connecticut, which was temporarily authorized to concurrently operate a second transmitter. Left and right audio was distributed to the two transmitters by dual microphones, placed about 7 inches (18 cm) apart in order to mimic the distance between a person's ears."Stereoscopic or Binaural Broadcasting in Experimental Use at New Haven" (correspondence from Franklin M. Doolittle), Cincinnati (Ohio) Enquirer, March 22, 1925, Section 6, page 6. Doolittle ended the experiments primarily because a lack of available frequencies on the congested AM broadcast band meant that it was not practical for stations to occupy two frequencies, plus it was cumbersome and expensive for listeners to operate two radio receivers. In 1925 it was reported that additional experimental stereo transmissions had been conducted in Berlin, Germany, again with two mediumwave transmissions. In December of that year the British Broadcasting Company's (BBC)