The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals. Since the sidebands are mirror images, which sideband is used is a matter of convention. In the early 1930s, single sideband and frequency modulation were invented by amateur radio operators. However, the modulator is followed by a special filter known as Vestigal sideband (VSB) filter. By either adding or subtracting the resulting signals, a lower or upper sideband signal results. There are several methods for eliminating the carrier and one sideband from the transmitted signal. In the U.S. beginning around 2010 using single sideband subcarriers for stereo was proposed. Telex systems were adapted to short-wave radio by sending tones over single sideband. This can be achieved by a combination of Doppler cooling and Resolved sideband cooling.