Monday, August 13, 2012

Duddell's singing voice

The first AM transmitters were arc transmitters aka arc converters. The first model to generate a continuous wave (CW) were invented separately by Elihu Thomson and William Du Bois Duddell. These models generate a carrier wave by continuously shocking a tuned circuit. This was the best we had until vacuum tube technology appeared on the scene.

Previous to these models transmitters were all spark gap transmitters. These just emitted a series of short arcs hence the term. These sparks were noisy and emitted broadband noise. Real improvements to these transmitters just timed the sparks so instead of emitted a series of damped waves, they edged closer to transmitting a continuous wave. This is what Marconi was using from about 1895 forward. He was still using them in 1901 for his public transatlantic experiments even when there was superior technology available.

Elihu Thomson had beat the hell out of that spark gap idea by 1892 (patent 500630). He discovered that a carbon arc with a tuned circuit would "sing" i.e. emit audio frequencies as well as radio frequencies. This is only a year after Tesla's invention HF coupled oscillatory circuit (patent 454622) in 1891. Tesla's was for arc lights, but it's actually closely related. Thomson's work also extended into arc lighting. This is the transmitter Reginald Fessenden used in his experiments.

It's worth noting that one of the reasons that it is believed that Fessenden may have broadcast voice and even music is that this arc converter couldn't be stopped and started (make/break) fast enough for practical Morse code. (This is widely debated) But it had other more serious problems. The intensity was variable as was the frequency. Fessenden made numerous improvements of his own mostly between 1900 and 1903. after that he moved onto rotating spark-gap transmitters that were generally inferior concepts.

But before Fessenden and after Thompson is where we squeeze in William D. Duddell. e was a a British physicist and electrical engineer. Like Thompson and Tesla he was working on arc lighting. In 1897 he invented an Oscillograph sensitive enough to see the shape of a wave with a frequency of up to 100 Hz. Tools matter. It was the first time we could see AC waveforms. In 1899 he discovered that arc lamps could emit frequencies of up to 1 MHz. His own oscilloscope couldn't quantify that, nor could any until after 1920. Around 1900 he invented an arc transmitter based around a carbon arc (like Thompson) but with a shunt to a resonant tuned circuit. This cancelled out the resistance of the RLC circuit. RLC stands for  Resistor, Inductor, and a Capacitor. [Engineers use the character "L" for inductance, in honor of the physicist Heinrich Lenz. But also because we are poor typists. ]  More here.

Duddell's Singing Arc Transmitter was only able to emit waves at about up to 15,000 Hz. Most of it's output was still audible sound. He had doubts that it could emit RF at all. He played "God Save The Queen" at a demonstration in 1901. This was the device that Valdemar Poulsen began improving up on in 1903 (patent 789449), and commercially feasible in 1908. The Poulsen arc transmitted continued to be used until about 1920. More here.