Multiplex Radio
U. S. Navy officials, visiting last fortnight at Gloucester, Mass., were treated to a demonstration remarkable even in this radio age. In the laboratory of John Hays Hammond Jr., they beheld that young wizard (aged 37) transmit eight radio messages simultaneously upon a single ether wave and receive them again, all separate and distinct, using single sending and receiving instruments at both ends of his operation.
A year ago (TIME, July 28,1924), Wizard Hammond made this demonstration before the Italian Government when he was winding up his deal with Premier Mussolini and the Marconi interests for a joint monopoly of Italian radio. Italy applauded but the inventor came home to perfect his work and offer it for purchase to the U. S.
The Multiplex System. Upon a 10-metre "carrier" wave, Mr. Hammond's multiplex sending instrument impresses modulations, much as a locksmith files teeth in a blank key. A set of eight modulations goes out with each compound signal of the set of eight messages. Physically, these modulations consist in alterations of the length of the carrier wave by fractions of a centimetre (down to 9.984 m., up to 10.016 m.). In the instrument that receives the multiplex or "scrambled" messages, one circuit is made sensitive to the carrier wave, other circuits to specific modulations thereon; much as the slot of a lock receives a key's blade and the tumblers are touched by the key's teeth.
The Significance is manifold. The spectrum of radio-carrying ether vibrations is, like the light spectrum, definitely limited in extent. Commercial broadcasting in the U. S. has utilized all the higher wave lengths, which, to avoid interference between stations, must be spaced about eight metres apart. So loaded is the air that identical wavelengths have already had to be assigned to pairs of stations, the one remaining silent during the other's program. At U. S. Secretary of Commerce Hoover's radio conference, called for early next month, wave-congestion will be the principal problem discussed.
On the short end of the radio spectrum, interference between the waves is avoidable through proportionately shorter spacing. Where only 12 stations could operate in a 100-metre range of the upper scale, many times that number can be accommodated by the first 100 metres of the lower scale. This fact, together with economy of power consumption and the proven superiority of short waves in penetrating belts of static, is what has led to exploration of the short-wave field. The Hammond system promises to multiply the multiple possibilities of this field, by 8 at least, probably by a higher number. It means for radio what the discovery that many simultaneous messages could be sent over one wire, meant for telegraphy and telephony.