Background To AT&T Research
After the work of John R. Carson in 1915 it became clear that multiplexed telephone transmissions could be greatly improved by the use of single sideband suppressed carrier (SSB) transmission. Compared to basic amplitude modulation (AM) SSB has the advantage of half the bandwidth and a fraction of the power (one sideband can have no more than 1/6 of the total power and would typically be a lot less). AM analysed in the frequency domain consists of a carrier and two sidebands. The carrier frequency in AM represents the majority of the transmitted power but contains no information whatsoever. The two sidebands both contain identical information so only one is required, at least from an information transmission point of view. Up to this point filtering had been by simple tuned circuits. However, SSB required a flat response over the sideband of interest and maximum rejection of the other sideband with a very sharp transition between the two. As the idea was to put another (completely different) signal in the slot vacated by the unwanted sideband it was important that all traces of it were removed to prevent crosstalk. At the same time minimum distortion (i.e. flat response) is obviously desirable for the sideband being retained. This requirement led to a big research effort in the design of electric wave filters.
| Electric wave filters |
| The term electric wave filter was much used around Zobel's time to mean a filter designed to pass or reject waves of specified frequencies across the band. It appears in numerous papers published in the early 20th century. Sometimes used to distinguish these more advanced designs from the simple tuned circuits which preceded them. In modern usage the simpler term filter would be used and is unambiguous within the field of electronics. |
George A. Campbell and Zobel worked on this problem of extracting a single sideband from an amplitude-modulated composite wave for use in multiplexing telephone channels and the related problem of extracting (de-multiplexing) the signal at the far end of the transmission.
Initially, the baseband pass range used was 200 Hz to 2500 Hz but later the International Telecommunication Union set a standard of 300 Hz to 3.4 kHz with 4 kHz spacing. Thus the filtering was required to go from fully pass to fully stop in the space of 900 Hz. This standard in telephony is still in use today and had remained widespread until it began to be supplanted by digital techniques from the 1980s onwards.
Campbell had previously utilised the condition discovered in the work of Oliver Heaviside for lossless transmission to improve the frequency response of transmission lines using lumped component inductors (loading coils). When Campbell started investigating electric wave filter design from 1910, this previous work naturally led him to filters using ladder network topology using capacitors and inductors. Low-pass, high-pass and band-pass filters were designed. Sharper cut-offs and higher stop-band rejection to any arbitrary design specification could be achieved merely by increasing the length of the ladder. The filter designs used by Campbell were described by Zobel as constant k filters although this was not a term used by Campbell himself.
Read more about this topic: Otto Julius Zobel
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