464 BELL SYSTEM TECHNICAL JOURNAL 



Syllabic Vodas — Type B Control Terminal 



The desire for a cheaper control terminal than the Type A led to 

 the development of a second type, known as Type B, in which the 

 vodas employs the same fundamental principles. In this vodas added 

 protection against false operation from line noise is secured by the use 

 of a new principle in voice-operated devices, called "syllabic" 

 operation. 



It is observed that in many types of noise a large component of the 

 long-time average power is steady. Speech, however, comes as a 

 series of wave combinations of relatively short duration. These 

 facts suggested a device which distinguishes between the rates of 

 variation of the envelopes of the impressed waves. This is accom- 

 plished by a filter in the detector circuit which passes the intermodu- 

 lated components of speech in the syllabic range, but suppresses those 

 of line noise which are above or below this range. 



Figure 5 shows a schematic diagram of the application of this device 

 to a Type B control terminal. The privacy switching circuits are 

 omitted from this drawing, as are also the circuits for delaying the 

 release of the relays. In comparing this drawing with Fig. 2, it will 

 be seen that relays 1, 2 and 3 perform the same functions, but the 

 transmitting branch of the vodas consists of two portions, one a 

 sensitive detector with a syllabic frequency filter, which on operation 

 increases the sensitivity of the second portion. 



Considering the action of Fig. 5 on transmitted speech, the output 

 of the sensitive detector of the syllabic device is a complex function 

 of the applied wave having intermodulated components in the range 

 passed by the tuned input circuit, together with a d-c. component and 

 various low frequency components set up by the syllabic nature of 

 the speech. There are also various components of any noise waves 

 which may be present including a d-c. component. The first step in 

 getting rid of the noise is to pass the detector output through a re- 

 peating coil which blocks the d-c. component of both the speech and 

 noise, but passes frequencies above about 3^ cycle per second. The 

 resulting waves enter the low-pass filter, the output of which contains 

 frequencies between 3^ and 25 cycles per second, which "syllabic 

 range" is between the d-c. component of zero frequency and the 

 fundamental frequency of the line noise. These syllabic frequency 

 currents cause momentary operations of relays (/) and (F). Relay (7) 

 operates when a speech wave is commencing and relay (F), which is 

 poled oppositely, operates while the impulse is dying out, thus sending 

 current out of the filter in the opposite direction. Operation of either 

 (7) or (F) effectively inserts gain ahead of the upper detector, thereby 



