SLAC-LBL VIDEO/AUDIO LINE

Transcription

1 SLAC-TN-754 W. C. Struven SLAC-LBL VIDEO/AUDIO LINE The joint PEP proposal submitted to ERDA (AEC) by SLAC and LBL has required a great deal of discussion between groups at SLAC and LBL. As detail design continues, conferences between many groups at both SLAC and LBL will continue. In an effort to conserve gasoline and peoples time, it was suggested by Drs. Rees at SLAC and Elioff at LBL that conferences might be held using a duplex (two way) video and audio system connecting a conference room in Berkeley with a conference room at SLAC. The requirements of a system and approximate costs were compiled early in The estimated cost of the system was $43,500. The system, as initially defined, was to be simple to operate and capable of simultaneous transmission of a TV picture and one channel of high fidelity audio in both directions. The system was to be capable of operation without the services of an operator. Two TV options were proposed, one to view people seated around a conference table and a second to view a selected portion of the chalkboard in each conference room. The audio was to be a hands off operation with a single microphone or multiple microphones as required by the size of the group. The initial system was to have one video and one audio channel each way but was to be expandable to several duplex voice grade audio channels and one or two duplex data channels. Path and Frequency Studies Path studies and requests for frequencies were initiated in February Path studies included such features as path clearance, path length and attenuation calculations, reflected ray and direct ray calculations (made more interesting due to the fact that the reflection point which fell out in the San Francisco Bay, was subject to the rise and fall of the tide). In addition, the proposed path went through the approach path to San Francisco Airport and directly over the end of the runway of Oakland Airport. Initial calculations were confirmed by the system supplier as the first part of his contract. Frequencies were requested at 2Gc, 7Gc and 14Gc but two frequencies were finally selected in the 7Gc band. This band offers a reasonable path attenuation,

2 -2- freedom from long fades, freedom from rain attenuation, reasonable dish size and moderate equipment costs. RF Terminal Equipment The successful bidder for the microwave equipment was Microwave Associates. They proposed their MA7G duplex FM microwave system. This system will support up to four subcarrier program channels above a standard 525 line video signal. The transmitter and receiver are connected using a microwave combiner which feeds a six-foot Prodelin dish on the roof at each end of the 31.8 mile path. The transmitters supply about 2.5 watts to the two six-foot dishes to provide a 45 db fade margin for the system. The baseband equipment is a Coastcom SBC400 multiplex subcarrier system. One program channel is provided around a 6.2 MHz carrier. This channel transmits 15 khz audio and is reproduced using a flat response speaker at each studio. Flat response* microphones were selected to minimize audio peaks which could lead to feedback. Studio microphone amplifiers and speaker amplifiers were provided with selective bandpass filters to?une the conference rooms so that the audio level without feedback could be maximized. A gated AGC system is used at the LBL end to further reduce the chance of feedback. This system has the characteristic that the microphone channel is held at a fixed gain during quiet periods, that level being determined by the last audio peak level which was sent through the microphone channel. The baseband system can be expanded, if desired, to provide a 9600 baud duplex data channel, a 4800 baud duplex data channel and up to six duplex voice grade channels frequency modulated around a 6.8 MHz carrier. Additional data and voice channels could be added around 7.5 MHz and 8.2 MHz carriers if further expansion is desired. Conference Room (Studio) Design Each conference room has been provided with two cameras, one or two TV monitors, a speaker and enclosure and one or more microphones for conference table use. In addition, a small control box has been provided to turn the system on and off, to switch cameras and at the SLAC end to control speaker audio level. One gain control at the SLAC end is the one necessary control to reduce the gain if feedback occurs. In practice, only occasional adjustment of this

3 -3- control is necessary. The system has been designed to require very little adjustment thus allowing the chairmen to concentrate on the meeting and not the TV/audio equipment. For this reason pan, tilt and zoom facilities have not been included, Push-to-talk microphone controls have been eliminated for the same reason. The users have found that this philosophy has allowed them to use the system in a relatively transparent manner. Expansion Capability In addition to the expansion noted above (six duplex voice channels and two duplex data channels), there remains the possibility of adding a very high speed duplex data chanuel. We are testing the prototype of a 1.5 megabit per second data system on the microwave system at present. If this system is successful, it will be operable on a noninterfering basis with our present duplex video and audio channels. This high speed data system will only be practical if the number and length of signal fades is suitably low. We are collecting path data at this time to determine the number and length of signal outages. Several pictures have been included to illustrate the features of our system.

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9 FIG. g--the microwave dish at the SLAC end.