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Planning to Go Remote?: Here's Help!

 SHOULD YOU lease wire circuits or install your own radio link? Or, is it wise to even consider going remote at all. Much depends upon economics, and the quality and reliability of available leased circuits. Where circuits of adequate quality are available or can be made available without paying excessive construction costs, leased circuits are usually less costly.

A radio link installation, on the other hand, requires a substantial initial investment, but recurring costs are generally lower. It provides considerable flexibility; it can reach into areas where link services are not available; and it enables expansion of remote control facilities without increasing the cost of the transmission medium.



Circuit Requirements  

Four or five circuits are usually required from the transmitter site to the remote control point. As shown in Fig. 1, these include (1) a control channel, (2) a telemetering channel, (3) an order wire, (4) a program circuit, and, if stereo facilities are required, (5) a second program circuit.

The program circuits already exist if the studio and transmitter are at different locations. Don't forget to include the present cost of leasing one or more program circuits when you consider the total cost of a radio link against the total cost of leasing all of the required circuits.

Control Channel  

The control channel may be a so-called 0-15 cycle circuit which may be a true metallic hookup capable of passing DC, or a derived circuit which does not pass DC but provides a relay at the transmitter site with control contacts at the remote point. These two types of circuits are illustrated in Fig. 2. Or, a voice grade telephone circuit may be used to provide the equivalent of up to 32 circuits of the type shown in Fig. 3, by employing tone multiplexing.

Telemetering Channel  

The true metallic circuit in Fig.2 can be used for direct analog measurement. A derived 0-15 cycle circuit, which does not provide a DC path, can be used for remote measurements of transmitter currents and voltages by converting them into coded pulses. A single circuit of either type can be used for both control and telemetering. The cost of leasing such a line is usually around $1.50 per mile per month. A voice grade telephone circuit, costing around $3 to $5 per mile per month, can be used to provide up to 32 telemetering channels by employing tone multiplexing. 

Order Wire   

The order wire may simply be a common carrier telephone hookup between the transmitter site and studio. Or, a separate voice grade telephone circuit may be leased, terminated at each end in a local battery, magneto telephone, or intercom unit of the type ordinarily used ror remote control of mobile radio system base stations, as illustrated in Fig. 4. On the other hand, the order wire may consist of a single-channel, reversible twoway radio link, as illustrated in Fig. 5, preferably equipped with tone squelch to mute the receivers except for desired signals.

Broadcasters are eligible for station licenses in the Business Radio Service and Citizens Radio Service for other than program transmission. Thus, the radio transmitter-receiver at each location may be a Class-D Citizens radio unit, or a 5-30 watt AM unit operated on one of the 27-me business channels. A 3-watt (or lower power) unit may be operated on one of the low-power business channels in the 5-50 mc, 150-173 mc, or 450-470 mc mobile radio bands, on which fixed communications are permitted. Or, a UHF/. FM unit may be operated on any one of the 48 Class-A Citizens channels in the 450-470 mc band with input power up to 60 watts.

Program Channel  

The program channel (two for stereo) must meet the frequency range requirements stipulated by the FCC for the type of broadcast station. A leased program circuit generally consists of an equalized telephone circuit. Rates are higher than for voice grade circuits, even if it's simply an ordinary telephone circuit with one or more equalizers. An 890-960 mc band radio link, licensed under Part 4 of the Rules, may be used to provide one or more program channels.

Combined Circuits  

A single broadband circuit, if available, can be used to provide one or two program channels plus the control, telemeter, and order wire circuits by employing frequency division multiplexing. The broadband circuit must have adequate frequency bandwidth to accommodate all of the channels, as provided under telpak tariffs. When the broadcaster has a right of way where he can install a suspended or buried coaxial cable, all of the channels can be accommodated by a single cable, as shown in Fig. 6. If such a transmission medium is not available, the program channel (s ) may be independent and one voice grade telephone circuit may be employed for control, tele metering, and order wire purposes.



DC or 15-Cycle Circuits 

When a DC or 0-15 cycle circuit is used, remote control is achieved by dialing specific numbers to perform various control functions, such as turning on transmitter filaments and transmitter plate voltages, reducing power, increasing or decreasing plate and filament voltages in discrete steps, switching over from the main transmitter to standby, etc. The same circuit can also be used to observe power line, filament, and plate voltages, plate and antenna current, frequency and modulation monitor readings, and for determining if the tower lights and flashers are functioning properly. Each circuit to be monitored is dialed. Selectors at the transmitter site connect the remote metering circuit to the various circuits in response to dialed commands. Fig. 7 is a functional block diagram of a remote control and telemetering system using a metallic circuit.

Voice Grade Circuits   

A single voice grade telephone circuit may be employed for simultaneous remote control and tele metering of several circuits by tone mu tiplexing. By using tones and sequential control and tele metering circuits, the same line can also be used as an order wire.

Tone MIJItiplex 

An on-off (AM) tone channel consists of a tone transmitter operating at a frequency in the audio range and a zone receiver tuned to the same frequency. As shown in Fig. 8A, ckesing switch S causes a tone to be transmitted. When rectified by the distant tone receiver, output relay (K) pulls in. In the absence of tone, the relay is de-energized.

A two-state FSK (frequency shift keyed) or FM tone channel is illustrated in Fig. B. When Sclosed, the frequency of the transmitted tone is shifted, causing relay K to pull in. The relay drops out again when the tone shifts back to its normal frequency. One tone or the other is always present. In a three-state FSK tone channel (Fig. 8C), a tone at the channet frequency is transmitted when S is in its center position. When S closes one contact pair, the tone is shifted up; when in the opposite position, tone decreases in frequency. The output relay (K) may be a three-position differential or polar relay, which is normally in its center position and is pulled one direction or the other, depending on whether the frequency shifts up or down. 

Or, separate output relays may be provided for all three tone frequencies, as shown in Fig. 8D. When Si is closed, relay K1 pulls in; K2 pulls in when S2 is closed. Both switches should not be closed at the same time. Relay K3 operates whenever the tone frequency is shifted. Since each tone channel operates on a different frequency, several tone channels may operate simultaneously on the same circuit. Up to 32 tones can be transmitted, in either or both directions, over a voice grade circuit.

  Speech Plus Tones

  Three or more tones may be transmitted along with speech over a voice grade circuit without mutual interference by using filters as shown in Fig. 9. The filters may cut a slot in the voice band at around 2000 cps or attenuate frequencies above 2600 cps or higher, allowing room for tones at the top of the voice band. ON-OFF and FSK tone equipment is available from several manufacturers. Any combination of ON-OFF and FSK tone transmitters and receivers and common power supply may be stacked in a 19-inch relay rack to accommodate the desired number of modules. In lieu of the power supply module, or as its standby, a 12-volt battery may be used as the power source.

An ON-OFF or two-state FSK tone channel can be used to transmit GO-NO/GO intelligence (mark and space signals). A three-state FSK tone channel can be used to transmit such commands as forward-reverse, up-down, fast-slow, increase-decrease, etc. A combination of these systems can be used to transmit more complex intelligence, including quantitative information, by coding the tone pulses or varying their duration, repetition rate or relationship. In addition to keyed tones, there are tone systems which convey quantitative information by stepless variation of the tone f requency. As shown in Fig. 10, the tone frequency is varied by changing the voltage applied to the tone transmitter. The output of the tone receiver is a DC voltage which is proportional to frequency. For telemetering, the DC voltage to be measured (reduced if necessary) is applied to the tone transmitter and the value of the voltage is read on a meter connected to the output of the tone receiver.

Current is measured in the same manner by connecting the tone transmitter input to a series resistance in the circuit being monitored. RF and AC can be measured by rectifying it. A variable frequency tone channel occupies more space than a keyed tone, and thus fewer can be accommodated within the same transmission band. 

 


 

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