System Design: Tips & Tricks
This page provides tips & tricks for system design by experienced RTS® Digital Matrix Intercom technicians.
- Overview
- Because of design and installation issues specific to the brand of intercom matrix used, it is now necessary to talk in some detail about the specifics of the RTS products, including Zeus, ADAM-CS and ADAM intercom matrices, as well as some accessories. When I refer to the ADAM series of intercoms in the following portions of the chapter, unless otherwise noted, the comments also apply to ADAM-CS and Zeus intercom systems.
Previously, we discussed the analogy between telephone systems and matrix intercom systems – the analogy is not correct in all cases, here are some exceptions.
In ADAM matrix intercom systems, the connection between the matrix and keypanel is normally via three twisted pairs of unshielded cable.
As shown in Figure 1, one pair carries balanced audio from the keypanel to the matrix, one pair does audio in the opposite direction, and one pair is a RS-485 data signal which is shared among 8 panels in a group.
IMPORTANT
As eight panels share one physical data line, the matrix must have some means of identifying which panel is sending data to it, and also have some means of addressing messages to one specific panel of the eight. The key word in the previous sentence is "addressing." Each keypanel in the system must be assigned an address by one means or another. On some keypanels this involves setting "dip switches" to select a "one of eight address" via binary code (KP-9x family of panels). On other keypanels, the address is set via rotary switch on the keypanel (KP-32 and Low Cost Series of Panels). And, on others the means is via menus and firmware (KP-12 series of panels). In all cases, the factory set default address has one chance in eight of being set correctly "out of the box."
If a separate keypanel is attached to each of the 8 ports which share a data line, each panel must have a unique address set which matches the physical port to which the panel is connected. Having a panel with an address different from the physical port to which it is connected will render that panel unusable (in a practical sense, even though the panel may receive audio). Having two or more panels in a given group of 8 with the same address will disrupt all eight panels in that group by causing data collisions on the common data line. This is so important that I will repeat it. Having two or more panels in a given group of eight with the same address will disrupt all eight panels in that group by causing data collisions on the common data line.
At time of initial installation, or system modification, the great majority of anomalies can be traced to improper addressing.
Figure 1
↑Back to Top
- Let's get started
- How many individual locations and/or persons need to communicate with one another? Write them down. Organize them by logical grouping or location such as:
Studio A
Floor
Lighting Director
Camera 1
Camera 2
Camera 3
Floor Director
TelePrompTer
Anchor A
Anchor B
Anchor C
Weather
Control Room
Director
Producer
TD
PA 1
PA 2
Segment Producer
Audio Operator
News Computer Operator
Font Operator
Other
Green Room
Makeup
Do this for all locations; it will give you a quick "port count" for your system, which will have a significant impact on size of the matrix, and, as a result, the cost. I presume that even if you do work for MegaMedia Corporate Conglomerate Entertainment Enterprises Ltd., you do not have an unlimited budget (Shame, really).
Next, figure out what external "stuff" you need to deal with, such as:
• Interface to allow access to telephone lines – How many? How capable?
• Interface to TW (party-line) intercom systems.
• Relays and GPI/O for external devices.
• Interface(s) for remote locations such as:
• Transmitter.
• News Bureaus in other cities.
• ENG vans.
• Interface to other matrix intercom system (trunking). Now, it is time to put some detail on the above requirements. For each identified user, you need to know certain things, such as:
• How many other users will he (or she) need to readily communicate with at one "sitting" – this will determine the number of keys required on the keypanel.
• Does the identity of the key assignments change? If not, a keypanel without displays, which relies on labeling strips, will save money.
• Does the user want, need, or deserve the ability to reprogram their keypanel features, key assignments and defaults? If yes, a more complex panel may be required, and chance for errors is increased, but the user can make changes without involving you or some other expert.
• Does the user regularly need the ability to adjust individual volumes of the keys (not to be confused with the overall volume control which all panels have)? If yes, a Level Control Panel should be added to their station.
• Is space an issue? Can a smaller panel be chosen which meets the other requirements?
• Does the user really just need to be part of a given conference at all times? If yes, then putting that user and the other members of that conference on a TW channel and interfacing that channel to the matrix may make more sense.
• Does the user need to be untethered? If yes, a wireless beltpack is required.
• Is the user really "two-way", or are they listen only – such as the paging speaker in the green room, or the earpiece (IFB) for the talent.
• Is the user of sufficient stature that they will get "the top of the line" regardless? Those of you that have done systems design before have likely encountered this phenomenon.
Those of you who haven't encountered this previously would do well to ask yourself if the CEO of MegaMedia Corporate Conglomerate Entertainment Enterprises Ltd. Really needs that Pentium VIII 35 GHz computer with the 30" monitor on his or her desk just to read weekly reports from the boys in marketing – The answer will enlighten you.
In undertaking this exercise, it helps to have a catalog of available products (see Figures 1-7) from your vendor of choice in front of you to assist you in categorizing which panels you will assume are suited for the intended user. A copy of the current (as of the publication date of this edition) RTS Matrix catalog, as well as the RadioCom, Audiocom, and RTS TW catalogs are on the included CD.
A wide variety of keypanel options exist. Here we have a selection of RTS keypanels that fit a range of needs. Small keypanels such as the (Figure 1) KP-12LK and (Figure 2) WKP-4 provide an interface for those with limited keypanel needs. The (Figure 7) KP-96-7, a medium sized unit, was the workhorse of the RTS keypanel line until the 1980's and 1990's. The (Figure 3) KP-32 is the top of the line keypanel, and can be enhanced through additional options, such as the (Figure 4) EKP-32 expansion panel, and the (Figure 6) LCP-32/16 level control panel. The (Figure 5) KP-8T is an example of a specialty keypanel that makes use of an empty bay in a Tektronix vectorscope.
Let's proceed on the basis that you have now compiled a list of needed equipment, have gotten approvals, placed the order, and are now ready to begin the installation of your system.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
↑Back to Top
- Audio and Data Considerations
- One of the benefits of the signal format described above is that generally, it does not matter how the audio and data signals get from the keypanel to the matrix. If you want to have a keypanel used in a Broadcast booth at the top of a football stadium which then is connected to an ADAM matrix in the Sports Truck below, it is perfectly OK to have prewired a small adapter to let you transport the three balanced signals (audio in, audio out, and data) over three microphone cables in the audio harness which is already run between the locations.
In addition, if you want to "piggyback" the audio and data on an existing corporate WAN running between two buildings on a campus, there should be no problem. The maker of your WAN hardware, no doubt, has modules available for your system that let you feed the balanced audio and data into an adapter that create appropriate format data to be merged into the WAN data stream, thus, you have eliminated the need to install any cables!
If you have "dark fiber" available to you, Telecast Fiber and others make adapters which can take the audio and data, and run them down the fiber, even while running other audio, video and data down the same fiber for other purposes.
Need to be able to "dial in" with a keypanel from a remote location to a matrix somewhere? Multi-Tech and other modem manufacturers make voice over data modems that can do the job. Intraplex and others make equipment that can take the voice and data signals and send them via ISDN or switched 56.
Do both locations have bi-directional radio equipment? For example, satellite uplinks and downlinks, microwave studio-transmitter links (STLs), or wideband full duplex two-way radios. These will also work with appropriate modulators.
Again, with one possible concern, which is discussed in the next section, it does not matter how you get the signals between keypanel and the matrix, simply that you do.
↑Back to Top
- Back-to-Basics
- As discussed previously, a modern matrix intercom system is very similar to a telephone system. It is comprised of, in its most basic form, a Central office switch (the matrix); interconnect wiring, and telephones (user stations). Most of the concepts and some of the terminology is common to both. Calls can be made, busy signals encountered, "call waiting" exists, conference calling is possible, unlisted numbers can exist, calls can be blocked (incoming and outgoing), and long distance (trunking) is possible.
The following examples will use the Telex RTS ADAM intercom matrix, unless otherwise noted. Most matrices on the market today will have similar features, but unlike Telex products, the competitors' units are not designed to also prevent dandruff, solve the meaning of life, the universe and everything (with apologies to Douglas Adams), and achieve world peace. We would like you to believe that our products will do so. (And in writing that I felt a bit like Dogbert from Dilbert.)
↑Back to Top
- Cable Considerations
- Cabling types do vary considerably among the manufacturers of matrix intercom products, as do the signals transported by them. For that reason, the following discussion is somewhat specific to RTS Zeus, ADAM-CS and ADAM matrix intercom systems.
As noted earlier, RTS Matrix Intercom Systems typically use three twisted pair, unshielded cabling for interconnection. I use the word "typically," as coaxial cable adapters are available from Telex to allow keypanels to be connected via coax, but the standard twisted pair methodology is more cost effective in most cases. Telex allows the user to choose from two different connector styles for the three pair connection. The choice is simply a matter of preference by the user. RJ-12 connectors can be used, and these are readily available, low cost, and quick to assemble.
Note: RJ-12 connectors are sometimes incorrectly referred to as RJ-11 connectors. While they are the same size, RJ-11 connectors have four conductors and RJ-12 connectors have six conductors.
On the negative side, they are plastic, and not as robust as some installations demand. DB-9 (actually DE-9, the more proper name) connectors are also provided, and can be used. These will be more robust, but are also harder to wire, and more expensive. All Telex keypanels have both types of style connectors on them. The type of connector on the ADAM and ADAM-CS matrices must be specified at the time of order, and can be either the RJ-12 or the DE-9 style. Zeus comes with DE-9 only.
As seen in Figure 1, the wiring takes pin 1 to pin 1, pin 2 to pin 2, and so on, for both style connectors. What the drawing also shows, and is equally important, is that a given twisted pair cable carries both portions of the same signal. If you were to wire pin 1 to pin 1, and pin 2 to pin 2, etc., but had one of the wires in a twisted pair carrying +audio in, and the other wire of that pair carrying -data, the audio would be degraded by having "data buzz" audible in that audio signal. The data signal would not carry for as great of distances. This type of error is second in the top ten of initial installation problems, after addressing mistakes.
ADAM and ADAM-CS systems are also available with other wiring schemes, including multi-pin breakout to jackfields for monitoring and rapid changes and for use of 25 pair "Telco cable" for distribution.
Figure 1
↑Back to Top
- How can I interface RTS TW to RTS four-wire systems?
- • The RTS Model SSA-324 interface system provides two-wire (RTS, Audiocom, and Clear-Com) to four-wire interface with nulling controls and gain adjust to and from the RTS-TW line.
• The Model SSA-324 Call Option Card interfaces RTS, Audiocom, and Clear-Com call signal to relay contact to be used with the four-wire system and two-way radio.
↑Back to Top
- Interconnecting Matrix, PL, and Wireless Systems
- As discussed earlier, the signal format for ADAM, ADAM-CS and Zeus Matrices is the same as used in the Telex RadioCom line of wireless intercoms. This makes interfacing between the two systems very easy, and in fact, Telex provides connectors specifically for this on the RadioCom products. To make the two systems work together, you simply connect two audio lines.
Since the RadioCom system is full duplex, with the base station transmitting continuously, there is no need for the matrix intercom to provide a PTT (Push To Talk) signal to the base.
In the case of radio systems where the base station is not transmitting continuously, the matrix must provide a logic signal corresponding to a user pushing an intercom key to talk to that wireless system. ADAM, ADAM-CS, and Zeus all come standard with logic signals, with open collector outputs for this purpose, and have available the UIO-256, as an accessory, which can provide an actual relay closure, if required.
As mentioned earlier, TW systems are, by definition, two-wire (one pair) communications systems, having both talk and listen present at the same time on the same conductors. In order to connect a TW intercom system to a four-wire system an interface is required. This interface is known by a number of different names, including: hybrid, two-wire to four-wire converter, and system interface. Regardless of the name, the function is simple, although the technology is not.
The hybrid, in Figure 5.12, acts as a "traffic cop" allowing the talk signal from the matrix to be applied to the bi-directional TW line while blocking its return when the talk signal from the TW is presented to the matrix. The effect of the blocking is termed "nulling", as it cancels of the return signal. The effectiveness of the cancellation is driven by many factors. Hybrids are generally available from many sources, including intercom manufacturers, Gentner, Telos, and others. Telex has two models available, the RTS SSA-324, and the RTS SSA-424. Both units are suitable for most applications. The primary difference is the SSA-424 is digital and auto-nulling, eliminating the need for manual setup and calibration.
Figure 1
Figure 2
Figure 3
↑Back to Top
- Interfacing
- It is rare that an intercom system is an island unto itself. Communications has become such a pervasive need and set of technologies that it is not a matter of IF you interconnect; it is more a matter of WHEN and HOW you interconnect.
If you doubt this, consider that today in your home, you may have a cable modem connecting your PC to your cable TV system; you may have your PC answering your phone and taking messages with an embedded voice mail system. Soon, you may have your refrigerator talking to the local supermarket over the Internet, ordering tomatoes and milk.
Some of the more common needs for interfacing, which are encountered when installing or modifying an intercom system, are presented in this chapter.
↑Back to Top
- Polling Issues
- Earlier, I mentioned one area of possible concern. In the examples I gave, where the distance between matrix and keypanels is large, the transit time can become problematic. If the distance is great enough, even the speed of light becomes a limiting factor.
Geo-synchronous satellites are 22,000 miles above the earth. To send a signal up to one, and back down again will take approximately a quarter of a second. To complete a round trip will take half of a second (500 milliseconds), at best. You may have heard this phenomenon on international telephone calls with your own voice coming back to you greatly delayed. While the voice delay can be distracting, the delays in data are the real problem. These data delays can become a problem even when the distance between the matrix and keypanel is "only" 3,000 miles – because the encoders, modems, muxes, etc. in that path also add delay; 30 milliseconds is typical.
We talked earlier about how addressing of keypanels is critical in the matrix intercom system. The way in which addresses work is as follows:
In a given group of eight panels sharing a common data line, the data is sent from a keypanel to and from a matrix by a process called polling. The matrix will broadcast a signal to all eight panels to the effect of "Panel Number 1, do you have any changes for me to act upon?" These changes could be as simple as a talk key having been pressed or as complex as the user wanting to see a list of all available party lines. The matrix expects an answer from the panel, either a simple "nope, nothing new to report" or a request for a specific action.
The matrix normally will not wait very long (less than 10 milliseconds) for an answer before deciding that the panel in question is not there, and moving onto panel number 2, and so on, up to panel 8, and then starting all over again at panel number 1. The short wait is mandated in order to assure quick response to panel requests. This 10 milliseconds is the "polling window", the 30 milliseconds between LA and NYC is the "polling delay".
To make such a system work without unduly slowing down all panels by globally increasing the system polling delay, you can use the AZedit configuration software to allow a longer poll delay (say 33 milliseconds) for one panel with no appreciable impact on other panels.
In the case of 250+ milliseconds delay due to satellite transit time, it is common practice to make sure the keypanel associated with the delay is in a group of eight ports where the delay is not important. For example, on ports that are used for paging outputs or IFBs, where there is no other data present.
In these ways, remote keypanels become very manageable and feasible, due in large part to their common format of standard balanced audio and RS-485 data.
↑Back to Top
- Signal Formats
- TW and Wireless systems often are tied to matrix intercom systems. A brief description of the signal formats of the various types of intercom systems is helpful.
In any intercommunications (intercom) system, the "inter" refers to two-way communication. For the purposes of this discussion, we label one of the directions as "talk" and the other as "listen". Obviously, either party in a conversation can be talking or listening at any time, or even at the same time. "Talk" or "Listen" is a matter of perspective. In a given two-way communication, what "talk" is to me is "listen" to you and vice versa. This is only a matter of semantics, as far as this discussion goes; what is key is both sides of the communications can be occurring simultaneously.
In a matrix intercom system of the type that RTS manufactures, the talk and listen signals are full duplex and travel on their individual pairs of wires.
In a TW system, regardless of the manufacturer, the communication is also full duplex, but both sides of the conversation travel on the same pair of wires.
In a wireless intercom system, the communication may be full duplex, with the two sides of the conversation carried on two separate frequencies. This is the case with all the Telex RadioCom products. In this way, the signal format is essentially the same as the matrix intercom system shown in Figure 1.
In some wireless communications systems (two-way radios for example), both talk and listen may share a single frequency, in which case the communication must be half duplex, with the users taking turns between talking and listening. A good example of such a system is low cost walkie-talkies, wherein the speaker you hear audio from doubles as the microphone you speak into when you press the transmit button. In the following discussion, we do not concern ourselves with that variety of two-way radio systems because those systems are rarely encountered in installations with intercom systems.
Figure 1
↑Back to Top
- Software Considerations
- Until now, we have concentrated on the physical and hardware issues for a matrix intercom system. As noted earlier, the intercom matrix itself is a matrix mixer, which is capable of mixing any combination of inputs to any output. A 50-port system is literally a 50-input by 50-output bus digital mixer. Firmware and software are what turns this digital mixer into an intercom system.
For the following discussion, it is helpful to understand the different roles played by the system firmware and software in a matrix intercom system. As system architectures vary, and some information is proprietary to each manufacturer, the information being presented here is specific to RTS ADAM, ADAM-CS and Zeus. The basic concepts hold for other matrix intercoms on the market.
A brief word on the architecture of the ADAM, ADAM-CS and Zeus matrix intercom systems will set the stage. Zeus is the entry-level matrix and is configured as 24 ports, and does not include power supply or controller redundancy. ADAM and ADAM-CS are expandable systems, and are standard with redundant power supplies and redundant auto-switching controllers. Apart from these differences, and the physical characteristics, the three matrices are very similar. Communications to and from the keypanels is handled by serial data ports, which are RS-485 based, and each port controls a group of eight keypanels. The need for addressing of the keypanels was covered earlier. The information sent to and received from the user stations is stored within the intercom matrix in non-volatile memory.
Note
The diagram in Figure 5.13 and the discussion that follows can also be applied, with a few minor exceptions, to the Zeus system.
As seen in Figure 1, the intercom system has provisions for an external PC, which is used to do initial setups and configurations, including: naming of ports, assigning of PLs, creation of IFBs, creation of ISOs, etc. The PC is also useful in monitoring system status and for other housekeeping functions.
The PC is not required for operation of the matrix, except in certain very rare circumstances where UPL statements need to act on files, or in response to date information. It is perfectly acceptable to use a PC to configure the intercom, and then remove the PC. Even without the PC connected, the intercom will function normally. The intercom recovers from power failures, and in the case of ADAM and ADAM-CS, primary controller failure, all without need for a connected PC.
Included on the enclosed CD is a copy of AZedit, the windows-based configuration programs for the RTS line of matrix intercom products. These programs can run without a connected matrix and the best way to learn the programs is to install them. An extensive help file is provided and the program is laid out in a logical manner.
Because the configuration software is run on a standard Windows PC, and communicates with the matrix via a standard serial RS-232 port, a number of possibilities exist for remote configuration, control, and monitoring. One option is to replace the PC with an autoanswering modem. This permits the PC, which is running AZedit, to connect from anywhere via telephone lines and remotely control and diagnose the intercom.
If that notion strikes you as just a bit too insecure, there are a number of available utilities such as PC-Anywhere, which can be used to accomplish the same thing in a different manner. Install a PC running AZedit and PC-Anywhere at the matrix location. Use another PC, running PC-Anywhere to dial into the PC at the matrix, running PCAnywhere, then supply the required login information, including security password, and again, you have full ability to control and monitor the matrix remotely.
As noted earlier, the differences between system architectures for control of matrix intercom systems from different manufacturers are significant. We do not go any further in describing them, except to point out that in the case of Telex RTS Intercom systems, the supplied software is included on the enclosed CD. You are encouraged to play with it.
Figure 1
Figure 2
↑Back to Top
- Very Large Systems, Split Operation and Trunking
- We have used the term trunking earlier and likened it to the long distance telephone system. In the case of RTS ADAM matrix intercom systems, that analogy is very lose to reality. Before we get deeply into trunking, let's discuss the different ways available to make large systems.
First, exactly what do we mean by a large system? How big is "BIG?" As we discussed earlier, with older technology (pre-TDM), systems were limited to a certain size (as a practical matter, in the "few" hundreds of ports) because of physical size and cost, not because of technological or logistic limitations.
Today, intercom matrices in general, and RTS intercom matrices in particular, have a higher absolute limit, and a larger "typical size". For example, in the early 1980s, a well-appointed high end Sports Truck, the type that would do an NFL game, likely had 12 or o channels of PL, 6 IFB channels and 6 ISO channels. Today, most "network size" trucks carry 64+ ports of ADAM matrix, and in some cases, over 100 ports. The intercoms have grown to carry program audio for monitoring, support 10, 15, or 20+ cameras, a host f graphics operators, and statistics personnel. Clearly, what is typical today was unimaginable less than 20 years ago.
Let's consider matrix sizes for a moment, again sticking to those I know best:
• RTS Zeus Matrix Intercom System: 24 ports fixed.
• RTS ADAM-CS Matrix Intercom System: 8 – 64 ports in groups of eight.
• RTS ADAM Matrix Intercom Single Frame: 8 – 136 ports in groups of eight.
• RTS ADAM Matrix Intercom Multiple Frames: 136 – 1,000 ports in groups of eight.
These are the numbers of ports that are available in a single RTS intercom matrix from Telex. Other manufacturers offer systems in sizes from eight to approximately 500 ports. As you can see, size is not a limitation in most cases. At the time of this writing, the largest known single matrix intercom system in service is a RTS ADAM system, which consists of 784 ports at both ESPN and NBC.
Size and capability are not the limiting factor in most cases. Many factors may guide the design in favor of smaller individual systems. If the system is needed for four separate studios in a facility, which never or very rarely work together, then it may make more sense to use four separate systems. Some very good reasons for doing this might include:
• Cost: Four 128-port systems cost less than one 512-port system.
• Reliability: A fire in one rack room will not destroy the entire system.
• Manageability: Four different control studios have four different crews affecting the setup of their operation.
• Shorter cable runs: The matrix for a given group of panels can be physically closer to those panels.
• Ease of Expansion: It is easier to expand a single matrix if the needs for one area grow.
Now, let's take the opposite tack; what would be the reasons for going to a single large matrix? Some of the reasons might include:
• Operations require ability for any of the 512 users to communicate with any of the other users.
• Desire for single point of administration, control, troubleshooting and monitoring.
• Design of the facility is highly decentralized operationally, and day to day, different portions of the facility must work together.
• Certain users must work with all the facilities, and giving them four separate keypanels (one per system) is not feasible. Now we have helped to identify whether to use one large matrix or a number of smaller ones. What happens when you get mixed answers to the questions above? Certain requirements drive you to use separate matrices, but one or two key factors seem to demand a single matrix.
A couple of different options or "hybrid designs" can be used in these cases.
The first and simplest is to define a few common points of contact between the intercom matrices. Take the following example, a television complex has two studios and two control rooms. Normally Control A works with Studio A, and Control B with Studio B. Occasionally, the wall between the two studios opens, (never mind how; that's the architects problem!) and there is a need for Control A to work with the cameras in the combined Studio AB.
Let's further presume the normal method of operation has the cameras in each studio receiving two channels of intercom; a "Technical PL" created in the intercom configuration, and a "Production PL" also created in the intercom configuration of the respective matrices for Studio A and Studio B.
A quick way of allowing the combined operation would be to configure (in AZedit) the Production and Technical PLs of each matrix to include two available sets of ports on a jackfield. Then, simply connect the output of Production PL from Studio A to the input of Production PL for Studio B, and conversely, connect the output of Production PL from Studio B to the input of Production PL for Studio A. Do the same for Technical PLs.
Now, any conversations on Production PL for A control will also be available to the Studio B cameras for both talking and listening, and the same is true for the Technical PL. Our problem is solved.
The technique described is called trunking; the two ports of each system assigned to PLs have been "trunked" to one another. For reasons that will become clear later, we refer to this as "dumb" or unintelligent trunking. That is not to say that it is not a brilliant idea or solution. It means that no system intelligence was employed in establishing the trunks.
To go back to our telephone system analogy, this is early-20th century technology, harkening back to the days of an operator in your hometown asking the long distance operator for a "trunk" to Chicago. That trunk then connected you to your Aunt in Chicago.
However, "wait," you say, "didn't the telephone make this much easier back in the fifties by going to long distance area codes and direct distance dialing?" Yes, you are correct, give the reader a prize!
Today, some intercom matrices (including at least one from someone other than Telex) offer varying degrees of improved trunking that eliminates the manual patching described above.
WARNING
Sales pitch coming – Telex has the largest, most intelligent, most proven trunking system available today, offering the ability to trunk more than 20 ADAM, ADAM-CS, or Zeus II systems together.
This can all be done without human intervention and in a system comparable to the long distance telephone system. Let's look at some of the features and attributes of the system.
Taking the example of the two Production and Technical party lines manually trunked together given earlier, let's make a couple small changes. Make the "trunking ports" assignable, and give them the designations "Trunk A" and "Trunk B," Connect a "computerized operator" between the two systems, communicating via a standard RS-232 serial port with both matrices. Let's call the computerized operator the "Trunk Master."
Now, all we need to do is assign "area codes" to identify which matrix has which port. In actuality, in the Telex Intelligent Trunking system, the trunk master figures out which matrix has which ports and keeps track of it for you. If you assign "ADIR" from the Studio A matrix to a panel on the Matrix for Control Room B, the system "knows" that it will have to configure and establish a trunk to allow that conversation to take place. It does so automatically, establishing the trunk, monitoring trunk usage, and releasing the trunk when the conversation is completed.
"Great" you say, "Why not always trunk and avoid HUGE matrices?" I am glad you asked that question.
First, there is what I refer to as the "Mother's Day Syndrome." Mother's Day rolls around, and all good sons and daughters decide to call their dear, sweet mom and wish her the best, and many of them do not get through. They hear a nice recording of someone saying, "All circuits are busy, please try your call again later." If you think about it, you have probably gotten that message a few times in your life when calling long distance, and never when calling someone down the block. This is because local calls (large metropolitan areas excluded) go through a single matrix (single central office), and there is a dedicated crosspoint (or a close equivalent) for each path. You get the message when calling long distance because there are a limited, finite number of long distance trunks available, and the heavy traffic volume keeps all of them busy at times.
Looking at the last example, imagine what would happen when the first person from Matrix A calls someone in Matrix B, Trunk A (or B) gets assigned, and life is good. A second person (maybe from B calling A this time) initiates a call, the other trunk is assigned and life is good. Now a third person in Matrix A decides to try to call someone in Matrix B. Oops, "All circuits are busy, please try your call again later."
In actuality, no voice is heard, but the calling party does get a busy indication on their panel, and the call does not go through. Therefore, we can see that trunking systems need to be sized appropriately for the anticipated traffic. Appropriately is the key. The Telephone Company (actually "companies" in the post-AT&T breakup era) set aside enough trunks to handle all of the traffic most of the time – sounds suspiciously like "You can fool all the people some of the time," doesn't it?
Telex® Intelligent Trunking shares something else in common with the telephone company, the trunk master continuously monitors and reports on status of trunk utilization. The telephone companies do it in great "war rooms" with multi-story maps with lighted paths. Telex does it with a constantly updated and logged report of trunk utilization on a conventional PC. It keeps track of (amongst other things) the maximum number of trunks you use simultaneously in the past x amount of time. With good historical data, you can determine the number of trunks you set aside for trunking.
However clever you think you are in setting aside trunks, there will always exist the unforeseen possibility that you may run out of trunks at some point.
For example, you have two studios, trunked together with five trunks, and in the past year have never used more than four at one time. Today, both studios are manned, and in Studio B is a news program being directed by Steven Spielberg, produced by George Lucas, with Tom Brokaw interviewing Madonna and Jerry Falwell (it could happen!). Studio A is busy doing a documentary on the history of dental appliances in South America. Care to take a guess how many of the crew in studio A will decide to listen in to the director, producer, talent IFB, program audio, cameras from B? All at the same time? Know what will happen? Yep, "All circuits are busy, please try your call again later."
The other significant limitation may be for each trunk you assign (which requires a port), you give up a port that be used for two keypanels (one at each matrix). Make your system too long distance "friendly" by allocating a lot of ports as trunks, and you either limit the number of keypanels on each matrix or spend more money to buy additional ports for each matrix.
All of these limitations aside, trunking can be a very good solution for many applications. Trunking works best when limited numbers of trunks are required to support occasional usage. Trunking works very well when many matrices need to be interconnected. As noted earlier, Telex Intelligent Trunking can simultaneously handle automated routing between more than 30 matrices. A side benefit of such a multiple matrix trunked system is that the trunk master can figure out and establish trunk paths via multiple hops if needed due to trunk usage. If the trunks from Matrix A to B (see Figure 5.8) are all in use, the possibility exists for the trunk master to route a signal from A to C and from C to B, thereby bypassing the bottleneck.
Another advantage of trunking is that there is no requirement for the individual matrices involved to be in close proximity. Systems, which are hundreds or even thousands of miles away, have been successfully trunked using techniques described earlier with respect to remote keypanels. Trunking is nearly identical to those situations, requiring the transmission of a single data signal and the appropriate number of audio signals.
Note
A series of articles written by Andy Morris and Ralph Strader, on trunking at NBC, appeared in Broadcast Engineering magazine in 1996. These articles, as well as an article on Trunking Supervisory Systems by Robert Streeter and Thom Drewke of NBC, in PDF format, are included on the CD.
A final methodology for distributing large matrices is a function of the manner in which multiple ADAM frames are interconnected. When two ADAM frames, each 128 ports, are connected together, they become a single 256-port intercom system. The interconnect between the two frames is through a Bus Expander, which transports all 128 ports between the two frames without rendering any of them unusable for keypanels.
The physical interconnect between the frames with bus expanders can either be via a pair of coaxial cables, which can be used for distances up to 1,000 feet, or via a pair of fiber optic cables, which can run for over 1,000 meters. The signal sent over the fiber or coax is a multiplexed data stream, running at approximately 220 megabits/second. Since this data rate is lower than the 270 megabit CCIR-601 serial digital video standard, many of the asynchronous devices that can transport serial digital video can be used for this signal to achieve even greater distances.
By using the Bus Expander with multiple ADAM frames, a single electrical matrix can be located floors or buildings apart within a complex, and yet function as a single large matrix.
Now that we have discussed a number of different methods used to create large intercoms, and how to interconnect smaller intercoms into a single system, let's move onto interfacing and accessories.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
↑Back to Top
RSS | Site Map