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requirements from an intercom. There is a current trend to put a sports pundit out in the stands with an EVS operator and a telestrator. They need to be in constant contact – but you should not risk confusing them with an over-complicated control panel. The great advantage of an IP system is that you can use intelligent control panels which can be dynamically configured as required. So on a simple touchscreen panel you can put the people most likely to be called from that location. The next panel will have a completely different set of contacts. That satisfies the degree of simplicity that many people on the project require: a big button for push to talk, along with a rotary control to turn up the volume to hear the response. Even the names on the buttons will change. While in racks the video engineer might want to talk to camera 1, the director – who works with the same crew week in, week out – will want to talk to John. Same destination: just different names on the buttons. All this is set up once then stored as a configuration file, one for each regular event. So set the intercom up for the way the rugby director likes to work, shoot the game and store the configuration. The truck goes off to do an opera and some light snooker, but the next time it pulls up to Twickenham you load a file and the intercom is instantly set up again, from IP addresses to names. Can IP intercoms cope with a lot of simultaneous conversations? Because it is a peer-to-peer network, in theory any station can talk to any other, and everyone can be doing it at the same time. It is very unlikely that this would ever happen, but we have planned for it. Typically this will be 7kHz quality, but you can trade off latency and audio quality for bandwidth, for instance if you want to link to SNG trucks over satellite. The architecture of our Gemini system is based on a 32 x 32 matrix. Any of those ports can be an interactive intercom panel, or a headset, or a wireless link, or even a simple SIP phone (you can buy them for tens of pounds). If you have Wifi then an app on a mobile can turn it into a SIP phone, which means it can be a station on the intercom. 20kHz audio – can we use this for programme sound? Up to eight of these matrices can be connected together on a single network, giving you a 256 x 256 system which can support 238 simultaneous conversations – everyone to everyone. If you need more than 256 stations just connect to another network, the way you connect to other networks like the stadium or the broadcast centre. The high quality network will be dedicated to the intercom, so latency and jitter should not be a problem. What happens if the network goes down? There are two levels of redundancy. First, there are two networks; second, each network is a ring, so if it is broken at any point the data simply goes the other way around to reach each matrix. The first ring uses a proprietary format, which gives 20kHz audio quality for each channel (48kHz 16 bit). That allows it to carry programme audio as a cue feed, for example. To support all those simultaneous conversations, this high quality circuit is a 270Mb/s ASI stream, over Cat 5 copper or fibre. The second network is used for the control and configuration messages over IP, and as a back-up audio path using the codecs I talked about earlier. No reason at all why you should not carry programme audio over the intercom network, as a backup to the normal embedded signal or audio router. What about quality of service? Over the secondary network, which is likely to be shared with other enterprise connectivity, then care has to be taken. IP was developed on the basis of getting the data from A to B by any means possible and reassembling it when it gets there. It does not really matter if the packets making up an email travel by different routes and arrive in the wrong order, because the IP format means it can be corrected before it is presented. In a realtime application like an intercom then jitter and packet delays can cause problems. Putting in a big buffer eliminates the problems but introduces so much latency that conversations become difficult. The solution is to use small packets – the Trilogy system has just 2ms of audio in each packet – and enterprise class switches that support prioritised packet switching and differentiated services. Is this available now? Yes, all of this is proven technology which is delivered and in service with major broadcast players. For us at Trilogy it is the second generation IP intercom. If you have a first generation IP intercom it can be integrated simply, and earlier analogue and digital systems can also be interconnected. The real benefits, though, will come when everyone migrates to IP connectivity. Thanks to the use of open standards – which the EBU is actively promoting – it will become common practice to link intercoms so that there will be no barriers to communication. ASK OUR EXPERTS POST YOUR QUESTION ONLINE: Search ‘tvbay’ Tel. +44 (0)1635 237 237 Email. TV-BAY MAGAZINE | 47