Remote cue, clean feed, mix-minus and telephone balance units
Four mysterious terms you never to dared ask, but always wondered about?
It is often necessary to make a contribution to a studio from a remote location, either for live broadcasting, or to be recorded for broadcasting later, or in such applications as voice-overs for advertisements or film sound-tracks. Such contributions can range from a simple phone-in from a member of the public, through a sports report in high quality audio, to a satellite feed from a combat zone. Contributions can involve both sound and picture but in all cases it is necessary for the contributor to be able to respond to the studio at the appropriate time. This will usually involve sending some sort of signal to the remote contributor in the form of an audio or visual 'cue'. In most situations today this signal is likely to be an audio cue.
At simplest this may simply be a mobile or other phone; the studio shouts 'go', the remote contributor responds. This is not enough in many situations, particularly if the session is interactive. In such cases, it is desirable that the remote contributor should be able to hear either the output of the studio or a member of the production team at the studio end.
With a traditional live radio broadcast, for example, the contributor simply listens to the radio programme using a radio. This is fed through headphones to avoid a feedback loop which would cause 'howl' or colouration. If the contribution is not taking place live on the radio or TV, then this signal is not available, and the cue feed must be sent by alternative means. In addition, if there is a significant delay in the transmission path, say with satellite links or even digital systems, the delay will confuse the contributor when the voice is heard delayed in the headphones compared to the actual sound. Again an alternative method must be found.
When separate, dedicated analogue land-lines were used by broadcasters to send the studio output for reverse cue this delay problem would be removed for sensibly short links, but another problem would arise, depending on the crosstalk performance of the circuits. The contributors output would travel to the studio and then be routed back down the reverse cue line. This is, in effect, a frequency-dependant feedback loop. If the cross talk was bad enough, audible colouration or even howl could be expected. (The same problem applies to analogue telephone lines when the full or even partial mixer output is sent back to the contributor as cue.)
A solution is to use a 'clean-feed'. This is a dedicated mix of the studio output without the contribution, fed back to the contributor so the activity in the studio can be heard. (Care must still be taken to avoid crosstalk causing colouration of the studio signal, rather than the contribution!) By the use of a clean feed the contributors can hear the studio activity and respond appropriately. The studio will, of course, have to deal with the consequences of the various delays leading to periods of silence.
A clean feed is traditionally a separate mix in the studio console, with its own mixing bus fed post-fade from all faders except the one for the remote location. Often these facilities are not included in small mixers, although sometimes an auxiliary or foldback mix is available, which may be used to generate a clean feed.
A 'mix-minus' feed is akin to this, but generated differently. It is often used in less traditional mixers to reduce cost. Here the entire output of the studio console is summed with the post fade output of the remote contribution in such a way that the remote signal is subtracted or 'nulled' from the resulting 'mix-minus' feed. This can work well if the communication circuits in use do not have any peculiar characteristics. The more 'perfect' the links are, the better the performance. It is a good solution where the main problem is simply time delay, and may often be applied using outboard adapters; 'mix-minus generators'.
Most of the above discussed a situation where, at the studio, there were two communication paths, one incoming from, and one outgoing to the remote contribution site. A variant of this is where one analogue telephone line is used for both those paths. It is also probably the most common. Despite today's digital communications using fibre, microwave links and satellites, subscriber phone lines are mostly analogue, with at least the last few hundred metres delivered over a pair of wires, some not even twisted. So despite any high technology at the studio site, there will be part of the path where the incoming and outgoing signals are carried on the same pair of wires, which, if the wires were a perfect bi-directional transmission line, would constitute 100% feedback.
Even if the clean-feed or mix-minus system were perfect, some of the the output of the mixer is still being sent out down the pair of wires that is supposed to be incoming contribution and hence comes back in through the 'phone' fader, often louder than the phone signal itself. This problem is addressed by 'telephone balance units'. These vary greatly in complexity. At the most simple, it is possible to wire two transformers and some resistors to produce a 'hybrid' transformer, which provides three ports. One connects directly to the bidirectional phone line, another is the input for the cue to the phone line, the third provides the wanted signal back to the mixer. The transformers are wired to reject the unwanted signal coming back to the mixer. If all impedances and levels are correct, this can work very well, but since the actual phone line is a very imperfect example of a transmission line, with frequency dependant impedance, and the phone lines in use will all be different, the thrill of adjusting such hybrids manually soon wanes...
Modern TBUs use a variety of techniques, digital or otherwise, to compensate for different line conditions automatically, and to provide to the console as 'clean' a remote signal as is possible by removing the cue feed electronically. There is little that can be done about echos of the cue signal coming back up the line, but the designers certainly try! Some TBUs have their own mix-minus facility built in. If not, a clean-feed or mix-minus output will be required from the console, or an external mix-minus unit must be used.