Studio Audio Connections

Mix-Minus

The Hx must be fed send-to-caller audio that is free of the caller audio, commonly referred to as a "mix minus" feed. A mixminus is a mix of all of your audio sources that will be placed on-air (or recorded) except the caller audio – thus the "mixminus" designation. The European term M-1 (mixminus one) is perhaps a clearer term. A mixminus is also sometimes referred to as a ‘clean feed’. The important thing to remember is that the hybrid must not "chase its tail" – the condition that occurs when its output makes its way somehow back to the input.

Hot Tip: Many hybrid installation problems are caused by an inadvertent signal path that forms a loop from the hybrid’s output back to its own input. Some consoles allow this when certain control combinations are selected by the user. In some cases, it may be as simple a mistake as assigning the hybrid to whichever bus is feeding the hybrid. This is the first place to look when strange or erratic performance is experienced. The quickest test is to bring up only the hybrid in question on the board and select a line. Dial tone should not appear on the send meter of the hybrid in question.  

Console Connections

Using a Modern Broadcast Console's Mix-Minus Capability

Most modern broadcast consoles have provision for multiple mixminus busses. The best consoles allow selective feeds to the phone system. This is useful since sometimes you want only one microphone feeding the phone, but sometimes you want two, three or four mics (during the morning show, for instance), and sometimes you want to play some audio piece that callers need to hear and react to such as contest sound effects, etc. Some even provide for separate ‘on-air’ and ‘off-line’ (recording) telephone modes.

"Making Do" With an Older Console

Consoles made before around 1990 rarely had good support for mixminuses, and almost never for more than one or two. With one of these oldsters, some clever improvisation is needed.

Here we describe a possible scenario that can be used as a starting point for your situation. We assume an older console with Program and Audition as the main busses. There is another bus of some kind that can be adapted for mixminus application. We’ll call this the ‘Utility’ bus. All sources, including the hybrid, will be assigned to Program so that the audience can hear them. We will also assign most of these sources to Utility as well, just never the fader with the hybrid’s own audio.

This arrangement is flexible, allowing the operator to place any or all sources in Utility for the caller to hear. In our example we are fortunate in that the console permits the Utility bus to be fed pre-fader, letting the announcer easily use the telephone system for off-air conversations.

A recorder can be attached to the Utility and hybrid outputs to record announcer + phone audio. This is often done as shown here, with each signal to a separate track. One drawback is the potential for the operator to accidentally put the hybrid in Utility, in which case it is no longer a mixminus. To avoid this error, the signal path could be permanently disconnected by removing the summing resistors or by some such creative operation.

If no bus is available to feed the Hx, you could use an external mixer that bridges the microphone inputs to achieve the same effect.

The Hx2 has multiple hybrids and works best if two faders can be assigned to the telephone system with two associated mixminuses, one for each telephone line. This is probably not going to be easy with an older console, but the Hx2 has an option to work with a single external mixminus by making an internal cross-connection of the hybrids. See the section on Hx2 Internal Mix-Minus for details.

Using a Small Mixer

A small audio mixer is sometimes used to record interviews from the telephone line using a single hybrid. The mixer’s main bus is fed to the recording device. Both the microphone and the hybrid will be brought up on the faders so the interview can be recorded. Most small mixers (such as those made by Mackie) have one or two Aux send busses, so we will use these to feed the telephone system. We will turn up Aux for the microphone but we will make sure it is turned fully off for each channel that has the corresponding caller audio.

Important: Before connecting your Hx hybrid to any utility mixer, make sure to disable phantom microphone power (if so equipped) from the mixer's mic preamp. Voltage from phantom power will damage the hybrid. Factory repairs to units damaged by phantom power will not be covered under warranty.

Using a Production-Style Console

Production-style consoles often used for TV audio will have multiple Aux send busses that can be used in a similar way to the small mixer example above. Each hybrid is sent from an Aux bus and everything the caller needs to hear is mixed into that bus, taking care to keep the hybrid itself off the bus.

Phones and Remotes

When on remote, calls are usually received at the studio rather than at the remote site to save money and hassle. In this situation, caller audio must be fed to the remote talent so that they can hear and respond to callers. Moreover, the callers need to hear the talent. In many cases, the remotes are sufficiently distant that talent cannot monitor the station for the caller feed. Even if they could, the profanity delay would be a problem, since the talent needs to hear the callers pre-delay.

All perceptual codecs (such as the Telos Zephyr or X-Stream) and any IP codec have too much delay for talent at remote locations to hear themselves via a round-trip loop. Therefore, another mixminus is required to feed a codec and the talent hears callers via the codec return path. As before, you feed this return with mixminus, a mix of everything on the program bus minus the remote audio. As for the second half of the equation, the callers hear the talent because the remote feed is added to the telephone mixminus bus. This is no problem if you have a set-up that permits selective assignment to the hybrid mixminus.

One problem with this arrangement is a result of a hybrid with too much leakage combined with the system delay. If the hybrid isn’t doing a good job of preventing the send audio from leaking to its output, the special remote send mixminus is corrupted. Remember, if any of the announcer audio from the remote site is returned via the monitor feed, it will be delayed by the digital link, causing an echo effect. The Telos Hx really shows its stuff in this situation. Because it has such good trans-hybrid loss, leakage is not at all likely to be a problem. And should there ever be a problem, you can solve it by increasing the amount of ducking.

Note -  The Hx has the more common pin-outs used for three-pin XLR inputs and outputs. One trick for remembering the correct pin-out when wiring connectors is to use the phrase "George Washington Bridge." Pin 1 = Ground, Pin 2 = White, and Pin 3 = Black. Note that most mic cables use a white conductor, through some use red.  

Hx2 Internal Mix-Minus

The two hybrids in the Hx2 unit can be configured so that a single mixminus feed is used for both hybrids with each hybrid’s output fed into the other’s input internally at unity gain. This way, the callers can hear each other and the outputs are summed with the audio from the console's mixminus output. Only the SEND IN #1 input is used to feed both hybrids. Both hybrid outputs still function independently. The two hybrid outputs are NOT summed together, so you should provide a fader for each hybrid. The console's mixminus must be configured so that no hybrid’s output gets sent to its own input.

Figure 4 above illustrates which signals are combined together inside the Digital Signal Processor and routed to each connector when the Hx2 internal mixminus feature is enabled. Use this option if you only have a single mixminus available from your console and you wish to use both hybrids to conference callers on the air. You’ll still need a fader for each hybrid output. Bit #6 in the ‘OPTIONS’ DIP switch bank controls the mixminus feature. The internal mixminus feature is enabled when the switch is ON. The default factory setting is OFF.