By Allen H. Burdick
Radio talk shows are currently enjoying unprecedented success. Witness the phenomenal success of hosts such as Dr. Dean Edell, Rush Limbaugh, Larry King, not to mention comedy car repair experts. Is it any wonder that local hosts are clamoring for the limelight as well? If your station has plans to add a talkshow, you will face the problem of creating a mix-minus, a signal that returns all of the program audio, except the caller's own voice, to the caller. Otherwise, echo, caused by Telco system signal delays, can confuse the caller.
If you have a production console with auxiliary buses that can be assigned to create a mix-minus, you are in luck. However, most simple on-air broadcast consoles do not have that luxury. And note that it takes one auxiliary mix-bus per phone line to create the mix-minus within the console. The IFA-10 is the answer for a simple two or three phone line talk show mix-minus. The IFA-10 is a 4 IN by 4 OUT assignable mixer, using removable internal jumpers to create the desired mixing array.
Fig. 1.0 A Two Phone Stereo Mix-Minus
Figure 1.0 above shows a block diagram of a mix-minus system for use with a stereo console, with the IFA-10 downstream from the console. All of the audio for the host and local guests is mixed in the console. The output of the console is fed to the IFA-10 at the Program Left and Right input positions. By following the schematic diagram you will see that the program audio follows across to the respective Program outputs. Additionally, telco audio is mixed with program audio. At the Tel Line 1 and 2 outputs you will see that they receive the program audio and the audio from the other Tel lines, but not audio from their own line. Of course you will need a Telco "Hybrid" for each of the phone lines and appropriate controls for the phone lines. Gain control for the incoming telco levels is also important. Metering at the program outputs is a must. Figure 2.0 below shows the configuration for a mono mix-minus and Fig. 3.0 shows what can be done by interconnecting two IFA-10s.
Fig. 2.0 A Three Phone Mono Mix-Minus
Fig 3.0 A Four Phone Stereo Mix-Minus
Larger mix-minus systems are easily possible using System 1000™ technology. For example, we recently designed a 48 by 20 system for one customer and we can do the same for you!