Controlling The Stereo Audio Signal Path

Introduction

Over the years we have received numerous requests for products with stereo audio mode control functions. We have responded and these products include the Jr. Audio Director™; the Jr. Audio Director Plus™ (stand-alone products); the APA-102 audio proc amp, a System 1000™ module; the MTX-02, a daughterboard for various System 1000™ modules such as the DA-102 stereo DA; and last but not least the RS-414, a routing switcher module for the System 1000™, which is examined in Stereo Mode Control Using the RS-414 Router by Jack Wells of KLRU-TV Austin TX.

Why have Mode Control?

Mode control is the ability to change the configuration of a stereo audio signal. That signal may be coming in from a satellite feed, from a Telco feed, or from various audio and video tape recorders, such as camcorders used for new gathering. Often the levels need to be controlled. Less often, but on occasion the positions of the Left and Right signals have been inadvertently interchanged and must be corrected. A devastating problem for broadcasters is the inversion of signal polarity since this causes the L+R signal to be placed on the subcarrier while the L-R signal is placed onto the main carrier. When this happens the poor guy listening in mono has most of his (her) audio signal disappear, not exactly what the station manager wants to hear about during ratings week.

Occasionally with a news recording the announcer will be recorded on one channel of the tape while background sounds are recorded on the other channel. Then at the TV station these two signals must be mixed and the levels of each channel carefully controlled. Some times a tape or satellite feed will only have one channel of information and it will need to be placed on both of the stereo audio channels.

One method of recording audio that is compatible with stereo broadcasting, and is now gaining favor, uses MS microphones. The MS microphone is a sum (L+R) and difference (L-R) microphone. Decoding the sum and difference signals after preamplification is necessary to get the normal Left and Right channel signals we need for recording and monitoring.

These stereo audio modes can be controlled by Benchmark Mode Controllers. We will next look at the various mode controllers that are available from Benchmark and discuss the features and limitations of each.

The Jr. Audio Director™

With balanced inputs and outputs, the Jr. Audio Director™ provides output source selection between Stereo, Mono, Left Only, Right Only, and Stereo Reverse. Additionally, Right Channel Polarity control is provided to give the mono signal back to the mono broadcast listener. The device has manual gain of both Left and Right inputs. This feature provides control over the mix in the Mono mode. It also provides independent gain for each channel when the audio source equipment isn't physically next to the Jr. Audio Director™.

The chassis is rack mountable with the optional RM-1 rack mount panel. LEDs indicate the devices operating mode. The mono mode has built-in gain reduction to correct for the natural summing level increase. It is front panel adjustable with a range of -6 to -3 dB. Installation is a "snap" using the plug-in Euro-style barrier strip. Power for Jr. comes from a PS-1, the Benchmark wall mount ±16 volt regulated supply.

The Jr. Audio Director™ Plus

The unique Benchmark Jr. Audio Director™ and the Jr. Audio Director Plus™ are proving to be very popular. Their ability to quickly correct assignment, mode and polarity problems is a godsend to the harried operator. You can view the block diagram of the Jr. Audio Director Plus™ which illustrates the versatile signal flow capability of this device.

Four primary inputs are available to both channels via internal 0.025" post shorting jumpers. Or you may easily remove the jumpers and use the secondary inputs to bring in four additional inputs for each channel, or even have a combination of both. 1 k resistors are at the input of each source selection switch. These allow passive mixing of more than one input by simply depressing two or more pushbuttons simultaneously.

Additionally, there is a concentric input gain pot that provides control of mix ratios, a front panel recessed potentiometer for adjustable mono gain of -3 to -6 dB, and a high current stereo headphone amplifier for outstanding sonic integrity under high levels.

As with all Benchmark products, the performance is second to none, and up to two units may be rack mounted side-by-side with the inexpensive RM-2 rack mount panel.

Applications include TVRO audio subcarrier assignment, D2 and D3 audio channel assignment, mono confidence monitors, etc.

These and other mode control functions are also available with the System 1000. MS decoding will require either the MTX-02 mode control daughterboard mounted on a MDA-102 mic-preamp, or the line level APA-102 Audio Proc Amp. Call the sales department today for more information.