Stereo Mode Control Using the RS-414 Router

By Jack Wells, V.P. Engineering, KLRU-TV Austin, TX

For me, it all started before our conversion to MTS stereo, while listening to one of Austin's network stations broadcasting stereo. The stereo image contained some of the weirdest, most disconcerting effects I'd ever heard. After I noticed it the first time, I continued to listen for such idiosyncrasies in all stereo broadcasts, both television and radio. After awhile, I determined that all the TV stations broadcasting in stereo had similar problems to one degree or another.

After some research I learned that the effects were being generated by the stereo synthesizer. Some of the problems were caused by the synthesizing itself, but most were related to the sensing circuits that determine whether the box is receiving mono left, mono right, dual channel mono or true stereo. At that time, I decided that when KLRU implemented stereo, some means to deliver either true stereo or dual channel mono, with no synthesizer involved, would be our tack for MTS.

The next event leading to our Audio Solutions was when I read a paper presented at NAB 1988 entitled "A long-term MTS system design for the broadcast studio Facility", written by Rick Craig at WGN-TV in Chicago. This paper documented their experiences with a stereo/SAP installation and was very enlightening. One of their philosophies that I strongly agreed with was that the SAP channel should always have audio modulation that was related to the Video modulation. If a second language was available, then it would be broadcast on SAP. But if no unique audio track was available, then SAP would have an L+R mono version of their stereo channels.

Their system used Benchmark Media System's DAs, each DA being equipped with left, right and L+R outputs. In addition to the three outputs, each DA had switching FETs allowing remote control of which input showed up at each output. The left channel could be sent to both outputs (and thus, left plus right). The right channel could be sent to both outputs, etc. The sum of left and right could also be sent to both outputs as well as true stereo, or left/right stereo reverse. Each DA could also invert the polarity relationship between ch 1 and ch 2.

Every VTR was fitted with two DAs; the first, or DA "A" received VTR ch 1 and VTR ch 2. They then fed the L+R from DA "A" to DA "B"s ch 1 input. The DA "B"s ch 2 input was fed from the VTR's channel 3 output. The VTR operator then manually selected which of two tracks went to left and right, then either ch 3 or L+R mix to SAP. He would do this as a matter of routine when setting up his TBC's tracking and other VTR playback adjustments.

This system was extremely powerful because it allowed either audio channel (ch 1 or ch 2 or stereo) to be routed to left and right. It also allowed SAP to be programmed with either summed left, right or unique ch 3 audio. Consequently, when observing the L+R output, stereo phasing errors are hard to overlook.

Unfortunately, as powerful as this system was, it left some situations unaddressed. If a tape were to come into the house with ch 1 in English and ch 2 in Spanish, for example, some special arrangements would have to be made to accommodate it. If channels 1 and 2 were accidentally erased ,but with a mono ch 3 still intact, this system would lack the capabilities to air the tape in a straightforward manner. Also, there was nothing to address the 4 channel VTRs.

Since our stereo conversion timetable was still some time off, it allowed me time to look into other systems. While speaking with Allen Burdick from Benchmark Media Systems, at 1989's NAB, we determined that what I really wanted was a 4 by 4 Audio routing switcher. This switcher should be capable of summing any or all channels to any or all outputs. This led eventually to Allen developing the Benchmark Model RS-414 Mixing /Routing Amplifier. This piece of equipment was exactly what I had in mind. We wired them between the VTRs and the 3 channel Routing/Master control system. This allowed us to put any channel of audio from any 4 channel VTR to any routing level and thus any broadcast channel. If a tape is configured with stereo on ch 1 and ch 2 and timecode on ch 3, the VTR operator routes ch 1 to Left, ch 2 to Right and ch 1 plus ch 2 to SAP. With beta decks, the FM audio tracks #3 and #4 have better specs. Thus they are the standard choice as inputs to routing left and right. If, however, Beta tape from an industrial deck comes in with only ch 1 and 2, it's not a problem. The RS-414 is so versatile I've nicknamed it "the all things to all people audio switch box."

The RS-414 also incorporates some of Benchmark's features that Allen has developed over the years. One such feature is audio metering on the circuit card edge. This is useful for a technician trying to track signal loss to its source. One glance and the tech knows whether or not the signal is making it to the audio switcher. Also, the circuit card has daughter board capability to allow adding VCA gain stages, an oscillator, parametric equalization or other options.

Our system has been in place since 3/1/92 with no problems. The quality and technical specs of the RS-414, as all Benchmark equipment, are top notch. Allen Burdick at Benchmark is one engineer that never stops at good enough. He's constantly looking for ways to improve his products both specs wise and capabilities wise. That's old news to WGN-TV and anyone else currently using Benchmark equipment.