History of the TDM-
Personal Audio Monitor Mixing System

       My bride (I have called her that for the last 25 years or so) says the whole thing was her idea.  The way I remember things, she is probably right.  We both played instruments in our church band.  She played flute, and I played guitar or bass.  During one practice session we were all standing around as the sound man tried to get the monitor mix just right for each of the band members.  We were supposed to be rehearsing, but since no one could hear what they wanted or needed to hear, rehearsal was on hold until these adjustments could be made.  We went through this time-consuming process at every rehearsal, and this one was no exception.  It was at one of these moments when my bride said to me (as she remembers it), "You're an electrical engineer.  Surely you can come up with something better than this!"
       My mind began to whir.  Yes, I probably could.  And if I did, I would want each band member to have his very own mix control so that no matter what anyone else did, he could have the mix he wanted.  I would want the system so easy to install a child (well, maybe a teen-ager) could do it.  And I would want the mix control units to have just a single cable to connect them into the monitor system; no snakes, no "wall-warts" or power boxes or anything of the sort.  Just total monitor control at each musician's fingertips.
       I would like to say I went home and built a system that night, but that is not quite how it happened.  However, I did begin to think about simple ways to approach the problem.  The system would have a "head-end" called the Main Console. This Main Console would accept several signal inputs (channels), like 8 or 16.  The inputs could be anything; Hi-Z, Lo-Z, "Y"-ed or loop-thru.  It would take these audio signals (voice, flute, guitar, etc.), and turn them into digital data.  This digital data would be formatted (encoded) into a data stream so that all the signals could travel together simultaneously down a single coax cable.  The coax cable would be a piece of ordinary, economical, household TV cable.  At the other end of this coax cable a "decoder unit", called an Output Mixer Station, would then sort the individual signals back out so the user could adjust the listening volume of each channel he wanted to hear.  The power to operate each Output Mixer Station would be sent from the Main Console down that same coax cable.  The Main Console would have provisions for driving as many coax cables and Output Mixer Stations as anyone could possibly want, and each Mixer Station user could adjust his mix independent of any other user.
       I began to design and experiment in my laboratory, and the various parts of the system slowly came together.  After much time and several tests, I felt I was ready to try out the system with the church worship group.  I wrote my Pastor at Grace Outreach Center, Gerald Brooks, a letter explaining what I had developed and what I wanted to do with it.  He agreed to let me install my prototype system on the condition that the present monitoring equipment was not disturbed.  If this new contraption didn't work out, he wanted to be sure we could just revert back to the old way and no one would notice.  I had to agree with him!
       I clearly remember the day we installed that first system.  It was Saturday, October 30, 1993.  The system consisted of a TDM-8 Main Console and four Output Mixer Stations, one of which was equipped with the PSM speaker for our lady piano player.  The other three Output Mixer Stations were connected to headphones for the bass, drum and guitar players.  The inputs to the system were lead vocal, lead keyboard, electronic drums 1 & 2, bass, guitar, piano, and aux. mic.  After a few initial tests, our sound man and I were satisfied that the system was working and left the building with a sense of excitement about how the Sunday morning rehearsal session would go.
       The next morning, I explained to the musicians how the system was supposed to work and that I would need a few minutes of practice time to set the channel levels on the Main Console.  With that first system, adjusting the levels meant slowly bringing up each channel until the red "clip" light came on, then backing off a bit.  (There were no green "channel active" lights as yet).  Within a few seconds after everybody started playing the first song I had the levels set.  The response of the musicians to this new gizmo was fantastic!  Everyone started adjusting his own mix just the way he wanted it, and the sound man didn't have to do anything!  He was free to look after the singers and the house mix.  By the time church was over everyone knew we had a winner.
       There were a few technical things I wanted to improve on, and that may just be part of my perfectionist nature.  As I mentioned, there were no green indicators on the channel levels, only red ones.  I wanted to add green lights to make the set-up easier.  Also, the channel inputs on the back of the Main Console had a switch to select between Hi-Z and Lo-Z.  After one switch accidentally got flipped the wrong way, I decided I wanted to design the switches out to eliminate that possible trouble spot.  And since we had instantly used up all the available inputs, I knew it was time to design a 16-channel version of the TDM-system.
       But I had to admit to a great sense of satisfaction, too.  My bride was right; there was something better, and I could (and did) come up with it.  And that satisfaction is renewed every time I ship one of my TDM-systems to another satisfied church customer.

(P.S. As of this writing, that first system is still in use in the church building where it was originally installed!)