It is now one year since I wrote Acoustic Antenna Part 1. The acoustic antenna is intended to follow the path of any non-muffled engine-powered model by means of an array of 8 microphones. The signals from these mikes go into a computer, which computes the phase difference between the mikes and hence hopefully determines the trajectory, in three dimensions, and airspeed.

In that article, I indicated that the computer sound card was a good prospect for use as a multi-channel very fast analog to digital converter. After initial success installing two sound cards and reading 2 microphones via Quick Basic commands, my efforts were crowned with the thorns of failure. When Windows wasn't actually crashing, it seemed to be fighting all my attempts to read the line-in and microphone channels separately. It's a long story, but basically I gave up.

Until, that is, I had a chat about my problems with Stuart Maxwell. Stuart's idea was for me to provide my own analog to digital converter, and read it into the computer via the ISA bus. If you look inside your computer, you will find some cards plugged into various slots: these are the ISA and PCI card slots. The PCI slots are difficult to address from Quick Basic, but the ISA slots are simplicity itself.

Only two commands are required, OUT and INP. OUT writes a byte onto the ISA port, and INP reads a byte off the ISA port. Now all you need is something to OUT and INP to! As it happens, there was published in Electronics Australia November 1996 the details of a construction project called "Improved 24-line I/O card for PC's". This card plugs into the ISA slot, and loves nothing better than receiving some OUT and INP commands. Hence it is ideal for controlling an analog to digital converter (ADC) such as the 7822 produced by Analog Devices (www.analog.com). What is needed then is a microphone signal which one requires to feed into the computer.

I have been using microphone kits from Oatley electronics, and these worked fine, especially with the Dick Smith pre-Champ amplifier. However, the acoustic antenna is required to detect sound from the model when it may be half a mile away, or up close; such is the range of sound level that this may be difficult.

In using Doppler effect for measuring RPM and airspeed, I was using Digitor tape recorders, and found that they did very well at picking up the weak sounds from a very distant model. They contain a device for amplifying weak sounds and at the same time not overloading on loud sounds. This is an Automatic Gain Control (AGC); I needed one badly.

So I was pleasantly surprised to find a suite of chips on the ANALOG web site, which included a microphone preamplifier with a built in AGC. This sounded ideal, a purpose built device for my application: perhaps the fates were smiling on me at last. Also in the suite were line drivers that would enable me to send the signal from the antenna to the base computer, up to 1000 feet away. How could I go wrong? Little did I know.

Firstly, I tried to order these chips from the local suppliers. These rascally resellers were asking so much money for the chips I literally could not afford to buy them. They were asking up to 4 times the US list price, for chips sold in the USA as "economically priced". So I purchased them in the USA, and landed them here at half the local price, except for the ADC's, which they threw in for nothing! So far so good.

Chips in hand, I proceeded to build a single channel of the antenna, comprising an electret microphone, SSM2165 microphone preamplifier and SSM2142 line driver. It all worked, except for one thing. It was nearly deaf: I had to shout for it to hear me, and then it had only a small signal output. No problem with the output, I could easily boost that with a 741 op amp.

However, this thing seemed to need an amp in front of the preamp! Hell, thats nuts! On reading further the spec sheet, I realised the chip included a non-adjustable input signal threshold of 500 microvolts. Doesn't sound like much, but the microphone produces at best about 30 millivolts, which is also not much. OK, I could live with an amp in front of the preamp, but then I found that the AGC was pretty hopeless as well.

Now you may think I'm a bit of a fussbudget, just because the SSM2165 is a hopeless chip. But its worse than that. The chip is whats called a surface-mount chip. It is half the size of a regular DIL chip, and the pins don't line up with VERO board. So before I could even mount the chip, I had to pay out several hundred dollars to have made some tiny circuit boards to convert the surface mount to match my boards. Not so much fun now, is it?

I had been on the look out for several years (unsuccessfully) for an AGC circuit. I asked everyone in sight, with no joy at all. Then I met a guy who suggested one could couple two circuits together using an LED on one and a photo resistor on the other, and this could be made to work as an AGC. Fair enough, I'll give that a go.

Then I remembered the Laser Communicator I purchased from Oatley Electronics some time ago. It had LED's and photo resistors, so I dug out the circuit and what did I find? A beautiful AGC right there on the transmitter board. As it happened, it did not use the LED/photo-diode set-up, but a FET in the feedback loop of the microphone. With nothing to lose, I pulled off the 2165 and built the whole laser front-end onto my board.

Instant gratification! The thing detected the sound of my breathing, and didn't overload when I yelled at it! It turns out that this circuit is very similar to that found in tape recorders: at last I was on to a winner.

This left only to fire up the ADC. I built it onto VERO board, using 1 byte on the I/O card for reading the data using INP, and 1 byte for controlling the ADC read/address requirements using the OUT command. Since I need eight microphones sampled simultaneously in the antenna, I needed a 3 of 8-decoder chip, a 74138.

The board didn't work. Not again!! I was baffled, and by now my sound card based oscilloscope was no longer up to the task. I pulled out my SoftMark oscilloscope card and it didn't work either, so it was off to Altronics to buy a nice 2-channel Hung Chang oscilloscope. Now we were cooking with gas! No food in the house, mind you, but now I could see what I was doing. And what I could see wasn't pretty.

Part of the circuitry required for the ADC is a level shifter. I built it from the Analog Devices circuit diagram, and it didn't work. By now I was beginning to think I was dealing with Microsoft rubbish, but no, this was the mighty Analog Devices. And their circuit was simply wrong.

But they got one thing right. Yes, the disclaimer. There in the corner of the sheet, " Information furnished by Analog Devices is believed to be accurate and reliable.....". Why do they think it is accurate and reliable? They clearly never built and ran the circuit! Is their disclaimer based on a revelation from the divine?

Well, with everything fixed, patched and redesigned, the channel now works, as a prototype anyway. There is noise on the ADC, probably the result of the high inductance of the VERO board layout. Sadly, VERO wiring kits, which produce very low inductance circuit layouts, seem no longer to be available. Indeed, I haven't found anyone old enough to remember them. So I shall have to make my own, which means back to the injection moulder to make the wiring combs.

Performance of the system is looking very promising. I am getting over 100,000 samples per channel in Quick Basic, and over 200,000 samples per channel in Power Basic. This should yield excellent spatial resolution for the antenna, for models doing up to 300 MPH. Yes Yes Yes!

Lets hope its not another year before the next epistle. Cheers from Joe Supercool.  

Photo Description	Link
8-bit ADC board layout for low inductance wiring. Very fast ADC chip permits digitisation of signal from microphone.
Vero board circuit test layout for ADC. ADC conversion time of order 2 microseconds.
Reverse side of ADC board layout for low inductance showing wiring dress. Wiring is wire-wrap hook-up wire, soldered to pins.
I/O card plugs into ISA slot on computer motherboard. Permits computer to control address and reading of the ADC card.
LED circuit which permitted easy testing of the I/O card. I/O card is not conspicuously easy to program.
Analog card provides the acoustic antenna transducer. Excellent AGC circuit from Oatley Laser Communicator kit.

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