Laser Communications System

Results

What would a project be without results?

By using the built in hardware UART to communicate with Hyperterm on a PC we have tested our project to transmit text at up to 38.4 kbps. At this rate it is entirely error free. The system can likely transmit much faster, but for text applications anything faster is overkill.

We can send arbitrary data at a rate of up to 27.7 kbps. Although we could not pin point the exact reason why the reliability of our audio transmission fell drastically at bauds higher than 27.7 kbps, we do suspect that is has something to do with a timing problem involving the time required by the ADC to complete a conversion since the biggest difference between the audio code and the text code is that the audio utilizes the ADC and transmits continuously. We did not have sufficient time to test this hypothesis, but as we did have the ADC clock turned up as fast as it could go (125 kHz) we do not believe there would have been much we could do about this problem anyway. In the end, 27.7 kbps is plenty fast enough to transmit our voltage values if the sampling rate is 3 kHz. So ultimately the baud limitation did not degrade the quality of the transmitted audio.

On the other hand we were somewhat surprised by exactly just how poor the 8-bit audio sampled at 3 kHz sounded (even though we knew it would not be great). Even after some liberal low pass filtering the audio was still of poor quality. One definitely would not be able to hear a pin drop over this line.

Performance

If the laser is properly aimed at the phototransistor as discussed in the mechanics section, the bit stream is received perfectly. As such text transmission works flawlessly at our largest test distance, across the Digital Systems Lab. Unfortunately, the quality of our focus application (voice transmission) is limited by 3 kHz sampling and 8 bit sound. The speaker on one end is clearly audible on the other, but it takes a trained ear to decipher what he or she is saying.

The Next Go Round

All of our dreams and wishes for the next go round lie mainly with the ADC. If we had more time we would do well to utilize all 10 bits of resolution that the ADC affords instead of just 8. It would be fairly simple to modify the UART code to transmit 10 bits instead of 8 (although this would be a departure from the serial standard). Also (though we realize ADCs are more difficult to build than DACs) we would wish for a faster ADC. This would allow us to sample faster and, if our suspicions are correct, to transmit at a higher baud. All of these would improve the quality of the sound.

Another thing we would have done differently would be to implement a full duplex software UART in order to establish a 2-way link. This would require the UART to both receive and transmit simultaneously and would likely involve some clever use of interrupts. Because our budget and the availability of only one collimating lens only allowed us to build a 1-way link in the first place we did not pursue the full duplex software UART idea. (Note that we could have implemented a 2-way link using the Mega32 hardware UART which is full duplex, so the decision to make a 1-way link was purely because of a budget and supply limitation.)

One wish we have that would not require new hardware would be to spend more time optimizing the use of the 8 bits of information we do have. As of now only about 4 or 5 bits are effectively being used with the microphone gain settings as they are. If we were to use closer to 8 bits the audio quality would improve noticeably (for a while we were only using 3 or 4 bits and were able to improve it slightly by fiddling with the gain resistors in the microphone circuit). This is a problem which just involves time spent fiddling around with values to get the bit usage just right and would be especially important to do in order to gain anything if 10 bits were used instead of 8.

Interference, Safety and Usability

We did experience some interference in the final testing of our system, although the interference did not come from within our system, nor did it come from other peoples’ projects. It came from other people. As one might expect, when we tested our system to see if it was capable of trans-room operation if someone walked through the laser beam the transmission was interrupted. The interruption was only momentary and had no lasting effects. Obviously if such laser communication systems are to ever become widely used they will have to be placed where they will not be obstructed.

Having people walk through the beam brings up another point: safety. We did our best to emphasize the dangers to everyone in the room at the time (with the help of Prof. Land) to be sure no one accidentally looked into the laser beam of any of its reflections. Laser light can be very harmful to the eye if shined directly into it and so we had to be very careful with this aspect of our system.

The most difficult thing to master about the usability of our system was aiming the laser diode at the photodiode on the receiver. Once the laser was aimed and appropriately focused the use of our system was fairly straightforward: talk into the microphone and listen at the other end.

Intellectual Property Considerations

The only intellectual property consideration is to not that some of the concepts we utilized in the development of our software UART were borrowed from the Atmel App Note 305 (Half Duplex Compact Software UART) at http://www.atmel.com/dyn/products/app_notes.asp?family_id=607. As mentioned before, the code provided was written entirely in assembler language and so we decided to develop our own version in C.

Task Division

We worked very closely throughout this whole project, and as such a division of tasks is hard to do. Keith probably spent more time on the actual design of the web page while Mike spent more time writing the content. Keith spent more time figuring out the analog components of the project while Mike was more the software guru. We both feel very comfortable with the amount of work each of us put in.

Final Conclusions

Ultimately we believe that we achieved what we initially set out to do. Even though the sound quality was not as good as we may have liked we did still transmit audio over the link and so we feel quite satisfied with what we have accomplished.