To design and build a wireless transmitter that works over the FM frequency and allows the transfer of a video/audio signal over a certain distance to a FM tuner.
In the fast-paced world, there is little time for inconveniences and a greater need for portability and adaptability. The idea for an Audio/Video transmitter stems from this need. There have probably been times when you've wanted to changed video tapes in your VCR without having to get off the comfortable spot on the couch. An Audio/Video transmitter will let you do just that. But it would offer other conveniences as well. For example, it would allow you to set up security cameras around your home w hich would send video signals directly to a television or VCR that was tuned to the same frequency. In addition, there would be no need to deal with cumbersome wires and cables.
Other uses for an A/V transmitter include creating a sister receiver and having truly portable speakers for your audio system. An A/V transmitter creates an infinite number of possibilities.
Design & Development (What we did):
The most difficult part of this project was coming up with a design that would work. Because both of us had very little experience with RF signal systems we had to learn, basically, from scratch. The approach we took, was to first create a video transm itter, then later add the audio transmitter. This way we could test each component individually and then integrate later when we knew both parts were working correctly.
We first went to the Grainger Library to research various transmitters designs and how they were built. Although all the books were very old, we were able to gather useful information from various sources. Most of the sources involved sending audio tra nsmission and had very little information on designing the transfer of video signals using the FM frequency. Unfortunately for the books that do show designs, the text were very outdated.
But we found some interesting standards that greatly help explain how the television works. This was not too far from our original intentions of building two different types of transmitters. Let us first look at the basic block diagram of what and how A udio/Video transmission works.
From the book: Television Electronics by Kiver and Kaufman (8th ed.) Copyright 1983; there is a block diagram of the television transmitter (page 9, Kiver and Kaufman).
As we can see, television transmission operates as two separate sections. One for the video and the other for sound. And just like our project, two different devices were needed to be built. And as noted before, most books in Grainger Library were older than us, so all parts used (tubes and such) were outdated and not readily available. So the search goes on to find another solution.
Let us look at some of the industry standards that might help shed some light on this project. From the book: Radio Frequency Transmission Systems by Whitaker (1st ed.) Copyright 1991; we see some of the standards set by the U.S. Federal Communications Commission (page 44, Whitaker).
|Low-band VHF||2 through 6||(54-72 Mhz and 76-88 Mhz)|
|High-band VHF||7 through 13||(174-216 Mhz)|
|UHF||14 through 69||(470-806 Mhz)|
|UHF||70-83||(806-890 Mhz) currently assigned to land mobile radio services|
Below is a table that show the specific frequency band assignment to the channels designations (page 45, Whitaker).
From above, we see that all channels assignments are 6 Mhz apart. And from Kiver and Kaufman (page 20-21) there is a listing of all of the corresponding television channels to their frequencies with much greater detail showing the picture carrier and the sound carrier assignments. Although based on cable standards, it is identical to the airwave standards set by the FCC.
Within the 6 Mhz range the picture and sound carrier are within 2 Mhz from the ends and also about 2 Mhz apart from each other. This leaves about 4 Mhz in between each channels. This is all illustrated in the following figure.
Implementation (How we did it):
Well, after searching high and low we have come across a diagram of some audio transmission which was claimed to also be capable of transmitting video signals. And it was labeled as a repeater. In the following pages we have printed up the schematics that we used to build our devices. Two devices were built (as noted above as to why two different transmitters are needed). We had various difficulties with implementing the video transmitter design. We discovered that a 9v battery did not deliver enough power to produce a strong signal. In addition, we came ill prepared because we needed an insulated crossdriver to tune our design to the correct frequency. But for an alternative remedy, we used an old clock/radio/TV to test our design. It provided an analog tuner which allowed us to fine tune the receiver in search of the correct frequency.
The audio transmitter eventually worked although the transmission was wrought with interference and fluctuations dependent on the distance between the transmitter, the receiver and the position of the transmitter's antenna. Also, since the devices built were very crude, it was very prone to large noise production (or at least highly distorted signals).
Audio/Video transmitter schematic
There was another design for video transmission found from the book: The Giant Book of Electronics Projects by The Editors of 73 Magazine (1st ed. 16th printing) Copyright 1982 (page 464).
We didn't build this design since we didn't know some of the undefined values (or at least they were not properly determined and purposely left undefined).
Overall we learned a great deal about RF signals relative to how much we knew before hand. We recommend taking an RF signal class such as ECE353 before undertaking any sort of RF project.
This project can be greatly improved on for those interested in RF transmission design which most people take for granted when listening to their favorite band on the radio or watching football games on the TV.