اليكم احبائى اعضاء المنتدى الموضوع بالكامل مع الدائرة .
Video/Audio Wireless Transmitter
Nick Shin
Henry Hwangbo
Project Report
Advanced Digital Systems Laboratory
FALL 1996
Abstract:
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.
Introduction:
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were very outdated.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).
Band Channels Frequency 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).
Conclusion (Results):
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
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