Orthogonal Frequency Division Multiplex (OFDM)
Live analog TV in a voice bandwidth!

Getting Started     Receiving     Picture Replay


There is a dedicated transmit program for each mode. Start the transmit program TX_48x48.exe, TX_96x72.exe or TX_RGGB.exe. All the transmitters look the same, except the RGGB program has no B&W option.

When you start the program, you will see a small box which shows the available sound cards. Select one (easy if you have only one!), press APPLY and then OK. You will then see the transmit program window (shown to the right, with the default 'Still picture' tab selected).

There are four tabs which can be selected: 'Still picture', 'Moving GIF', 'AVI/MPG' and 'Webcam'. You can select any one of these and transmit the appropriate image. You can also 'preload' some of these for instant switching and seamless transmission. Whichever tab is currently showing is the one which will be transmitted in the next frame. All the tabs except Webcam accept 'drag and drop' selection of pictures (just drag the picture from an Explorer folder onto the appropriate tab). You must 'drop' the correct type of picture or the program will complain.

On most tabs the current file name is displayed at the bottom of the tab.

Except in the 48 x 48 mode, the whole picture seen on the tab is transmitted. It does not matter what size the original picture is, as the program will resize it to suit (although it does not change the aspect ratio). In 48 x 48 mode, the full height of the picture is transmitted, but only a square in the centre (chopping off the sides) is sent. Even when transmitting B&W, the transmit program still shows the picture being transmitted in colour.

If the picture has an aspect ratio other than 1:1 (48 x 48) or 4:3 (96 x 72), the picture will be adjusted to fit the width or height of the frame appropriately, and the unused space transmitted as plain background colour. In other words, if the picture is very long and thin, the top and bottom will have unused space, and if it is tall and narrow (portait shape for example), the sides will have unused space. A 4:3 (landscape) picture, such as a camera or TV picture, fits perfectly. The default background colour is white, but you can change the background colour with the 'B/G colour' item on the menu.

Still Picture Tab
With the mouse, you can 'drag and drop' any still image onto the Still picture tab to be transmitted. This tab accepts all the following image formats:

Remember that it takes some time to transmit each picture, and there is a delay from the time you drop the picture to when it is transmitted, so if you drop them too often, some will be missed. No partial images will be transmitted - each picture is completed before the next is started.

Moving GIF Tab
Do you know about 'moving GIFs'? They are multiple run-length encoded frames stored as a sequence in a single .GIF file, such as the example to the right (you may not see it moving - depends on your browser). These frames are transmitted over and over, one frame at a time. Really cool for an ID test sequence. You can download and test this example (or right-click and select 'save picture as...). Check out the cool moving .GIF of calvin and his yoyo at the bottom of the page! .

This tab will play movies! The transmission is of course very slow, but at the receiving end the frames can be recorded and then replayed at full speed. You can set (from 'Frame rate' on the menu) the frame rate of the transmission (frames per second). For example, with a 3-sec frame rate on a 9-sec transmission, two out of every three frames in the original file are skipped.

Open the file to be played from the File menu (top left). The controls at the bottom start/stop the AVI file and show/adjust the position within the file. MPEG1 movies can also be sent, but the video accelerator must be set to minimum, so that the pixels can be captured off the TX screen.

Webcam Tab
This tab uses any TWAIN compliant video source, such as a USB 'web' camera, TV capture card or TV receiver card. It will even work with a scanner! With the TV devices you can use images from conventional video input from a camera or digital still camera, live off-air TV, or TV camera with RF modulator. If you plan to use a USB webcam, make sure you plug it in and allow it to initialize before you start the TX program, or it won't be recognised.

From these devices one frame is captured whenever it is needed, so many are missed. The Webcam tab shows the video source with colour and full motion, but remember that that the transmitted frame rate is slow, so don't pan the camera around or expect to transmit movement.

Two extra buttons appear at the bottom of this tab - one to select the video source (if you have more than one), the other to select video format. Any format should work, and you don't need more than 288 x 384 pixel resolution. Higher resolutions require more processing power from the computer, but don't change the transmitted picture size or frame rate.

96 x 72 B&W transmission, live
from a digital still camera
Always on top
To the right of the Transmitter applet window there is a check box labelled 'Always on top'. When this is ticked (and it is by default), the transmitter applet stays above all the other windows on the screen. If you uncheck this option, the transmitter can be buried under other applications.

About the only advantage of this is that you can then transmit pictures of formats not supported by the program, through the simple expedient of dragging them over the top of the TX window. Remember that the image has to be small enough to fit on the transmitter active area. It is the top of several layered application windows that will be visible to the transmitter. This technique is a bit tricky, as you cannot see where to place the window. The technique overrides all the transmitter tabs.

Starting the Transmitter

Select PTT from the menu, and select the port you have chosen to control the transmitter PTT circuit. This is whatever port you use for other digital modes. If you plan to use VOX (and you can), leave this setting at 'none'. If you want to test at any time without transmitting, you can also set this value to 'none', but leave your VOX off. The value you set is not remembered by the program, and the default value is 'none'.

Start transmission by pressing the 'TX' button (now that wasn't hard, was it?) The button changes to read 'RX', and the transmitter will come on immediately, but the video modulation will be delayed by one frame period. As it starts, you should hear a buzzing sound from your computer speakers. This is the TV transmitter signal. Once there is output, adjust the soundcard level for a suitable transmitter power. You can pull up the sound card Output Volume applet from the menu.

Most 100W transmitters can produce on average about 20W of video. You will see the power vary up and down as the pictures and colours change. Do not attempt to increase the power as the transmitted output will be very distorted (and unusable) and the transmission will be very broad. Have a friend check your output quality and bandwidth, and if necessary reduce power. You'll be surprised how far your signal will be received on quite modest power.

These transmissions use OFDM, with many carriers transmitted at the same time. Since the phase of the many carriers is unpredictable, it is possible for them to all momentarily line up in phase to require more than full transmitter output. In other words these modes have a high crest factor, or peak to average ratio. So take care not to overdrive!
The simplest way to test transmission is to drop an image from your 'My Pictures' onto the 'Still picture' tab. Once you are organized, you will want to put together a series of neat slides for a slide show. You can easily make 'CQ', 'QRA' and '73' slides using a graphics editor. Try to avoid large areas of highly saturated red, green or blue in your images as they tax the transmitter linearity more than other colours.

Prepared slide examples. Note the different aspect ratios.

Once transmitting, you can switch between tabs to send different types of images. Remember that it takes up to nine seconds to send each image, and each new image is not 'grabbed' until the previous one is completed, so you need to change tabs or slides slowly. It is a good idea to 'preload' the tabs with what you intend to transmit before you start the transmission.

To stop the transmission, simply press the TX/RX button again.

Getting Started     Receiving     Picture Replay

Copyright © Con Wassilieff and Murray Greenman 2004-2009. All rights reserved.