ZL2AFP Narrow Band Hybrid NBTV

Conventional FM subcarrier as in SSTV, but with secure digital sync
Introduction     Receiving     Picture Replay


Transmitting Hybrid NBTV is quite straightforward. The controls are very similar to the OFDM NBTV system, and operation is also similar. The program includes drag-and-drop capability, and can transmit still pictures, movies and live camera (e.g. webcam) images.

The transmitter program window

'File/Exit' closes the program. 'Soundcard/Audio properties' and 'Soundcard/Mixer Output' allow you to select and adjust the computer sound card for transmission from within the program.

In movie mode, this program is capable of transmitting MPEG1 and MPEG2, but for this you may need to change the video accelerator setting, hence the extra 'Video Accelerator' menu item. 'Movie frame rate' allows you to select a replay speed, or synchronize it to the source movie, one frame at a time.

There are three tabs which can be selected: 'Still picture', 'Movie' 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 the 'Still picture' and 'Movie' tabs the current file name is displayed at the bottom of the tab.

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). Even if a B&W image is selected, the transmit program still transmits colour information.

If the picture has an aspect ratio other than 4:3 (landscape mode), 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 (4:3 portait shape for example, as shown in picture on right), the sides will have unused space. A 4:3 (landscape) picture, such as a camera or TV picture, fits perfectly. The background colour transmitted for the unused space is the same grey as the window background.

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. Watch the progress bar to the right of the image, and drop the new image just before the last one completes. No partial images will be transmitted - each picture is completed before the next is started.

Movie Tab
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. There is also a 'Sync frames with sender' option.

Open the file to be played from the File menu (top left), or drop it onto the tab from a folder list. The controls at the bottom start/stop the AVI file and show/adjust the position within the file. While the program is in pause (the '||' button pressed), transmission continues: the frame showing during pause will be transmitted over and over.

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. In addition, in order to use the webcam, click on the 'Enable Webcam' button at the bottom right of the window. You need a dual-core Pentium 4 3GHz computer to use this tab. The extra button is added to allow slower computers to operate with still pictures without the extra burden of the Webcam driver software.

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 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.

Always on top
Under the Transmitter applet picture pane 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.

Negative modulation
In some circumstances noise on received pictures will be less if the modulation sense is reversed. This of course needs to be coordinated with the receiving station. The default is negative modulation.

Preserve still picture aspect ratio
It was mentioned under the Still picture tab information that portait format pictures will be transmitted with grey space on either side, since the transmissions are always landscape format. This is the default procedure. You can however, unclick the 'Preserve still picture aspect ratio' control, and transmit such pictures stretched full width (example on right). This has the advantage of using all the available image resolution. The receiving station can restore the correct appearance by manually (with a graphics editor) changing the image aspect ratio back to 3:4 once it has been saved.

Starting the Transmitter

Type the number of the com port you have chosen to control the transmitter PTT circuit into the box next to the 'Set comport' button, then press the button. This is presumably whatever port you use for other digital modes. If you plan to use VOX (and you can), leave this setting at '1'. If you want to test at any time without transmitting, just leave your VOX off. The value you set is not remembered by the program, and the default value is '1'.

Start transmission by pressing the 'TX' button (now that wasn't hard, was it?) The button changes to read 'Turn Off', and the transmitter will come on immediately. As it starts, you should hear a typical SSTV sound from your computer speakers. Once there is output, adjust the soundcard level for a suitable transmitter power, using the sound card Output Volume applet.

Most 100W transmitters can run full power with conventional SSTV, but don't attempt this with Hybrid NBTV. 20W should be plenty. This is partly because there is envelope modulation on the PN sequence, which is BPSK modulated, and any non-linearity will cause this to be very broad. In addition, with long transmissions over multiple frames, your transmitter will become very hot. 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.

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.

Prepared slide examples. Note the different aspect ratios.
(Sorry, this program won't transmit Calvin's yoyo skills as a moving .GIF!)

Once transmitting, you can switch between tabs to send different types of images. Remember that it takes 10 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.

You can follow the progress of each transmitted image from the progress bar to the right of the tab image pane (blue and while line in picture above). The blue part of the line moves downward, charting the position in the transmitted image (which is transmitted from top to bottom, the same as regular SSTV).

To stop the transmission, simply press the 'Turn Off' button again. Try to time end of transmission so the progress bar has reached the bottom (end of frame) before switching over.

Introduction     Receiving     Picture Replay

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