The NBTV receiver programs allow you to save what you receive quite efficiently as multiple frame standard .AVI files (like movies). These can be retransmitted or viewed with the standard Windows™ Media Player, but the special AVI Viewer.exe replay program provided has features designed for making the most of NBTV. These include frame averaging (effective in countering noise), adjustable brightness, contrast, frame rate, the ability to save the adjusted file, and a manual slider which you can use to slide back and forth to view individual or averaged groups of frames.
Although the OFDM NBTV system cannot transmit movies in real time, it can transmit them slowly, and this viewer can be used to play the received pictures back at motion speed.
Open AVI Viewer.exe and you will see a window as shown to the right. Press the 'Load AVI' button or use the menu 'File/Open' item to open the file dialog, where you select a suitable .AVI file. Once the file is selected, the program is in pause, and you may press the 'View AVI' button to watch the movie, or adjust the slider under the picture window to see individual frames.
When you press 'View AVI', the program plays the file over and over until stopped by the 'Pause AVI' button or a new file is loaded.
If you pause the replay and then enter a value in 'Frames to average', successive frames will be mathematically processed to remove noise. The more frames averaged, the better the result. It is also creates a pleasant 'fade' between different sets of identical frames.
Frame Averaging is especially effective when multiple frames are the same, or when you wish to enhance the background where there is movement in the foreground. The technique is not so useful where there is colour banding, such as is caused by lightning or short fades.
Averaging moving or different sequential images makes no sense of course, although used sparingly (average = 1 ot 2) gives a nice fade effect when playing a series of slides at speed. The default value is 0 (no averaging).
Frame averaging is also very effective when viewing frames during pause, and averaged AVIs can be saved.
The default frame rate when viewing is defined by the AVI file, typically 10 frames/sec. This option is selected by default, or by the 'Select frame rate/AVI speed' menu item. The frame rate box is normally greyed out. You can set other frame rates (even fractional rates such as 0.2) by entering a value in the 'New frame rate' box. You have to first select 'Select frame rate/Select own speed' from the menu to enable this box. You then have to fiddle with 'Pause AVI' and 'View AVI' a few times for the new frame to work.
The Picture Adjust menu item pops up Brightness and Contrast sliders for picture adjustment. There is also a 'Default' button.
The 'Convert AVI' button allows you to save the adjusted view (altered frame rate, frame averaging, brightness and contrast) as another AVI file. It brings up the usual file save dialog.
The VHF compressed mode RX_RGGB.exe receiver, for various technical reasons, saves files in a different format. This format, the '.tv' format, is proprietary, and not compatible with Windows Media Player or AVI Viewer. Use the alternative RGGB Viewer.exe program to view these files. The program operation is generally similar, although the picture window and the control window are separated.
The program starts with the 'File Open' dialog ready to go. Just select a file, and you're away. The transmitter programs are not capable of replaying the .tv files.
The RGGB Viewer in action (Con ZL2AFP behind the camera)
Previous attempts at Narrow Band TV (i.e. less than 3kHz) have not been particularly successful. The first systems were AM with Baird-type scanning disc video sources, which could produce good pictures, but required at least 10kHz bandwidth. Later attempts used SSTV type FSK modulation, but had limited data rate and poor performance under multi-path conditions. One way or another, the various systems suffered from excessive bandwidth, poor performance and generally poor picture quality, which made them unpopular with DX TV enthusiasts. Until the arrival of this system, NBTV experiments were limited to recordings, as radio transmission was not practical, especially on HF.
Con Wassilieff ZL2AFP is the foremost NBTV pioneer - he started with a Baird system, and has progressed though all the various techniques, to arrive at the present system described here. This system has several advantages -
The signal is an Orthogonal Frequency Division Multiplex (OFDM) system consisting of 48 or 72 carriers from 400 to 2400Hz, one carrier per (horizontal) picture line. The lowest frequency carrier carries the lowest line in the picture. The carriers are 42Hz or 27Hz apart, are generated using digital oscillators, and modulated using differential narrow frequency shift keying in response to signal brightness. The pixel rate is about 40 pixels per second, 256 sound card samples per pixel, and each field takes one second to three seconds to transmit, depending on the resolution. The sampling rate is 11.025kHz.
- The transmitted signal is only 2kHz wide, and suited to SSB transmission.
- No sync information is necessary, so all the bandwidth is devoted to picture information.
- The low data rate (48 symbols/sec) gives the system good noise rejection, and especially gives it excellent multi-path and fading performance.
- FM modulation of the OFDM subcarriers gives good noise suppression with modest transmitter power.
- Using modern Windows™ software, facilities such as web cameras and TV/video capture cards can be easily used as real-time image sources.
- Because the system is computer sound card based, advanced modulation, demodulation, digital signal processing and image enhancement techniques are available.
Colour is transmitted in three Red, Green and Blue sequential fields. While this is slower than other methods, it is the most robust under noisy conditions. A vertical dotted line is added to the start of the red field so that the frame start can be identified. An extra pilot carrier, placed in the centre of the video carriers, provides a means of very accurate tuning. This carrier has low frequency sine wave FM modulation.
The receiver uses a Fast Fourier Transform (FFT) process, making 96 FFT's per second at an 11.025kHz sampling rate. The samples are decimated by a factor of two, and the FFTs achieve a bin resolution of 43Hz or 27Hz depending on the mode. So that the bins can exactly match the received carriers, the fine tuning is relatively important to give good grey-scale rendering. Both transmitter and receiver also need to be reasonably stable of course.
The FFT samples are converted to brightness information, using one bin per pixel. The brightness information is coded in the difference between adjacent carriers, in order to remove the colour shift effect of mistuning, and minimize the overlap of modulation sidebands in the receiver FFT. Because there is still some signal spread across the bins, there is some cross-talk between pixels, which is not noticed except when receiving a test pattern. Each FFT results in a single vertical image stripe of 48 or 72 pixels. 96 new stripes can be displayed each second, since the transmitted pixels are oversampled to ensure there is no (well, limited) Nyquist sampling error.
Adjacent row information can also be interpolated to provide an in-between averaged value to give 96 apparent vertical pixels. Sequential frame data is stored in a buffer, and subsequent frame data can be synchronously averaged with it, allowing the displayed image to have data enhanced and transient effects such as impulse noise, selective fades and white noise to be averaged out. For reasons related to recording the image in real time, this feature is currently available only in the viewer. The result is very effective.
Sure, the results are modest by any measurement, and especially so when compared with colour SSTV, or fast scan (wide band) TV, but with the ability to deliver a colour picture in three seconds in 2kHz bandwidth under noisy NVIS conditions, this system is unique. You really wouldn't credit how good the pictures are, given the limited picture resolution.
Murray Greenman ZL1BPU
Copyright © Con Wassilieff and Murray Greenman 2004-2009. All rights reserved.