NBTV (Narrow Band Television) is a technique with some similarity to SSTV, and some to conventional FSTV (Fast Scan TV). Like SSTV, it operates in a narrow bandwidth, suited to an HF SSB transceiver, but rather than send individual image frames, NBTV is designed to send multiple frames one after the other, in much the same way as FSTV, but with considerably slower frame rate. This idea allows frame-averaging to be used on noisy channels, and enables the transmission of moving images.
You can imagine that to transmit moving images in a bandwidth of under 2kHz (NBTV) rather than 7MHz (FSTV), something has to give! For the technique to be practical, the frame rate must of course be much slower, the image resolution is much reduced, and various compression techniques must be used, along with a range of strategies designed to reduce or eliminate the effects of propagation, especially noise and multi-path fading and timing errors.
The image resolution is generally lower than it is for SSTV, but this is made up for by the motion effects, and also by the faster frame rate, typically 1 - 10 seconds per frame, rather than 30 seconds or more for SSTV.
Three systems are described here. They are broadly, analog, digital, and hybrid designs, and all were developed by Con Wassilieff ZL2AFP, the acknowledged world expert on NBTV, and to date the only developer of modes that are successful on HF (there have been modes which work on VHF or on telephone lines). These new systems use sophisticated techniques, including digital signal processing, forward error correction, image compression and even path equalization. The three modes are:
With all three of these systems you can transmit live pictures from a web camera, TV capture card or TV camera, send still frames in any image format and any size, and transmit movie clips in several formats. Because the frame rate is low, the receiving programs allow transmissions to be recorded, and played back later at movie speed.
- Orthogonal Frequency Division Multiplex Analog OFDM NBTV
- Single-tone 2000 baud PSK modem Digital NBTV
- Hybrid FM NBTV with PN-sequence synchronization
Compared with SSTV, which sends even more slowly (and still has trouble with multi-path propagation tearing the images) NBTV must send faster, and is therefore even more prone to ionospheric effects. Thus the unconventional nature of these designs is explained by the emphasis on managing or countering these effects.
These different NBTV programs enable low and modest resolution B&W and colour TV images to be transmitted at modest power, and received via a limited performance HF radio channel. The systems has been optimized for specific radio conditions; most are designed for NVIS conditions (such as the Amateur 80m band at night); but there are other versions with better resolution, more suited to higher bands; and at least one faster version for VHF. In most cases the pictures are easily tuned with practice, as the software provides special tuning aids.
All three systems operate using a conventional digital modes setup: PC, sound card, radio interface with audio and PTT functions, and an SSB or (on VHF) FM transceiver. Use the links below to explore these modes further.