High speed PSK modem, image compression and noise-free pictures
Introduction     Transmitting     Receiving     Picture Replay


A number of problems experienced with the OFDM NBTV system led to the development of this Digital NBTV mode. First, the OFDM signal is very difficult to tune, especially for beginners, and requires extreme transceiver accuracy and stability. Then in addition, the pictures tend to be noisy and individual frames can be marred by multi-path effects, especially when used on the lower bands.

The approach taken for the Digital NBTV mode is completely different, in many senses:

So nothing could be more different! The transmission consists of a stream of packets, each containing data for a small number of image lines. The amount of data in the packet varies according to the image complexity, the compression level and the level of forward error correction included, but the packet size is constant at 256 symbols plus an 80 symbol header.

Digital NBTV uses a modulation technique which is widely used by high speed HF radio modems. A single 1500Hz phase modulated carrier is used to send both packet sync and payload. Using BPSK modulation, a pseudo-random binary (PN) sequence starts each packet, and is used to identify an exact point in the transmission from which the data can be synchronized. A cross-correlator is used in the receiver to locate the one point in the whole message where the sequence matches up with the local copy of the sequence. The cross-correlator works with a known pattern to look for, and is a very powerful and sensitive tool.

These radio modems use the PN sequence technique to enable complex high speed data to be decoded accurately - the ranging information determined from the cross-correlator is used to correct the received data timing to reduce errors induced by the ionosphere, using a signal processing device called an 'equalizer'. The equalizer also corrects for Doppler errors which affect carrier phase, making the use of 8-PSK practical.

Digital NBTV uses a 31-bit PN sequence borrowed from STANAG 4285, with one chance in two billion of a perfect score being caused by noise. It uses 80 symbols (modulation time slots) to send this sequence about 2.5 times. Each packet is contained in a 336 symbol frame. 256 symbols are used for image data and FEC information. Since 4-PSK is used for the data, each packet could contain 512 bits of image data, or 3047 bps raw data rate. The data symbols are scrambled in an 8-PSK pattern to improve resistance to selective fades.

Each Digital NBTV packet takes 168ms to transmit

Like the STANAG 4285 system, single-tone PSK Digital NBTV can also operate at 2400 baud, using a sub-carrier frequency of 1800Hz. The corresponding bandwidth (just under 3kHz) is too much for most HF transceivers, but quite suitable for VHF, and gives a worthwhile speed improvement. However, to fit the signal into a normal amateur transceiver IF, it is usually operated at 2000 baud using a 1500Hz sub-carrier.

The modem section of the transmitter or receiver converts digital data into PSK audio for the transmitter, or received audio into digital data, respectively. The receiver modem also has to manage sync and equalization.

Each transmitted packet commences with a BPSK pseudo-random (PN) sequence (same sequence as STANAG 4285), which is used to synchronize the receiver timing with the start of the packet, and also serves as a measuring point for the receiver equalizer software which measures and compensates for frequency offset and drift, and other code which compensates for timing errors. Detection of the PN sequence is achieved using a cross-correlator. This technique is extremely sensitive, so no matter how weak the signal is, packet synchronization is secure.

The packet data payload is transmitted as 4-PSK, to ensure a high data rate. There are nearly six packets per second, using a 2000 baud modem.

Because a digital system is inherently much less bandwidth-efficient than an analog one, in order to achieve even reasonable frame rate, considerable effort must be made to reduce the amount of data transmitted to a minimum. Two coding and decoding (codec) strategies are used: a pixel interpolation technique, and a wavelet compression technique.

The Digital NBTV system offers some flexibility of image size:

48 x 48 pixels zoomed
64 x 64 pixels
96 x 96 pixels zoomed
128 x 128 pixels
256 x 256 pixels
The 48 x 48 and 96 x 96 images are zoomed-in versions of the next size up.

All transmitted pictures are square (1:1) in pixel ratio, but due to interpolation techniques used, are generated from 4:3 ratio images, and result in received images that are again conventional 4:3 landscape format. This gives a built-in compression to 3/4 of the original data. The images are always displayed the same size, but of course vary in resolution. The image at the top of this page shows the receiver image view pane, with a 64 x 64 pixel image displayed. The image was received over a 500km path on 80m at night, and is completely noise-free.

Standard image compression algorithms such as JPEG, JP2 and MPEG are designed for significantly higher image resolution than those used here, and do not work well on such small (low resolution) images. A special series of 'Wavelet' compression algorithms was therefore developed, tuned for small images. The wavelet transforms to work at their highest efficiency when operating with images that are 2n (a power of two) wide and high, which is accomodated by the interpolation to 1:1 aspect ratio and the choice of image sizes. Three alternative transforms are offered:

Daubechies D4
Cohen-Daubechies-Feaveau 9/7 CDF97 (default)
The last of these is the default and generally gives the best images. Haar is useful for images with high contrast, such as text. Because the transmitter software shows the effect of image size and compression in real time, it is easy to select the most suitable for any image. The CDF97 wavelet is used in the JPEG2000 image compression system, but has been adapted for this particular application. Wavelet compression, as with any image compression system, merely reduces the number of bits per pixel and requires additional 'packing' of the bits to achieve the high compression ratios. Arithmetic coding is used to compress the data.

The transmitter codec illustrating transmission in 128 x 128 mode

In the above screen-shot of the transmitter codec program, you see at the left the image from the camera, already squashed horizontally by arithmetic compression into a 1:1 ratio. To the right of this is the actual image transmitted, and you can see the artifacts and blurriness generated by reducing the original large image to 128 x 128, and the application of the CDF97 compression. This will be exactly the same as the received image. To the right of this picture are the codec controls, which set the frame size, the compression transform used, and (further to the right) a vertical bar which shows how much of the available space in each packet is occupied by data (green) and Forward Error Correction (FEC) data (red). The green bar changes as you change picture size, and you can drag the red bar up and down to change the strength of error correction. All these controls can be used 'on the fly' - the change the appearance of the displayed picture immediately, and affect the transmitted pictures from the start of the next frame.

Error Correction
A digital system, especially one with compression, cannot recover an image at all if it is not received correctly. This is a tall order on HF without some sort of error correction. The technique used here is the Reed-Solomon (RS) technique, which is ideally suited to packetized data, and also has the advantage that the level of correction strength depends on how much correction data is transmitted. The FEC decoder will simply throw away any image that cannot be completely corrected. Thus reception using this NBTV system is always error free. You might not get many pictures, especially if conditions are poor, but every picture received will be perfect. Two levels of Reed Solomon coding are used: an errors-only rate 2/3 RS on the data within each packet, and an errors and erasures variable rate RS on the data (which have been interleaved and thus spread over several packets to further reduce the effect of burst errors).

Data Transmission
Because this is a completely digital system, it is no trouble at all to also transmit text and other data. In this software two simple options are offered. You can tag every packet with a callsign. There is no overhead involved, as there is free space in each packet to allow this to be included. You can also send a text message up to 39 characters long. This is sent as a special text packet, once every new image. This does slow down the image transmission rate slightly.

Transmission Speed
The actual time taken to send each frame depends on the complexity of the image, the compression used, and the amount of FEC used. Image complexity alone can account for a wide variation. The table below shows typical values, provided you leave the compression transform and error correction level at the default settings.

Mode TX Size Image Size Pixels/Frame Frame Rate
  48 48x48 48x64 2304 ~9 sec
  64 64x64 64x85 3072 ~10 sec
  96 96x96 96x128 9216 ~13 sec
 128 128x128 128x171 16384 ~15 sec
 256 256x256 256x341 65536 ~25 sec
Digital NBTV Mode Summary

This performance seems at first glance to be not much better than SSTV image rates, but that's the price paid for using a digital (and noise free) system. Note also that the time taken to send each frame is not in direct proportion to the number of pixels in the image, as the wavelet compression is much more efficient on higher resolution images. The transmissions are always in colour, as the compression ratio for B&W is little better and thus the option is not justified.

The ITU 'Emission Designation' for all these modes is 2K40G1FNF.

Digital NBTV Software

Software can be downloaded free from this website. It is designed for Windows™ 98 and Windows™ XP. At present there is no software for other operating systems. A Celeron™ 2.4GHz or better computer is required. To operate the transmitter and receiver at the same time requires a Pentium™ 4 2.4GHz (preferably dual core) or better. A USB web-cam or TV receiver/video capture card is required for live pictures.

Download the archive (link below) and unzip it into a suitable folder. Create shortcuts to the transmitter program (Video_send_JP2_movie.exe), the receiver program (Modem_RX_JP2.exe) and the viewer (Viewer_drag_drop.exe).

    ZL2AFP Digital NBTV Software (D-NBTV.zip, 265kB)
    Example Digital NBTV transmission (D-NBTV.avi, 7.35MB) Captured on 80m, 500km range.

Introduction     Transmitting     Receiving     Picture Replay

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