ZL2AFP Narrow Band Hybrid NBTV

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


Although the Hybrid NBTV system cannot transmit movies in real time, it can transmit them slowly. The receiver allows you to view the incoming pictures in real time, and to record a series of frames, so a Viewer can then be used to play the received pictures back at motion speed.

Start the receiving modem program RX_NBTV_128x96.exe. The program should look like the picture below.

The Hybrid receiver program

Receiver Indicators and Controls

At the top of the receiver window is the 'Correlation' pane. This is a type of tuning display, but not like any you've seen before! Rather than a frequency vs time display (spectrogram) this is a correlation strength vs time display (correlogram). Time is represented vertically. Now, this is where the difficult bit starts!

The receiver cross-correlator knows the PN sequence used in the transmission, and is continuously comparing the last 80 bits received from the phase detector with the known sequence. Each of the received bits that is the same as the same bit in the PN sequence scores '1', while each one that is different scores '-1'. Most of the time the cross-correlator is comparing FM image signal or incorrectly timed PN bits with the sequence, and so the score will be low, close to zero. However, just once per data packet the score will match well. It may not be perfect due to noise and other errors, but will be clearly much higher than at any other time in the packet. This allows the receiver software to determine easily where the data starts within the packet.

Now, imagine we draw a graph of the scores with time (e.g. across the length of a packet):

The correlator response over time

Note that there are two smaller spikes as well as the main peak. This is because the PN sequence is sent more than once. Now the horizontal direction of this little graph is also time, time over an interval of one packet. We can use this information to create the correlogram, and that is what happens. The data from each packet's correlation scores is plotted as a line on the correlogram, with the spikes showing darker. The Correlogram plot (top picture, top pane) moves down in time with every packet, and then wraps around from the top.

Of course the point at which the samples start is arbitrary, and so the three spikes may not conveniently appear in the middle as shown here, or approximately so, as in the top picture, and may appear anywhere on the Correlogram. It doesn't actually matter.

After all this effort (it seems!) the sole purpose of the Correlogram is to indicate when you have a real Hybrid NBTV signal (as nothing else will show correlation). However you also use this display to fine tune the receiver - simply adjust the receiver tuning to maximize the contrast of the three lines compared with the noise background. The tuning range over which the Correlogram lines are visible is about 50Hz. As you maximize the signal, you'll notice the green tuning line to the right of the Correlogram also grows.

Slant Adjust
The slider under the Correlogram allows you to modify the receiver sample rate very slightly. This is done to compensate for errors in the sound card sampling reference oscillator (yours or the other guy's). If there is any significant error, the lines in the Correlogram will slant to the left or right. Use this slider to straighten the Correlogram. The receiver performs better with the slant corrected, but a small slant will not affect results greatly. You will notice the green tuning line also grows as you get close to the correct sampling rate.

The box on the right labelled 'Tuning' shows how well the signal is tuned in. Once you have it roughly tuned using the Correlogram and its tuning bar (described above), then watch the red line on the graph in the 'Tuning' box. This shows the receiver tuning error, and has a range of 35Hz. You can use the receiver RIT to centre the signal, but it's just as easy to use the 'Carrier tune' slider under the red-lined tuning display.

Picture Control

The main picture pane (bottom left) has two sections. The upper third displays the current image data as it is received. The picture is wider than the number of transmitted samples because the signal is oversampled to prevent aliasing effects. Whenever an image is completed (the correct number of lines have been received), the image is interpolated vertically (more lines created by inserting averaged pixels) and transferred to the lower two thirds, resulting in an image which is the correct aspect ratio. Unlike the upper image, which is always changing, this image remains static until the next complete picture is received.

Vertical Sync
No vertical sync is transmitted, so it is very likely that when a transmission starts the first picture received will not be phased correctly - as in the first example below (left).

To correct this, you simply click the mouse on the correct point, which is the top of the lower part of the image or the bottom of the upper part (see arrow). This is usually easy enough to find since most often the top of the picture is ligher than the bottom. Once you click on the image, the next frame will be realigned correctly, as in the picture on the right. Sometimes it is difficult to see where to click, so it may not be right first time. If it's not quite correct, simply click again.

Sync misaligned (left - not arrow), and correctly aligned (right)

Once the vertical alignment is correct (this needs to be repeated at the start of every transmission), it stays correct for hours on end.

Mixer Control
The 'Mixer' button calls up the Windows™ Recording Control applet, so you can select the correct sound card and input, and adjust the input level. Select the Line Input (or wherever you connected the receiver audio) and adjust the level to show some noise on the Correlogram. You can adjust further later. The adjustment is non-critical, and has no effect on the received picture brightness or contrast.

Colour Control
The 'Colour Off' button suppresses the colour information that is received. If the signal is especially noisy, or if the transmitted image lacks colour, pressing the button may improve image quality by removing colour noise.

Save Video
Incoming frames can be saved to file (in proprietary .TV format) by pressing the 'Save video' button. All frames are saved until the button is pressed again. The saved files have an arbitrary file name based on the date and time, and are stored in the program folder. They can be played back in real time or sped up to simulate motion using the Viewer program. When you press the 'Record' button, all frames correctly received will be added to a .TV format movie file. This continues until you press the button again. The files become quite large fairly quickly if reception is good. The file created has an arbitrary file name based on the date and time. You cannot replay the files from the receiver, but an excellent viewer is supplied with the software.

Introduction     Transmitting     Picture Replay

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