Orthogonal Frequency Division Multiplex (OFDM)
Live analog TV in a voice bandwidth!

Getting Started     Transmitting     Picture Replay

Guided tour of Receiver

Receiving NBTV is not as simple as you might imagine, but with the correct equipment and a little patience and practice, you'll soon become familiar with the process. The problem is that (on HF SSB at least) the transmitter and receiver need to be very stable, and the receiver needs to be very closely tuned to the correct transmitter frequency. The receiver need not have fine tuning steps (even 100Hz will do) provided the RIT (Clarifier) range is sufficient and can tune slowly. Final tuning is ALWAYS done with the software.

You CANNOT successfully use a VFO rig for this mode (on HF SSB), and some older synthesized rigs may not be sufficiently stable. Tuning must be achieved to within 1Hz, and the stability of the transceiver is thus very important.

On VHF (or anywhere you use an AM or FM transmitter) tuning and stability are not so important. Within 100Hz should be good enough, and you should not need to adjust the software tuning. The same applies when testing at audio frequency between computers. You can't reliably make tape recordings of these signals, but digital recordings should be OK.

Take a look at the picture to the right (handsome old guy, eh?) All three receiver programs are similar. There are three important panes to the receiver window:

When no signal is received, all these panes should show noise, coloured in the case of the main pane, when in colour mode.

Below these panes you find the 'Record' button, which allows you to record what you are receiving as an AVI movie. You can also select colour and B&W reception here, in the 'Mode' panel. In Mono mode, the 'Sync-Setting' pane switches to show just one field, and the frame reception rate speeds up.

Obviously 'Colour' mode won't do anything useful for a B&W transmission (except slow reception down), but selecting 'Mono' mode may help reception of a noisy colour signal. At the very bottom is the software 'Fine Tune' slider control.

It's difficult to move the 'Fine Tune' control in very fine steps. Once you've clicked on it with the mouse, however, you can fine tune in very tiny steps using the keyboard 'Left' (<--) and 'Right' (-->) keys.
The 'Sync-Setting' and 'Tuning' panes have mouse-click action. If you can see a green sine wave line in the 'Tuning' pane, you can click on it to center the tuning. A red line will appear, which indicates how far off the tuning was, but which you can otherwise ignore. In practice it is easier to just watch this pane and adjust the 'Fine Tune' slider so the sine wave is exactly centered under the yellow line (as in the picture above). The other green wobbly lines are caused by the video carriers adjacent to the pilot carrier, and if you're mistuned by 30Hz or so you will see the sinewave on one of these lines instead. Until you have tuned correctly, you may also see (if you look carefully) the pilot carrier as a dotted line through the pictures in the 'Sync' pane.

When you click in the 'Sync-Setting' pane, you set the point in the received picture that becomes the left side of the main picture. As the next complete picture is received, it will be aligned with the point on which you have clicked. In this pane there is one image in B&W mode, and three in Colour mode (as shown above). You need to click just to the left of the most obvious vertical dotted line between the small B&W images in this pane. If reception is good, you should clearly see the divisions between the three images, and one will have a wider dotted line (at the far left in the picture above).


Start the program RX_48x48.exe, RX_96x72.exe or RX_RGGB.exe as appropriate. Check that the correct sound card input is selected, by selecting 'Soundcard/Select soundcard' and 'Soundcard/Adjust' input from the menu. If you have the correct input you should see noise on the screen in the lower and upper panels.

The receiving program can be run even while you are transmitting, in fact it's a useful way to test your transmissions and become familiar with the receiving software. To do so, you need to set your Recording Applet (opened with Adjust input from the menu) to allow the transmitted signal to be one of the input sources. There should be a device available called 'Output Mix' or 'Wave Out Mix', which you should enable. Once you do (and assuming the transmitter program is running, but not necessarily transmitting), you should see the transmitted picture on the receiver screen, as in the example on the right. You may need to adjust the sample rate (menu 'Sound card/calibrate sound card') as the timing can be off when running both programs at once, especially on slower computers and sub-standard sound cards. You may never achieve particularly good pictures from your own computer, but don't lose heart, as the other guy will!

Now try tuning in a real signal. The best way to tune first of all is to tune the receiver main dial to give the most natural sound to the other guy's voice, and then leave the main dial alone. It's best (if you have the capability) to use a preset frequency. The software tuning range is quite small.

Coarse Tuning
If you don't have the opportunity to hear the other guy speaking, or start receiving in the middle of a transmission, try adjusting the receiver main tuning so that the B&W images in the sync pane contain only picture, with no black area below, and no noisy area above. If you can see it, adjust with the RIT so the horizontal dotted line caused by the pilot carrier is in the centre of the picture. You might even be able to make it suddenly disappear with the RIT. Tune very slowly.

Fine Tuning
Once the picture is being received (you'll hear a buzzing sound), adjust the receiver RIT very slowly until you are able to see the green sine wave on the tuning pane. Do not touch the main tuning dial! You may need to tune 100Hz either way with the RIT. After each adjustment, wait a few seconds for the pilot sine wave to build up. If your RIT goes in steps rather than smoothly, you may not see the sine wave anywhere near the centre of the tuning pane. Once you can see the sine wave, bring it near the centre if you can, and then very slowly adjust the Fine Tune slider on the receive software to exactly center the sine wave on the yellow line. Until you can do this, you won't see a usable picture.

Sync adjustment
Once you have the tuning correct (major achievement!), you can then focus on getting the sync correct. Look closely at the image(s) in the 'Sync' pane, and click just to the left of the most obvious vertical dotted line. Wait a moment as the 'Sync' pane is redrawn in a different place, and soon you will see the main viewing pane redrawn in the selected position. It may take a few tries to get the setting correct, and the images shown while you are adjusting will have strange colours. Be patient!

If the colour image seems to be aligned correctly but the colours are strange, you may have selected the wrong vertical line, so simply try again. If the image appears to have no colour, or only coloured noise, it may be a B&W transmission, so select (from the menu) 'Picture properties/Colour mode/MONO' (or 'Mono' from the 'Mode' panel). If you are sure the picture is adjusted correctly but the colours still seem wierd, (and the picture is upside down, which is a dead giveaway!), try the other sideband and start again.

Sound Card Calibration
The OFDM NBTV system is used because (unlike SSTV) it is remarkably tolerant of multi-path reception and fading. However, it is not very tolerant of differences between sound cards. Some computers even have a difference between transmit and receive sampling rates! These differences cause the received images to slowly drift left or right, and the picture will appear lighter or darker at the top of the image from the bottom of the image, with colour shifts. The picture may also be quite noisy at the top or bottom.

An example of poor sound card sampling rate

The menu item 'Sound card/Calibrate sound card' can be used to adjust the receiver sampling rate slightly to compensate for these effects. Adjust for even brightness across the picture. Once you have done this, you will need to fine tune the signal again.

If the sound card (it might be the other guy's) is badly out, even though you should be able to achieve even brightness through calibration, the picture will still slowly drift left or right, so you'll need to readjust the sync every few minutes.

RGGB Reception

In many ways the RGGB VHF program is very similar to the other programs. It has however three pilot carriers instead of one (one top and bottom as well as in the middle) to give better tuning recovery with weak signals, and it has only two fields instead of three for colour reception. The frame period is 4.5 seconds.

The tuning technique is exactly the same as the other receivers. Of course if used on 10m or VHF FM, tuning won't even be necessary. The modulation is the same as the 96 x 72 version. The difference lies in the sync selection. There is no marked dotted line identifying the start of the frame, just a narrow dotted line between fields.

If you look closely at the sync pane in the accompanying picture, you will see that one of the small B&W images appears to be striped (especially noticeable on high saturation colours). It is the one on the right (look at the callsign). This is the Red and Blue field. You need to click just to the left of the narrow dotted line on the right of this field. The field is striped because each alternate line is a different colour. With practice you'll find it easy, and if you don't hit it first, time, keep trying.

This mode has slightly poorer vertical colour resolution, but since the eye is more sensitive to vertical than horizontal edges, the observed effect is minimal. The increased frame speed is a big advantage, and results on VHF FM are excellent. On HF however, this mode performs quite badly as it is very affected by colour noise.

There are two other differences with this receiver program. It has a built-in frame averaging noise filter, and it saves files in a different '.tv' format, not compatible with AVIs. Use the RGGB Viewer.exe program to view these files.

Getting Started     Transmitting     Picture Replay

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