Slow Scan Television or SSTV for short, is a method of transmitting still pictures over a very narrow bandwidth, on frequencies normally used for speech by Radio Amateurs .
SSTV was originally invented by Copthorne Macdonald and first used by Radio Amateurs, but now it is used by the Police & Security services. The original idea was to find a way to transmit a television picture over a single speech channel. This meant that a typical (at that time) 3MHz wide television picture had to be reduced to around 3kHz (1000:1 reduction). It was decided at the outset that the scanning rates must be very slow, which ruled out the use of moving pictures. The choice of time base for synchronising was the readily available domestic power supply at 50 or 60 Hz (depending on the country of origin).This gave a line speed of 16.6Hz and 120 or 128 lines per frame (against the then UK standard of 405 lines (now 625) per frame), giving a new picture frame every 7.2 or 8 seconds.
The original SSTV systems were based on ex-government Radar screens and long persistence cathode ray tubes. Now most systems are computer based for ease of use, giving full colour with up to 16.7 million colours and transmission times of up to several minutes for each picture.
SSTV does not have the benefit of error checking and this can cause problems espacially when you are trying receiving that 'perfect' picture from Japan and the last quarter of the picture is spoilt by someone calling CQ CQ or tuning over the top of the picture.
To get started on SSTV is really easy these days. All you need is a PC running MS DOS, Windows 9X, 2000, XP.
There are a number of SSTV programs available that allow you to transmit and receiver SSTV using your PC sound card to encode and decode high quality color pictures. Large numbers of hams build up simple interfaces to connect the radio to sound card in the PC.
The output of the sound card must be connected through an attenuator to the input of the transceiver. If you don't disable the microphone while transmitting SSTV pictures the microphone will pick up noise that puts black lines through your nice color picture. Most of the interfaces disable the microphone when transmitting a picture yet keeps the microphone ready for voice communications. The interfaces cover both HF SSB and VHF FM.
One of the most popular SSTV programs is MMSSTV written by JE3HHT - Makoto Mori. Its easy to use and configure. It has many good operating features such as overlay templates to allow you to customise your picture without actually changing the picture file. This means that you can drag a template onto a picture and send it with different information every time. Help files are available for download in many languages not just English from the MMSSTV website.
OK, now you have the PC and downloaded the Software from the MMSSTV website. The next thing you need is an interface to connect your sound card and you Radio or receiver. **If you are interested in having a go at making your own interface, scroll down the page and follow the email link and I'll be happy to help you out.**
You will need to adjust your clock frequency setting. MSSTV measures the SSTV signal timing with the sound card clock.
If the sound card clock is not correctly adjusted you will have a slanted image on transmit and receive.
Not all soundcards work the same (especially PCI soundcards) some have different frequencies for recording and playing. even if you have correctly adjusted the RX clock for receiving pictures in straight, you would still have a slanted image in the TX mode.
MMSSTV 1.11 has several ways to adjust the soundcard clock frequencies for TX and RX independently.
- High-accuracy slant adjustment
- Automatic slant adjustment
- Manual slant adjustment
High-accuracy slant adjustment and Automatic slant adjustment analyze the timing of the received signal to automatically adjust the slant. They, however, might not work well in the situations of QRM, multipath, or weak signals.
Receive Slant Correction
There are several ways to correct the receive slant adjustment, So that you may start receiving them pretty pictures in straight.
In normal situations, you would only have to adjust the slant by using the Sync window. If you want to calibrate your soundcard very accurately, listen to the standard radio or fax wave.1) Go Option, MMSSTV Setup, Misc page, and push Adj button.
You could use FAX broadcasting instead of WWV or JJY, but be sure it has exact timing. Refer to the MMTTY help for details.
This method seems to be used by a lot of folks. Get a friend that has his/her sstv program all calibrated and have him/her send a picture. At the end of his/her transmission. you will notice that the image may have came in slanted to one side.
In MMSSTV there are several ways to fix this.
One way is to correct it manually using the slant button under the sync tab. click on the slant button, then left click at the top of the image where the slants starts. drag the mouse down the image following the slant, once at the bottom left click again.
You will notice that the numbers have changed and the MEM button lit up click on the MEM button to record the new rx clock numbers into setup.
Have your friend send another picture to see if the program receives the image in straight. If not repeat the steps explain above until image finally comes in straight.
Second way is a lot easier and requires less work. Have your friend send a picture to you. after receiving it. click on the sync tab. you will see a smiley face next to the x4 button. simply click it and MSSTV will automatically adjust the slant for you.
The RX clock frequency numbers will change, click on the MEM button to record the new numbers into the setup. In most cases you will only have to do this only once depending on band conditions. have your friend send another picture to double check the new setting.
Failing to click on the MEM button to record the new RX clock frequency. will only apply the changes to the current received picture.
Auto Slant Adjust
Although you have the sampling frequency accurately calibrated, the station you are in contact with might not have it done correctly. To facilitate the slant adjustment, MMSSTV has an automatic slant adjustment function.
To activate this function:
Check "Auto slant" in the RX window of Setup MMSSTV under options up top of the main window or Right click the RX image window and check Auto slant adjustment.
It should be noted, however, that this function will not always give a satisfactory result when you have multipath or QRM, which causes jitters in the synchronization.
* Note - The automatic slant adjustment can be used only if the receive buffer is turned on.
* If you have a very slanted image with the default sampling frequency, try changing the source of the soundcard to Stereo (Left or Right) in the Misc page of the Setup window. Some soundcards have different sampling frequencies in Monaural and Stereo settings.
Adjustment of TX offset
Assuming that you have the RX clock calibrated, MMSSTV transmits the signal and receives it through the external loop back. After you have adjusted the slant (push the smiley button), push the TX button in the Sync window. MMSSTV automatically records the TX offset frequency.