There are lots of ways of making contacts on Amateur Radio.
You can obviously use voice
You can also use digital modes. These convert your typed message into audio, which is then sent out over the air. The other end detects the audio, and converts it back into text, in much the same way that modems do.
However, the difference between modems using phone lines, and digimodes using HF is that HF is much less reliable.
Some problems include:
- Signal fading (QSB)
- Interference from other stations (QRM)
- Interference from natural sources, such as lightning (QRN)
- Interference from man-made sources, such as electronics
This means that a good digimode has to have error correction in it, which usually takes the form of Forward
Error Correction. This is where extra data is sent, to allow the receiving end to reconstitute parts of the
message stream that were lost. Forward Error Correction avoids the need for requests for expensive and time
consuming resending of parts or all of the message.
The point of amateur radio is to make contacts. A contact isn't always a 15 minute natter. It's exchanging
information correctly and without errors.
So a good digimode in my mind has to have robust error correction.
Obviously, the more Forward Error Correction applied, the larger the message sent, and the longer it will take. However, I prefer a mode that sends slowly, but is correct over one that is fast, but has lots of uncorrected errors. Olivia is one of these modes.
One of my favourite modes is Olivia. It is a multi frequency-shift keying
It has various versions of the mode, which use differing number of tones, and differing bandwidth. These variations give a wide range of speeds and signal sensitivity.
I have compiled the following table which details the length of time in seconds to send a specific piece of text, and is ordered by the fastest Olivia mode first.
I can't quite see what the relationship between the tones, bandwidth and speed is, which is why I thought I'd publish this.
Here are the same modes, but sorted by maximum sensitivity (found on various pages
|Name||Tones||Bandwidth||Max S/N (dB)|
If anyone knows what the formula is for calculating the rest, get in touch.
Because Olivia can be successfully decoded down to the level where the signal is hard to hear with the ear or see
waterfall, the usual method of scanning around the frequencies searching for signals doesn't work as well.
For this reason, standard frequencies for Olivia have been adopted. Here is a table of the frequencies I am aware of.
Please note - these are centre frequencies. The centre frequency is your dial frequency, plus your audio offset. I don't understand why people give the frequency as dial frequency + audio offset.
|5368.0||Be careful on 60, as different countries have different allocations|
Even if you can't hear anything on the calling frequencies, leave your radio decooding on them. There might be a signal there that you can't hear.