The Gateway project explored several challenges, both technical and operational. These are discussed on this page.
All gateways detect the presence of signals on their input ports to determine the direction of audio flow to their output ports. At present, all of the amateur radio VoIP gateways already operating in the UK employ Frequency Modulation on the RF port. There is thus a clear method of switching the direction of the gateway, triggered by the presence of, or lack of, a control voltage that is derived from the squelch circuit of the radio receiver.
When operating a Single Sideband RF port on HF, the situation is more complex. Unwanted artefacts, such as data transmissions, ‘tuning up’ carriers, off-frequency speech, man-made noises and lightning static can all contribute to holding the gateway in a single direction. Fortunately, most modern HF transceivers provide a squelch status voltage (as with the IC775DSP proposed for this experiment) and it has been found that careful setting of this squelch and receiver AGC will satisfactorily mute the audio on background noise compared to the wanted signal of a participating station.
In the early stages of experimentation, the basic noise squelch would necessarily be the method of gateway switching and therefore the benefits of relatively clear frequencies (as mentioned above) can be appreciated. However, practical circuits have been identified that use speech-pattern recognition to greatly enhance the operation of HF squelch. Some will be familiar with the Icom VSC facility that exists on many of its products, and both Thales UK and Naval Electronics Inc. produce commercially available units. There is also a very affordable amateur construction project offered by AH6LE www.ah6le.net for exactly this purpose. Such circuits, effectively FM discriminators, ignore non-human audio signals.
An early introduction of such a circuit to this project was considered important. It may also be that some exploration of CTCSS or audio tone access systems could be conducted.
Although currently in its infancy, the future use of WinDRM or similar digital HF speech modes may again make the use of a simple noise squelch all that is necessary.
See the INTELLIGENT SQUELCH page on this website for more.
Modern transceivers offer the operator a very large number of parameters, many of which can be adjusted to optimise the transmitted and received signals. In terms of audio treatment, the settings of these parameters are likely to be different when the transceiver is being used for gateway operation as compared to direct microphone-to-air, particularly as signals are passing between the radio equipment and a computer.
Furthermore, there is a sub-layer of control signals (PTT and squelch status lines at least) which are also subject to the setting of various parameters (switching sense, delay aspects, hang-time).
On the Internet control side, the software similarly requires the careful setting of many compatible factors.
To be effective in longer-range emergency communications, an HF gateway will need to occasionally employ reasonable output power levels.
The proposed 21dB(W) here is an attempt to set a realistic upper limit for practical deployment although it is envisaged that much of the testing could be carried out at lower levels.
However, the use of higher power levels allows for understanding the issues that arise when significant radio frequency energy is being generated close to domestic (that is, unbalanced) audio installations.
It is important to be able to recognise the symptoms of RF induction and to understand techniques for minimising such problems, which can lead to substandard transmissions and audio distortion.
Communication via a radio transceiver and Internet-based speech circuits tend to employ different audio treatment.
When communicating via radio, clarity and ‘punch’ are important whereas when speaking from computer to computer a more natural (and fuller bandwidth) quality is sought. When these two systems are interconnected, treatment is necessary in order to optimise audio quality when bringing one domain into the other.
The proposal is to produce guidelines on level, equalisation and compression settings, as well as hints on good operating practice and microphone technique. In terms of the physical interface between computer and transceiver, the issues to explore include safety isolation, signal levels, noise and distortion.
There is no doubt that the operation of an HF gateway requires skills on behalf of the licensee/operator that can only be acquired through practice and having the opportunity to experience the problems that arise in real situations.
Many amateurs involved with talk-through operations for emergency communications will know how demanding this role can be. The gateway operator must be able to listen and talk to both sides of the gateway either singly or simultaneously, understanding that there are potentially two nets of operators that remain unaware of each other unless the gateway is invoked.
The operator must have the appropriate switching facilities to instantly control the gateway manually if necessary, particularly in the case of interference.
Once the gateway is operating, great skill in identification and control is required in order to avoid confusion and it is assumed that all gateway operations in this initial phase of experimentation would be conducted under a strictly controlled net.