E8D – Keying defects and overmodulation of digital signals; digital codes; spread spectrum
It is good amateur practice to ensure that the CW and digital signals you transmit are high quality. Perhaps the biggest problem that you’ll have when sending CW signals is key clicks. Key clicks are spurious signals that cause interference to other stations operating near your frequency. The generation of key clicks is the primary effect of extremely short rise or fall time on a CW signal. (E8D04) It follows, then that the most common method of reducing key clicks is to increase keying waveform rise and fall times. (E8D05) Fortunately, most modern transceivers allow you to set the rise and fall times of the CW signal, so this is an easy fix.
To ensure high-quality digital signals, such as when transmitting audio frequency shift signals, such as PSK31, you need to set the audio input level properly. A common cause of overmodulation of AFSK signals is excessive transmit audio levels. (E8D07). Strong ALC action indicates likely overmodulation of an AFSK signal such as PSK or MFSK. (E8D06)
Intermodulation Distortion (IMD) is a parameter that you can measure that might indicate that excessively high input levels are causing distortion in an AFSK signal. (E8D08) A good minimum IMD level for an idling PSK signal is -30 dB. (E8D09)
Digital codes
Although ASCII and Unicode have now become standard codes for sending textual information, we still use the Baudot code when sending and receiving RTTY. Some of the differences between the Baudot digital code and ASCII are that Baudot uses 5 data bits per character, ASCII uses 7 or 8; Baudot uses 2 characters as letters/figures shift codes, ASCII has no letters/figures shift code. (E8D10)
Even though it uses more bits per character, ASCII does have some advantages over Baudot. For example, one advantage of using ASCII code for data communications is that it is possible to transmit both upper and lower case text. (E8D11)
In an eight-bit ASCII character, the eighth bit is the partity bit. In systems that use even parity, the parity bit is set to either a one or a zero, so that the number of ones in the character is equal to an even number. In systems that use odd parity, the parity bit is set to either a one or a zero, so that the number of ones in the character is equal to an odd number. The advantage of including a parity bit with an ASCII character stream is that some types of errors can be detected. (E8D12)
Spread spectrum
Amateurs can now use spread-spectrum techniques on all bands above 420 MHz. The reason these bands are used is because spread-spectrum signals require more bandwidth than is available on the lower frequency bands.
Spread spectrum transmissions generally change frequency during a transmission. This is called frequency hopping. The way the spread spectrum technique of frequency hopping works is that the frequency of the transmitted signal is changed very rapidly according to a particular sequence also used by the receiving station. (E8D03) Direct sequence is a spread spectrum communications technique that uses a high speed binary bit stream to shift the phase of an RF carrier. (E8D02)
Because transmission and reception occur over a wide band of frequencies, spread spectrum communications are less susceptable to interference on a single frequency than are more conventional systems. Received spread spectrum signals are resistant to interference because signals not using the spread spectrum algorithm are suppressed in the receiver. (E8D01)
Dave New, N8SBE says
“Fortunately, most modern transceivers allow you to set the rise and fall times of the CW signal, so this is an easy fix.”
Actually I don’t recall hearing of any transceiver that has this feature. It might be hidden in some factory-only settings, or an operator might be able to change it by substituting some components in the keying circuit.
Do you have an example of a rig that offers this setting via a menu?
Dan KB6NU says
My old IC-746PRO lets you set the rise time to 2, 4, 6, or 8 ms. It’s one of the CW menu selections.