Lesson 24

Digital Communication Modes

Why use digital communication modes?  A significant benefit is their efficient use of bandwidth.   Let’s do a comparison of some digital modes. 

Bandwidth by mode

• The approximate bandwidth of a 13-WPM international Morse code transmission is 52 Hz. Bandwidth of a transmitted CW signal can be impacted by keying speed and shape factor (rise and fall time).
• The bandwidth of a 4800 Hz frequency shift, 9600-baud ASCII FM transmission is 15.36 kHz.
• PSK and FT8 are both very efficient and use little bandwidth. That makes them great for long distance and weak signal communications. PSK31 uses about 60 Hz. The bandwidth of an FT8 signal comes in at 50 Hz.

Digital modes have some items in common:

• Data rate can be increased without increasing bandwidth by using a more efficient digital code.
• They can implement the use of gray code. That is digital code that allows only one bit to change between sequential code values. This technique is great for catching errors, because if one more than bit changes, the data must be incorrect.
• Digital transmissions can be quantified with a symbol rate. Symbol rate is the rate at which the waveform changes to convey information. Know that when discussing symbol rate and baud, they are the same.

FT8, CW and PSK31 are all very efficient. How does a PSK31 signal gain its efficiency? A trick used in a PSK31 signal to minimize the bandwidth is the use of sinusoidal data pulses. Another efficiency is to change the phase of a PSK signal at the zero crossing of the RF signal. This allows it to minimize bandwidth.

Error Correction

There’s always a risk for errors in our communications. If you are receiving Morse Code a quick “T” sounds like an “E” to a different ear. During a noisy sideband contact you may need a call sign repeated. The use of digital communication provides the same risk of data errors. Digital modes have built in features to help minimize errors. Let’s go through a few methods.

One principle is called Forward Error Correction. With FEC you transmit a little extra data like a checksum. That can be used to detect and correct transmission errors. 

That FEC checksum can be used to trigger an ARQ or Automatic Repeat Request. With an ARQ the receiving station decodes the check. If errors are detected, a retransmission is requested from the sender. 

Some of the older digital modes used Baudot (pronounced Bo Do) code to send data to teletypes. Baudot was later replaced by the ASCII standard still in use today.  

Baudot code chart

Here are some of the differences between the Baudot digital code and ASCII code. 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.

ASCII code chart

It’s easy to see one reason ASCII has been adopted for data communications over Baudot. In ASCII it is possible to transmit both upper and lower case text. ASCII includes built-in error correction too.   Thanks to parity bits in ASCII characters some types of errors can be detected.

What about issues with other modes? Let’s start with CW. One issue occurs when there is an extremely short rise or fall time on a Morse code signal. The primary impact is the generation of key clicks. The most common method of reducing key clicks is to increase keying waveform rise and fall times.

Now a look at PSK31 and other AFSK signals. Overmodulation can occur, usually due to excessive transmit audio levels. Hams call that splattering. You can use the Intermodulation Distortion (IMD) parameter to evaluate this in AFSK signals. An acceptable maximum IMD level for an idling PSK signal is -30 dB.