MT63
Robust digital mode for high-noise HF keyboard conversations.
MT63 is a digital radio modulation mode built for transmitting in noisy conditions. It was created by amateur radio operator Pawel Jalocha (call sign SP9VRC), mainly for real-time keyboard-to-keyboard conversations on the HF amateur radio bands.
The mode spreads each character's encoding across a long time period and multiple tones. This spreading of the code and symbol makes it highly resistant to poor signal conditions. MT63 also handles mistuning well; under normal circumstances, most software can cope with tuning offsets of up to 120 Hz.
Users can choose between a short or long interleaver. The long interleaver improves resistance to interference but adds more latency.
On shortwave, MT63 was used by VOA Radiogram until 2017, though that software encoded text differently from the original Varicode design. Modern programs that support MT63, such as Fldigi, use base128 encoding, which is essentially the same as ASCII. MT63 has also been proposed as a modulation format for time signal stations, but that application does not use Varicode.
- Developer
- Pawel Jalocha (SP9VRC)
- Primary use
- Keyboard-to-keyboard conversations on HF amateur radio bands
- Tolerance
- Handles 120 Hz tuning offsets under normal conditions
- Interleaver options
- Short or long interleaver
Lore & Background
MT63 distributes the encoding of each character over a long time period and over several tones. This code and symbol spreading implementation is key to its robustness under less than ideal conditions. The mode is very tolerant of mistuning; most software will handle 120 Hz tuning offsets under normal conditions. MT63 can use either a short or long interleaver. The long interleaver makes the mode more robust against interference, at the cost of increasing latency. MT63 was used on shortwave by the VOA Radiogram until 2017, but the software used to encode the text was not using the Varicode that MT63 used in its original design. Modern software that supports MT63, such as Fldigi, uses base128, essentially the same as ASCII. MT63 has been promoted as a modulation format for time signal stations, but this system does not use Varicode.
Reader's Guide
MT63's significance lies in its robust performance under high-noise conditions, achieved through spreading each character's encoding over time and multiple tones. This design makes it particularly suitable for HF amateur radio bands where interference and signal degradation are common. The mode's tolerance of tuning offsets up to 120 Hz further enhances its practicality for real-world operation. Its use by VOA Radiogram until 2017 demonstrated its viability for shortwave broadcasting, though that implementation deviated from the original Varicode encoding. The adoption of base128 (similar to ASCII) in modern software like Fldigi reflects an evolution toward broader compatibility. The proposal of MT63 for time signal stations indicates its potential beyond keyboard conversations, though that application also does not use Varicode. The availability of both short and long interleaver options allows operators to trade off latency for robustness as needed.
Did You Know?
- MT63 was developed by Pawel Jalocha, call sign SP9VRC.
- The mode distributes each character's encoding over a long time period and several tones.
- Most software handles 120 Hz tuning offsets under normal conditions.
- MT63 was used on shortwave by VOA Radiogram until 2017.
More in Radio Modulation Modes, Part 2 1-24
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