Sandbox:TWINPLEX

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TWINPLEX, also known as F7B4 and TWINPLEX-SITOR, was a diplex 4-FSK ARQ mode developed by Mackay Radio and Telegraph Company, based in New York City, in the early 1950's. This mode was used by Interpol, the United Nations, and many government diplomatic services all over the world, including Australia, Denmark, The Netherlands, Norway, Pakistan and Spain.

TWINPLEX was developed to try to double the throughput of a single telegraph channel while still staying within a single channel's bandwidth. Two separate channels can be transmitted simultaneously using TWINPLEX; the two channels can even be at different data rates.'

Characteristics

TWINPLEX employs two channels that are transmitted via the 4 FSK methods. Each frequency used represents two symbols, each symbol representing either a mark or space for each of the two channels transmitted. TWINPLEX typically runs at 100 Bd, but rarely at 200 or 300 bd.

The bandwidth of TWINPLEX varies, from 450 Hz to 950 Hz depending on setup.

This is an example of a TWINPLEX signal with a setup of 115-170-115:

Each TWINPLEX transmission can be determined by the three shifts between the 4 frequencies used in the A-B-C format.

TWINPLEX stations only key the two inner frequencies, f2 and f3, during the IDLE state (no traffic) or during the RQ state. In these states, SITOR and TWINPLEX systems cannot be distinguished from each other.

Variants

The Mackay Radio and Telegraph Company also developed two derivative modes from TWINPLEX, dubbed TWINMODE and POLYPLEX.

TWINMODE

TWINMODE is the transmission of two TWINPLEX signals simultaneously. This allows for 4 simultaneous channels to be transmitted. The original TWINPLEX channels are transmitted as usual, but the additional two channels are AM Modulated in subcarriers flanking the original TWINPLEX signal at a frequency shift that allows ordinary TWINPLEX receivers to be able to decode, maintaining compatibility.

POLYPLEX

POLYPLEX is a further jump from TWINPLEX where up to 8 simultaneous channels are supported. It employs Twinplex equipment as basic building blocks, adding units for shifting a subcarrier at approximately 200 kHz by each Twinplex Combiner, mixing these together with a 1.8 Mhz frequency, selecting the upper side bands centered at 2 MHz, selecting the lower side bands centered at the operating frequency and amplifying it in SSB operation. Receivers are standard TWINPLEX receivers with their tuning adjusted to the offset of the desired pair of channels within the 1600 Hz - 3400 Hz pass band of their input filter. In this way, each telegraph channel has complete independence from all other channels.

Setups for TWINPLEX

The standard setups for TWINPLEX are:

• 100-100-100 Hz
• 200-400-200 Hz
• 170-170-170 Hz
• 115-170-115 Hz
• 200-200-200 Hz
• 115-170-515 Hz (Asymmetrical)
• 65-170-65 Hz

Samples

200-400-200 Denmark Diplomatic	115-170-115 Spain Diplomatic	115-170-515 Pakistani Diplomatic
<html5media>https://www.sigidwiki.com/images/2/2c/Danish_-TWPLEX.mp3</html5media>	<html5media>https://www.sigidwiki.com/images/1/1d/Spanish-TWINPLEX.mp3</html5media>	<html5media>https://www.sigidwiki.com/images/1/14/Pakistani-TWPLEX.mp3</html5media>

Additional Samples

Asymmetrical (all 100 Bd):
<html5media>https://www.sigidwiki.com/images/c/c7/TWINPLEX_Asymmetrical_100_Baud_greek.mp3</html5media>
<html5media>https://www.sigidwiki.com/images/b/b4/Twinplex%28115-170-515%29_asym.mp3</html5media>
<html5media>https://www.sigidwiki.com/images/a/a2/Twinplex_asymm_%28dat_11_02%29.mp3</html5media>

Decoding Software

Professional Equipment/Software

• Hoka Code 300-32
• Shoc go2decode
• Shoc go2monitor
• Wavecom W-Code
• Wavecom W-Spectra

Additional Links

• RadioReference.com TWINPLEX
• Digital Signals
• TWINPLEX: Electrical Communication, March 1952
• shoc Database: TWINPLEX
• WAVECOM Database: TWINPLEX
• IEEE Twinplex, Twinmode, and Polyplex Radiotelegraph Systems