RTTY Terminal Unit Comparison

This page attempts to pull together various terminal unit design features, especially with respect to the demodulator. The Relative Performance column shows how many perfect lines were received with this impaired audio (audio from the KFS SDR on 20 meters receiving W6IWI in Tucson running 90 watts to an inverted L antenna). The impaired audio includes fades, selective fades, noise bursts, and interfering signals). Results may be affected by the teleprinter being used for testing. Some machines may do a better job of handling a poor signal. Therefore, tests between different terminal units are most reliable when comparing terminal units that were tested by the same person with the same teleprinter.

ModelPre-Limiter BPFLimiter/AGCTone BPFData LPFSlicerRelative
Black Cat Systems Multimode 65. Tested by w7gz
Dovetron MPC-1000CR No No limiter or AGC, but some limiting may occur by overdriving the input. The unit generally operates in AM mode. Three section BPF for each tone. Default bandwidth is 170 Hz for 75 Baud. Mark and space filters are both 750 Hz. Sections are stagger tuned with frequencies of 750 Hz, 812 Hz, and 700 Hz. Section Q values are 4, 5, 4.5 for the default 150 Baud. Input signals are heterodyned to this frequency. Four pole Bessle LPF 88. Bad lines were 01, 03, 04, 06, 09, 11, 16, 17, 28, 39, 46, and 92. Tested by KA4RSZ
82 with or without regeneration. Tested by KE3BK.
Flesher TU-170 NoLimiter6 pole, Q=25, BW=83Hz45 Hz two pole LPFFixed threshold Schmitt trigger with trip points at ±1.9V with peak input data of ±5V.64 Tested by W6IWI.
FLDIGI 32. Test by KE3BK
HAL ST-6 260 Hz (170 Hz shift). LC circuit using three 88 mH loading coils wired for 22 mH. Switchable limiter (FM) or no limiter (AM). Limiter is open loop LM709. 70 Hz (170 Hz shift). Single LC for each tone using 88 mH loading coils. 27 Hz (45 baud) Open loop LM709 (OA302) with no hysteresis. Includes "slideback" automatic threshold control (C307, C308, and surrounding components). 90. Bad lines are 03, 11, 16, 17, 30, 33, 40, 48, 49, and 51. Test by KA4RSZ
HAL ST-8000 Four selectable input filters. For 45.45bps 170 Hz shift, FC is 2210 Hz, Q is 4.7, and bandwidth is 470 Hz. Selectable limiter or AGC Selectable four pole R6621 switched capacitor filters. For 45.45bps 170 Hz shift, the mark filter Q is 29.4, and the mark filter bandwidth is 72 Hz. The space filter Q is 32.7, and its bandwidth is 70 Hz. Reticon R5613 switched capacitor low pass filter Mark only, Space only, or Mark/Space automatic threshold (midway between peak levels of appropriate signals). Another mode is Digital Multipath Correction. DMPC looks at the mark only and space only signals. When they are different (mark or space true, but not both), the mark or space is latched into an RS flipflop. 85 (AM detection), 84 (FM detection). Test by KE3BK
HAL ST-8000A 54. Test by KE3BK
MMTTY 78. Test by KE3BK
DSP TU 220615NoneLimiter (copysign)6 pole, Q=25, BW=83Hz. Three cascaded biquad BPFs. 45 Hz two pole biquad LPF for each tone after full wave rectification (absolute value)Zero hysteresis comparator. Mark if MarkLpf-SpaceLpf>0).67. Tested by W6IWI
Filters for RTTY. Extensive discussion of RTTY filters in RTTY-Journal. Suggests wide pre-limiter filter (4*Nyquist) Suggests variable threshold system will outperform system with limiter 3 pole Butterworth with BW=1.2 * bitrate (54.54 Hz for 45.45 bps) 3 pole Butterworth LPF with cutoff at 0.6 * bitrate (27.27 Hz for 45.45 bps) Variable threshold should outperform limiter