RFT Smaragd BG 20/4

East German (GDR) valve reel-to-reel, a mid-series revision of the 'Smaragd' BG20 line (VEB Messgeratewerk Zwonitz / RFT), late 1950s-early 1960s. Series configuration: half-track (Doppelspur) mono, valve electronics, single-motor drive with a large flywheel, permalloy heads, IEC equalisation, reels up to 18 cm; leatherette-covered plywood case with Bakelite cover (also sold as a built-in chassis). Speeds 9.53 and 19.05 cm/s. Note: figures published specifically for the /4 variant are scarce. Across neighbouring variants frequency response ranged from about 60 Hz-12 kHz up to roughly 40 Hz-18 kHz depending on speed, with signal-to-noise near 45 dB - these are series-level references, not confirmed /4 values, so precise numbers are not assigned to this exact model.