Overview
IEC 60774-4:2002 specifies the S‑VHS ET (Extended Technology) mode for helical-scan video tape cassette systems using 12.65 mm (0.5 in) magnetic tape on type VHS. The ET mode records an S‑VHS signal format on a standard VHS cassette by switching key recording parameters so that the recorded signal characteristics are equivalent to S‑VHS on S‑VHS tape. Cassettes recorded in S‑VHS ET mode are playable on S‑VHS playback systems and on VHS recorders equipped with the SQPB (S‑VHS quasi-playback) function.
This part of IEC 60774 is intended for equipment designers, test engineers and manufacturers implementing compatibility between S‑VHS recording quality and VHS media.
Key Topics
- Signal compatibility: ET mode adjusts recording parameters so S‑VHS signals are reproduced correctly from VHS cassettes.
- Applicable systems: Supports both 525 line‑60 field and 625 line‑50 field recordings where specified; recording systems must comply with both VHS and S‑VHS standards.
- Pre‑emphasis and compensation filters: The standard provides compensation filter responses and pre‑emphasis frequency responses for the two signal families. Example compensation values include:
- 525 line/60 field (compensation filter response): 1.0 MHz: –0.2 ±0.3 dB; 2.0 MHz: –0.8 ±0.3 dB; 5.0 MHz: –3.4 ±0.3 dB.
- 625 line/50 field (compensation filter response): 1.0 MHz: –0.4 ±0.3 dB; 2.0 MHz: –1.3 ±0.3 dB; 5.0 MHz: –5.1 ±0.3 dB.
- Clipping levels: White clipping level is specified as 190 % ±10 % (S‑VHS spec notes 210 % ±10 %); dark clipping remains at –70 % ±10 % relative to sync tip.
- FM recording head current: Optimum recording current for the FM carrier band shall be set per cassette tape to achieve maximum playback output.
- Converted chrominance: Converted chrominance spurious component (fy ± 2fc) is specified at –20 dB to –25 dB relative to the fy playback level (fy = luminance centre frequency).
- Cassette discrimination: ET mode is selectable only when a VHS‑standard cassette is detected; playback mode detection uses the FM carrier frequency.
Applications
- Ensuring backward-compatible S‑VHS quality recording on consumer VHS cassettes.
- Guiding R&D and production for VCR manufacturers implementing ET mode and SQPB functions.
- Test and conformance verification of tape recording and playback parameters (pre‑emphasis, clipping, FM levels).
Related Standards
- IEC 60774-3:1993 - Helical‑scan video tape cassette system: S‑VHS (reference for terms and baseline S‑VHS parameters).
- IEC 61041-1:1990 - Non‑broadcast VTR measurement methods (referenced for red lustre signal definitions).
For implementers, IEC 60774-4:2002 must be used together with the general VHS and S‑VHS parts to ensure complete compliance and interoperability.