System A
405-line is System A in the CCIR assignment of broadcast systems. The audio uses Amplitude Modulation rather than the Frequency Modulation in use on modern analogue systems. In addition, the system was broadcast in an aspect ratio of 5:4 until 3 April 1950 when it changed to the more common 4:3 format.
All System A transmitters used vestigial sideband transmission, with the single exception of Alexandra Palace in London, which closed down in 1957 when it was replaced by Crystal Palace.
| System | Lines | Frame rate | Channel bandwidth (in MHz) | Visual bandwidth (in MHz) | Sound offset | Vestigial sideband | Vision mod. | Sound mod | Aspect ratio | Effective resolution (4:3). |
|---|---|---|---|---|---|---|---|---|---|---|
| System A | 405 | 25 | 5 | 3 | -3.5 | 0.75 | Pos. | AM | 4:3 (5:4 before 1950) | 503 x 377 (theoretical) |
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