What channels first aired on 31 December 1990 in the Southern Downs region?
CREATE TABLE "main_transmitters" (
  "region_served" text,
  "city" text,
  "channels_analog_digital" text,
  "first_air_date" text,
  "erp_analog_digital" text,
  "haat_analog_digital_1" text,
  "transmitter_location" text
);
SELECT "channels_analog_digital" FROM "main_transmitters" WHERE "first_air_date"='31 december 1990' AND "region_served"='southern downs';