How many launch dates are there for digital transmission?
CREATE TABLE "free_to_air_television" (
  "ch" text,
  "channel" text,
  "channel_content" text,
  "transmission" text,
  "format" text,
  "launch_date" text,
  "licence" text
);
SELECT COUNT("launch_date") FROM "free_to_air_television" WHERE "transmission"='Digital';