user: Question: How many launch dates are there for Digital transmission?
Columns: ['Ch №', 'Channel', 'Channel content', 'Transmission', 'Format', 'Launch Date', 'Licence']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['1 (A); 81 (D)', 'TVB Jade', "TVB's main Chinese ( Cantonese ) channel.", 'Analog & Digital', 'SDTV', '19 November 1967', 'TVB']
SQL:

assistant: SELECT COUNT("Launch Date") FROM "Table" WHERE "Transmission" = 'Digital';