Query: How many analogue transmitters were shut down on July 31, 2012?
Context: On July 17, 2012, the CRTC approved the shut down of CBC's analogue transmitters, noting that "while the Commission has the discretion to refuse to revoke broadcasting licences, even on application from a licensee, it cannot direct the CBC or any other broadcaster to continue to operate its stations and transmitters." On July 31, 2012, at around 11:59 p.m. in each time zone, the remaining 620 analogue transmitters were shut down, leaving the network with 27 digital television transmitters across the country, and some transmitters operated by some affiliated stations.