Wales will be the first nation in the UK to go completely digital
Swansea will become the first city in Wales to go totally digital when the analogue television signal is completely turned off later.
Everyone in Wales watching S4C on Freeview will also need to retune.
The first stage of the switch off began in Swansea last month when analogue users lost BBC Two Wales.
When completed around mid afternoon, areas such as Neath and Clydach will see the number of channels available free increase from five to around 20.
The rest of Wales will follow later this year and early next year, with every home going digital by March 2010.
Digital UK has said 90% of homes in Wales already have digital TV.
Digital TVs or analogue sets converted with a digital box will be able to receive BBC One, BBC Two, ITV1, S4C and Five, plus additional digital channels such as BBC Three, BBC Four, BBC News, ITV2, More 4 and digital text and radio services.
Emyr Byron Hughes, Digital UK's national manager for Wales, said: "From Wednesday many more viewers will be able to receive digital TV free via an aerial.
"People in Swansea responded really well in adapting to the change at stage one of switchover by preparing in good time.
"For those who haven't prepared for analogue to be permanently switched off on Wednesday needs to take last minute action."
A switchover roadshow is running in Swansea on Oxford Street this Wednesday while a helpline is available on 08456 505050.
The S4C Viewers' Hotline is available to deal with any enquiries on 0870 6004141.