Five and C4/S4C have submitted bids to Ofcom for the fourth HD channel on Multiplex B, which the regulator will weigh up before announcing its decision next month.
Three HD Freeview channels have already been given the go ahead: one from the BBC, one from Channel 3 licensees ITV, STV and UTV, and one from C4 and S4C.
Ofcom invited proposals for the remaining fourth slot from PSB broadcasters in December last year. The chosen channel will launch on Freeview at the end of 2010.
If C4 and S4C win a second HD licence, Five would be the only UK public service broadcaster without an HD offering on Freeview by the end of next year. Ofcom will establish a fifth HD slot by 2012, the content of which would be determined by the BBC as multiplex owner.
C4's proposed channel would start at 7am with the UK-wide broadcast of four hours of S4C children's shows, before switching to Film4 from 11am - two hours earlier than the film channel currently broadcasts in standard definition. Film 4 would then air to 4am.
From 4am to 7am, C4 would deliver a range of the most popular HD programmes from its own channels and third party acquisitions to PVRs.
In C4's first year of HD transmission on Sky ,it broadcast more than 150 films in HD, nearly 60% of them in peak slots.
The slots for children's programming are designed to prepare C4 for its expected new responsibilities as part of the government's proposed reshaping of C4 as a PSB rival to the BBC. As a means of securing UK-wide distribution for S4C children's content on DTT, it would also ensure an outlet for Welsh-language programming.
C4 said the remaining three hours of programming would “strengthen the competitiveness of the Freeview HD platform by including a video on demand element that makes use of the likely features of enhanced, HD-enabled receiving equipment.”
It said the proposal “would contribute to the purposes and characteristics of public service content by stimulating public interest in the arts, strengthening awareness of cultural identities and driving awareness of different cultures and viewpoints as well as delivering significant quantities of original, British made content.”
Five's HD channel would mirror the parent's schedule from 5pm to 1am, which accounts for around 70% of all viewing of the existing channel. Initially, 43% of programmes would be in HD and Five would gradually increase this to establish a fully HD schedule by 2018.
From 1am to 6pm, other services such as push video-on-demand from Five and other broadcasters would be offered, while another broadcaster's HD service could be supplied from 6am to 5pm.