Welsh-language channel S4C says it will lose "significant" amounts of revenue when it stops broadcasting English language programmes in 2009.
The drama series Con Passionante has been an S4C success
In its annual report, S4C chairman John Walter Jones said the change as a result of the switch to digital would pose challenges for the channel.
But the report also noted an increase of 11% in peak hour viewing figures for Welsh-language programmes.
S4C shows series such as Lost and Big Brother which also go out on Channel 4.
When the switchover to digital happens, S4C will broadcast only in Welsh, which Mr Jones said would have an impact on advertising revenue.
At present, S4C transmits on three channels. Its analogue service, under broadcasting law, has to show a certain amount of English-language output from Channel 4, as viewers in many parts of Wales cannot receive the station.
S4C Digidol is the main digital channel which broadcasts solely in Welsh and will be the station's principal channel when analogue broadcasting ends.
It also runs S4C2 to broadcast from the Welsh assembly and various public service programmes, including from some of Wales' main festivals and events.
Where the money will come from to bridge the gap in advertising revenue is unclear, but Mr Jones said he was confident that negotiations with the Department of Culture, Media and Sport (DCMS) over the next few years would ensure a "quality service".
He said he would not be going with "a begging bowl" to the department but that ministers were aware of the challenges facing S4C, adding: "Unlike other broadcasters, S4C has not benefited to date from any kind of digital payoff from the DCMS.
Rhys Ifans in Dal Yma Nawr
"We need to work out our plans before we go to them."
Mr Jones told BBC Wales: "It' not just changes in technology terms, it's a change in content terms. Whereas we have had the wraparound of Channel 4 programmes, now that will disappear by and large."
"We accept there will be a decrease in income generated by advertising.
"If we can get the viewing figures up as we have been doing, there's no reason why advertising revenue couldn't hold up after 2009."
The report by the S4C Authority points to an increase of 11% in its peak-hour viewing figures for 2005, achieved under the leadership of new chief executive Iona Jones.
The most popular programme on the channel continues to be the long-running soap Pobol y Cwm, which had a peak audience of 165,000.
Rugby is the second most popular form of programming - the Lions v Argentina drew an audience of 137,000.
However the report also shows that overall, viewing figures are down.
Mr Jones said: "In responding to the expectations of the audience and to huge changes within the broadcasting industry in general, creative excellence will be the aim for every member of staff as well as those who provide the channel with content."