Set top boxes, IDTVs and PVRs are now becoming available at retail for consumers to receive the Freeview HD service, which went technically live last December.
BBC HD and ITV1 HD are already available on the platform, and Howling confirmed that Channel 4 HD will join the lineup "before Easter".
In a speech today at the DTG Summit, Howling said that the £6m marketing campaign will launch in Easter and run to the start of the World Cup under the slogan, 'Buy today, watch today, free forever'.
As previously announced, Freeview is now on the main TV set in 10m homes in the UK, along with 18.2m sets in total.
Recent research by the firm indicated that 18% of its existing customers will be "likely to" switch their main set to the HD service within the next six months.
More interesting, though, is that 17% of Sky+ HD subscribers also expressed their willingness to switch to the subscription-free platform.
Howling said that sales of Freeview+ equipment have now reached 2.1m just two years after the timeshift service launched, with three quarters of Freeview+ boxes sitting on the main set.
"From analogue to digital, from linear to timeshift and in just a few weeks' time, from standard definition to high definition - this is what Freeview is all about," said Howling. "For millions of homes across the country, the time for high definition is now."
A Freeview HD receiver manufactured by Humax is already available at retail, priced at £179, while a Sony Bravia Freeview HD model also recently went on sale for around £480.
Howling said that Freeview+ HD products should come onto the market soon, priced at between £250 and £270, with all HD products incorporating an ethernet connection to "enable people to explore online TV and catchup alongside HD TV".
Welcoming the potential arrival of a Project Canvas platform, Howling said that up to 3m homes with Freeview on their main set would be "genuinely interested" in the service.
Howling further stated that the ability to offer video on-demand and internet services on top of the existing Freeview channels is a "natural way forward" for the digital TV platform.
However, she added that the large number of Freeview consumers intending to upgrade to the HD service will be happy with that in the medium-term.
Also at the session, the DTG's David Cutts said that the launch of BBC iPlayer on Freeview is "subject to stable receiver availability", but he can "pretty much promise" that it will happen.