Nielsen has unveiled an overhaul of its ratings measurement system aimed at countering criticism over its methods.

The data behemoth will begin supplementing homes that use either diaries or meters with a technology dubbed “code readers” in 20 markets beginning in the fourth quarter of the year. The code readers are VHS cassette-sized boxes that listen for audio watermarks embedded in TV programming.

In addition, the sample size will double in 12 of those markets that use diaries, while the sample size for the other eight using either set meters or local people meters will quadruple.

Data derived from the revamped efforts won’t be available until 2013. The changes won’t take effect in Nielsen’s other 190 markets until two years after the first wave of changes are evaluated.

Altogether, it’s the most comprehensive revamp of Nielsen’s oft-maligned measurement system, which has been plagued with technical hiccups resulting in delayed or missing data. That said, existing methods are not being phased out including the diary, which could be subject to future changes that would make maintaining it less onerous.

Also sticking around for further tweaking are set-top boxes, which, while rich in data, aren’t providing viewing measurements across all top pay TV providers.

Nielsen is hoping its hybrid measurement tools will also the foundation for cross-platform tracking — a big priority for the company as video consumption explodes on wireless devices.