By Aidan Malley

Published: 06:25 PM EST

Those expecting Blu-ray level video from Apple's new HD TV shows may be significantly disappointed -- though buyers will find a surprise in the form of a smaller version for their iPhones and iPods.

While Apple's announcement on Tuesday that it would start offering TV in 720p high-definition directly through iTunes has been hailed as a significant move towards better-quality video through the company's online store, Gizmodo today reminded would-be viewers not to expect a direct replacement for Blu-ray discs or even some purely digital formats.

A standard 720p file downloaded either through iTunes or an Apple TV consumes about 4Mbps of data, or just a tenth the total bit transfer rate of the optical format and a fifth of the nearly 20Mbps for over-the-air HDTV; even Xbox Video Marketplace video affords more, at 6.8Mbps. Some of this shrink in file size can be attributed to features left out of Apple's encoding, such as the 1080p resolution or 7.1-channel surround audio, but much of it is attributed to compression that can degrade the final picture quality significantly from the reference image.

Such reductions can often lead to smearing in videos, particularly in fast-moving scenes where the bandwidth allowed to the video isn't enough to keep up with the changes necessary for the picture.

The bitrate isn't an automatic gauge of image quality, however. As discussed in AppleInsider's Apple TV quality comparisons, video format choices and other factors can actually result in supposedly higher-bitrate cable TV sources faring worse than Apple's H.264 videos, which are more efficient at compressing data than the MPEG-2 format used by some TV providers and older Blu-ray titles.

Apple also isn't seen as having much choice in the matter due to the nature of Internet technology. An HD TV show on iTunes can be nearly three times the size of its standard-definition equivalent and downloads slowly enough on most American connections that it may be difficult to start watching in real-time, with an example show taking 40 minutes -- or nearly its full duration -- to finish downloading. Higher-quality video would both be impractical for some connections and quickly fill up hard drives.

As a compromise, Apple is known to be offering portability, something that most Blu-ray or cable video-on-demand services can't offer themselves. Ars Technica notes that Apple has quietly made sure that all HD downloads from iTunes also include an SD version as part of the purchase.

The lower resolution chews more disk space but also guarantees that buyers will have a version of their TV show choice already formatted for iPhones and video-capable iPods; none of these currently have the performance, the displays, or the capacity to practically support HD playback.

Bundling a second copy of a title also puts Apple slightly ahead of Blu-ray in HD TV bundles: while a handful of Blu-ray and DVD movies now offer an iTunes Digital Copy that achieves a similar effect to Apple's new venture, most TV series collections still largely include just the original video and can be difficult or impossible to copy.