Comcast is attempting to deliver more HD channels within their available bandwidth. To achieve that, they’ve recently started packing three HD channels into one 38.8 Mbps QAM channel instead of two, according to a post at the AVS Forum on Tuesday. The result, is a noticeable degradation of Comcast’s HD picture quality based on side-by-side comparisons with Verizon’s FiOS.

"Previously, Comcast allocated a maximum of two HD channels per 38.8 Mbps QAM, so each channel had the full 19.4 Mbps available if needed. Now, with the addition of new channels, Comcast is squeezing three HD channels into each 38.8Mbps QAM. Furthermore, some existing QAMs with two HD channels are being recompressed in preparation for new channel additions," the post from "bfdtv" said.

HD broadcast standards specify about 17-19 Mbps for a HD channel, so it’s possible to transmit two HD channels in a 38.8 Mbps bandwidth. As the forum post notes, additional compression over the ATSC standard would be required to transmit three.

"All of these channels are re-compressed at the Comcast Media Center (CMC) in Denver and uplinked to the AMC18 satellite for distribution to Comcast systems around the country. If your Comcast system has all (or most) of the channels listed above, then the added compression is likely in effect on your system," bfdtv wrote. The tripled channels were listed in the forum post and show, for example, Discovery, SciFi and USA all combined into one QAM channel.

The author conducted some download tests and compared the bit rates against Verizon’s FiOS network which uses optical fiber and does not use Comcast’s technique. For example, he found, for Discovery HD Theater: 17.45 Mbps for Verizon but 12.60 Mbps for Comcast. Captured images, displayed in the forum post, show noticeable degradation of the Comcast images compared to Verizon’s. Another Comcast competitor, uses a more efficient compression algorithm, MPEG-4, while Comcast still uses MPEG-2.

One viewer, snatta, who posted a comment confirmed that there was an apparent degradation of the Comcast HD channels over the weekend.

Comcast’s national public relations in Philadelphia, Penn. was contacted by iPO about this on Tuesday, but as of this writing has not yet responded to a request for comment.