Why is HD in a square box?

This morning I wrote an email to the local network affiliates, Comcast and to SharpUSA to find out why some of the programming on the Comcast HD channel doesn't fill the screen and shows up in a square box.

My letter:

Hi,
I was wondering why some HD programs on Comcast are full screen HD, while others are in a square box.

Do only shows that are shot in HD fill the screen?

I've got a Comcast HD DVR which is the Motorola DVR: Dual-Tuner DVR High-Definition Set-Top DCT6412 and a new Sharp LC37D40U connected via an HDMI cable with the stretch option turned on.

Thanks,
--Steve

Responses have started to come in.

Jack Barry from CBS4 writes:

Dear Steve,

You are correct. Only those shows that are true HD will fill a 16x9
screen. Non-HD shows will appear as a square box.

Jack Barry
Director of Broadcast Operations & Engineering
CBS4; UPN28; UPN38
1170 Soldiers Field Road
Boston, MA 02134
(617) 787-7100
www.cbs4Boston.com
www.UPN28.com
www.UPN38.com

Jim Shultis from WHDH writes:

Steve,

I received your question about Aspect Ratio on our HD feed. Your guess was correct; only material originated in the HD format fills the entire screen. So, any time the screen is filled, that is true HD (usually from NBC). When we up convert our standard definition material to HD (1080i) we have chosen to leave it in 4 X 3, and let the viewer expand it in their set if they would like.

best regards,
Jim Shultis
Director of Engineering