FAQ: 720p versus 1080i

[Richard]

As a Signal Source

DTV Broadcast: Currently 1080p is not used for broadcasting although 1080p24fps and 1080p30fps is supported by the system. At this time HD content uses either 720p or 1080i. Most stations are setup for 1080i rather than 720p.

DTV provides multi-channel capability and that means where the old system could only provide one standard definition channel the DTV system can provide up to 6 standard definition content channels on the same frequency appearing as dash numbers. For example, 16-1, 16-2, 16-3, ETC. To get the full benefit of an HD broadcast requires the full available bandwidth of the carrier frequency and that means local HD stations should not use multichannel capability. Ultimately they do and for a variety of reasons this may have little to no effect on the HD channel of a DTV signal.

1080i will have problems in a multi-channel application because it always uses the full bandwidth. That said static displays such as a weather channel have little effect while an additional channel of SD video content would have an impact. 720p on the other hand is a different story. If the 720p content is of the sports variety more then likely it will be 60 frame 720p in which case like 1080i it will need the full bandwidth as well. The catch is that non-sports programming is not 60 frame, it is 24 or 30 frame and under such circumstances requires only half of the bandwidth making quality HD content co-existing with SD content in a multi-channel application practical.

1080i is more popular than 720p for local DTV broadcasters. Nearly all cable and satellite premium HD channels are 1080i. ESPN uses 720p.

ABC and Fox distribute HD content in 720p and CBS and NBC distribute HD content in 1080i. Local affiliates can rebroadcast in which ever HD format they choose and are not required to duplicate the networks official choice although it is more common for them to do so.

As a Setting From an HD Source Product

For the most part with todays products the 1080i output setting of source equipment is used primarily for convenient interfacing with older displays that do not support 1080p as a signal or never were capable of it such as legacy CRT HDTV product. Nearly all DTV tuners support only the 1080i setting.

As an HD Standard

720p uses the coveted progressive signal and 60 frame format. This provides the best motion response for content such as sports while retaining significant detail response. 720p uses a square pixel matrix of 1280x720 providing 921,600 pixels.

1080i uses an old analog compression method called interlaced directly related to old CRT display technology which takes each frame and separates it into two fields cutting detail in half therefore reducing bandwidth by half. 1080i uses a square pixel matrix of 1920x1080 providing 2,073,600 pixels, double the detail response of 720p. That level of detail versus available bandwidth also prevents a frame rate of 60 creating a frame rate limit of 30. Whether DTV uses 1080i at 30 or 24 frames or 1080p at 30 or 24 frames the bandwidth requirements are similar but the progressive 1080p version will compress more efficiently. Ultimately 1080i or 1080p is a great format for most of the content we watch since nearly all movies and TV shows are captured in 24 or 30 frames. In this case 720p offers no surface advantage and 1080 offers twice the detail response.

1080i primarily exists because CRT technology is what was used to begin the HDTV revolution. Getting a CRT to scan at a 1080 progressive rate was a daunting challenge reserved for expensive data grade front projection displays so the 1080i format was purposely used to overcome that hurdle counting on source products to convert 720p to 1080i. In the beginning a native 720p consumer CRT display was difficult to find and within about 2-3 years not even available with either multiscan 480p/1080i or 1080i only CRT product taking over the market. During this interval 720p was available in plasma and digital MD ( micro-display) front projection technology, was very expensive and therefore limited in volume and few HDTV enthusiasts ever experienced a true 720p presentation. That changed about 2004 when 720p MD rear projection product became available. Unfortunately that lasted for a brief 2 years as 720p was replaced by 1080p technology highly coveted by HDTV enthusiasts and performance users due to detail response. Currently one piece true 720p displays do not exist having been replaced by oddball pixel matrixes and targeted for entry level lower tier product lines. 720p is also becoming rare for MD front projectors.

720p versus 1080i Performance

1080i CRT displays suffered from a lack of fine detail due to vertical filtering to prevent 1080i artifacts from appearing on screen along with horizontal filtering to prevent moire patterns from appearing due to the lenticular screen and a 1920 response. Even without a lenticular screen these products were simply not capable of a 1920 line response. Another artifact of 1080i is the small up and down picture bounce caused by the alternating fields of 1080i which is difficult to perceive until your eyes have become accustomed to the rock solid response of progressive imaging.

Current display technology is either 720p or 1080p with 1080i now in the dustbin of display history. Performance users are left with a number of 720p versus 1080i problems nonetheless.

One is the conversion of 1080i to 1080p which must occur to display a 1080i source on a 1080p or 720p display. This is called 3/2 pull down video processing and if not done properly detail may be removed or blurred. While vastly improved over just a few years ago this is an area where you will need to inspect what you expect. On the other hand that same detriment is a positive because along with less detail comes less noise and broadcast HD is not perfect. Ultimately 1080i performance on a 1080p display will always be directly related to the quality of conversion. While conversion is just as important for 720p displays 1080i performance on a 720p display will always be limited because it is half the resolution of the 1080 pixel format.

At this time it is quite difficult to experience the full benefits of 720p because regardless of the display being 720p or 1080p conversion will be taking place. For a 1080p display 720p must be upconverted and while 1080 may have double the pixels overall it does not have double the pixels in the vertical or horizontal planes and that causes blurring of detail when converting. 720p on a 720p display suffers from the same conversion problem because you can