[mythtv-users] HDTV Video Modes with MythTV

Hamish Moffatt hamish at cloud.net.au
Fri Aug 12 05:03:14 UTC 2005

On Thu, Aug 11, 2005 at 09:17:26AM -0500, James Oltman wrote:
> On 8/11/05, Hamish Moffatt <hamish at cloud.net.au> wrote:
> > However in 1080i you have 1080 lines, updated 30 times per second.
> > In 540p, you only have 540 lines! The resolution is halved, and it's
> > not much more than 480i.
> You are confusing frames, and fields.  In 1080i, you have 2 fields of
> 540 lines a piece.  The first field uses lines 1, 3, 5, etc all the
> way to 539 or whatever.  The second field uses lines 2, 4, 6, 8 etc
> all the way to 540.  Those two fields equal one frame of 1080i (i =
> interlaced).  When you have 540p you are actually writing all 540
> lines in both fields, thus making for a cleaner, smoother picture.  If

Yes, this is true. But, you only have 540 lines! You wrote them twice as
often, but there were only 540 unique lines. In 1080i mode, you have
1080 unique lines updated half as often.

Theoretically you could make it equivalent to 1080i by displaying
alternating lines (odd/even) of a 1080 line frame on your 540p display.
ie perform the interlacing in software.

> anyone can add/correct me, please chime in.  I am fairly certain that
> is how this works.  It's been about a year since I played with HDTV
> modelines.

That is how it works, but it does not make them equivalent.

Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>

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