[mythtv-users] OT: A question on the HD-PVR and non-HD content

[jarpublic at gmail.com]

On Thu, Feb 19, 2009 at 3:05 AM, Jean-Yves Avenard <jyavenard at gmail.com>wrote:

> Hi
>
> 2009/2/19 John P Poet <jppoet at gmail.com>:
> > resolution.  So, 480i can be configured to use a much lower bitrate
> > than 1080i, since the extra bits will not help the picture quality.
> >
>
> A bitrate is independent on the resolution being used ...
> it just indicate how much compressed (lossy) the video is.
>
> So I fail to see how those "extra bits" won't help the picture quality
> because it's 480i and not 1080i
>

You can' t create more information than you start with. It would probably be
more accurate to say those extra bits *probably* won't help. For a given
video there is a maximum bitrate such that going above that bit-rate will
produce no perceivable improvement in image quality. In fact going too high
can sometimes degrade the quality for certain sources. For example if there
is a noisy signal then those extra bits will be used to encode that noise,
where as a lower bit rate would actually 'ignore' or smooth out some of the
noise. So assuming you are using the ideal bitrate for a 1080i source, then
using the same bitrate for a 480i source (which will almost always contain
much much less information) will almost certainly be wasting bits, in the
sense that a significant number of those bits yield absolutely no
improvement in image quality. As an extreme case you could set the average
bit rate to be higher the than the bit rate of the source. Obviously those
extra bits are not just going to magically add information (ie image
quality) that wasn't present in the source.
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