[mythtv-users] High end, state of the art Myth Frontend

[Joseph Fry]

>>> Within the limits that the ATSC allows, it's true.  Theoretically,
>>> compressing a 1080p/60 video would use twice the bandwidth of
>>> compressing a 1080i/60 video at the same level of quality.  In reality
>>> that's not true because a) file size is not directly related to
>>> resolution, b) the higher amount of detail would allow a greater
>>> percentage of compression without a perceptible difference quality.
>>>
>>> But the fact of the matter is, compressing a 1080p/60 signal will
>>> result in a larger bandwidth file than compressing 1080i/60 to the
>>> same perceptible quality level.
>
> Incorrect, the EBU have been running a trade roadshow for many years disproving this commonly held belief.
>
> This is not a proper paper from the EBU but I am not permitted to pass on the studies this derives from, it covers the concept quite well I think and the engineers I saw watching this (if they understood what they were watching) left quite amazed. Sadly I was told that most visitors told the EBU they had the labels wrong :-(
>
> http://tech.ebu.ch/docs/techreview/trev_308-hdtv.pdf
>
> This is not a proper paper from the EBU but I am not permitted to pass on the studies this derives from.
>
> From the emission section: referring to the effect of high compression on 1080p50 over 1080i50 and 720p50
>
> "However, at the lower bitrates (i.e. <10...13 Mbit/s) the 1080p/50 encoder becomes more overloaded with information, depending on the content, and this information overload appears to become the dominant factor affecting quality. The impairments with high compression are not as bad as those for 1080i/25 but more visible than 720p/50."
>

Which exactly confirms what I said.  In order to have the same
perceived image quality, a 1080p signal requires more bandwidth than a
1080i stream.  This is contrary to one some people would think as it
suggests that interlacing, as a means of compression, results in
higher quality perception than what can be achieved with MPEG encoding
alone.

It also confirms what I said about the higher amount of detail
allowing a greater percentage of compression without perceptible
difference in quality, from the article, sentence before yours (which
you quote below): "1080p/50 provides more information in the
spatio-temporal domain and encoders can conduct the compression more
efficiently."

Essentially, the article seems to confirm everything I said... but of
course the MPEG4 used in the EU is different than MPEG2 used here in
the US.  I suspect that 1080p compressed with the MPEG2 settings
allowed here would be simply unwatchable.

>
>>>  Your starting with double the data,
>
> Which is irrelevant, please see the above techreview doc.
>
> "1080p/50 provides more information in the spatio-temporal domain [than 1080i] and encoders can conduct the compression more efficiently."
>

Yes, the compression is more efficient... but more efficient doesn't
mean that it can compress a stream to the same bandwidth at the same
quality.  Lets make up some numbers:

Say I am allowed to broadcast a 10Mbps stream, and I have to choose
between compressing a 20Mbps 1080i stream, or a 40Mbps 1080p stream...
I need to achiveve 50% compression on the interlaced and 75%
compression on the progressive streams and have the resulting stream
look comparable in quality.  This may not be possible as compressing
the stream by 75% will introduce too many artifacts, thus the 1080i
actually looks better because it started with 1/2 the data and thus
did not need to be compressed as much.

This diminishing return at greater compression can be summed from the
articles conclusion too: "The 1080p/50 format was rated equal or
better than 720p/50 for the higher bitrates – the extent depending on
the test sequence. However, 720p/50 was rated better than 1080p/50 at
the lower bitrates."

Essentially, starting with less information resulted in less
compression artifacts when going to a lower bitrate... and the
perceived quality is better, the same holds true for 1080i vs 1080p
compression; there is a level of compression where 1080p starts to
look worse than 1080i.. and I suspect that the bitrate that happens is
somewhat higher than what ATSC (US MPEG2 broadcast standard) allows.