2 pass

[speedo]

lets say i use a bitrate calculator for a 97 minuite film which tells me for a 700mb cd

bitrate 913
audio 96 kbs
screen size 512/288

what would a 2 pass do for it
make the file size smaller or make it better quality

[kinneera]

2-pass will improve the quality. Highly recommended for a bitrate that low.

[vhelp]

kinneera, or anyone w/ an understanding of min/ave/max. . .

I've ben puzzled about this 2pass thing too, but more on the
min/ave/max settings.

Can you explain in detail how to effeciently setup a scenario
per encoding project??

But, forget about space for the moment. I'm really concirned.
I realy want just the quality side of it, later I might consentrate
on the space saving.

TMPG:
-----
CQ_VBR - ie, if I want to set a max bitrate (but no more) for 2520
and a min but no less, for 1850
2pass - ie, if I want to set a max bitrate (but no more) for 2520
and a min but no less, for 1850
"Q" what? ? ? mine is usually 65. that right??

** there may be differences in each versions of TMPG. I have pretty
much all of them, just about.

CCE:
----
2pass - ie, if I want to set a max bitrate (but no more) for 2520
and a min but no less, for 1850, and an average of 2300

What would the optimum be for both, and why?

any shedding of light, or direction in this min/ave/max issue would be
greately appreciated from anyone w/ an understanding.

Thanks for the help.
-vhelp

[kinneera]

I'm not exactly sure what you're asking, but I'll give it a shot:

2-pass VBR allows you to specify an average bitrate. This average will provide predictability of final file size, comparable to a CBR encode set at that bitrate. What it will do is fluctuate above or below the specified average, within the constraints of the minimum and maximum you specify. This means you can achieve maximum quality with a predicatable output by using a bitrate calculator to determine the average bitrate necessary to fit a given length of video into a given space. For your specific case, you would specify 1850 as min, 2520 as max, and use a bitrate calculator to determine what the average should be based on the length of your video. In reality, it is generally better to specify a low or zero value for minimum when using 2-pass, as it will allow the encoder to perform better (ie. 300/avg/2520). When you raise the minimum, it means the encoder has less liberty to conserve bits in low-motion/complexity scenes to be applied later in high motion/complexity scenes. These considerations apply to both TMPGEnc and CCE, as both are equivalent in terms of setup for multi-pass encodes.

In TMPGEnc, the details of CQ mode are something of a mystery. However, as far I know, the proper definition of CQ is that the encoder will always try to maintain a constant quantization level throughout the video (quantization is a measure of quality vs complexity of some sort, I can't tell you much more than that). This means that in theory, an extremely complex or high motion scene could cause a gigantic bitrate spike, as maintaining a constant quality level would require a massive increase in bitrate to encode more complex scenes. In practice, this isn't real useful, since bitrate spikes will easily exceed playback limitations, so the safest assumption is that TMPGEnc employs a sort of "capped" CQ encoding, as determined by the maximum value you specify. The CQ "level" specified in TMPGEnc presumably sets internally what quantization level it attempts to maintain. A value of 100 by some reports will result in little more than a CBR encode. Anything above 50 in my experience has proven acceptable, but I still generally do not use it as the resultant filesize is unpredictable. You can only limit the maximum possible size, but if it turns out fewer bits were needed, you waste space. Hope this sheds some light on the situation.

[vhelp]

kinneera,

Thank you for helping to shed some light on this issue. Much appreciated.

Well, you can visit my <<DV cam - discusisons. . .>> thread for other dissucsions.

-vhelp

[Greg12]

for vcd, i set the max to 5 megs, my dvd players limit

[vhelp]

Greg12,

fewww! That's a lotta bits dude! he, he....

curious... what's YOUR source, that it takes you to the limit, hence 5mb/bts? ? ? ...and why???

-vhelp