[mythtv-users] bitrate questions

[Michael T. Dean]

Chris Petersen wrote:

>> I don't know what 3:2 pulldown is.  Is that the same or similar to 
>> two-pass encoding ?  Is this what prevents realtime encoding to make 
>> these smaller files ? 
>
> 3:2 pulldown (somehow related to, if not the same as, inverse telecine)

3:2 pulldown is "telecine"--changing 24 frames per second into 59.94 
fields per second--and "inverse telecine" (IVTC) is undoing the telecine 
process--returning to the original 24 frames per second.

3:2 pulldown gets it's name from the fact that film frames are repeated 
in a recurring 3:2 pattern--i.e. for frames A, B, C, and D, applying 3:2 
pulldown would use frames A, A, A, B, B, C, C, C, D, D (3 shots of A, 
followed by 2 of B, followed by 3 of C, followed by 2 of D).  Because 
NTSC is interlaced with video at 59.94 fields per second, this results 
in fields A1, A2, A1, B2, B1, C2, C1, C2, D1, D2, where A1 is the even 
horizontal scan lines of A and A2 is the odd horizontal scan lines of 
A.  Therefore, we have the even and odd lines of all four original 
frames--allowing us to recreate the material in its original source form.

Note that telecining is not always 3:2 pulldown.  For video source with 
a different number of frames per second or target formats other than 
NTSC, other telecine processes may be used.  For example, 2:2 pulldown 
is used to create PAL video from 24fps movies (50 fields per second, but 
instead of duplicating fields/inventing frames, the audio/video is sped 
up by 4% and audio is pitch-corrected).

> is a way to trim out

redundant fields/invented

> frames (thus losing quality by having a lower framerate)

but, if it truly is inverse telecine, the original source material 
started at this lower framerate, so in truth, you are not actually 
losing quality--and, depending on the player, may be increasing quality 
(if the player--your monitor--isn't constrained to NTSC video standards)

> to make bandwidth room for better-quality frames.

definitely--we'll have 24 frames per second to encode versus, assuming 
NTSC video, 59.94 fields/29.97 frames

>   So you end up with a lower framerate, but higher quality frames.

true

>   The "pulldown" is then reversed on playback so that it *appears* to 
> be full framerate.

vv -- pulldown or telecine is applied to allow playback at NTSC standard 
rates (which is actually greater than the "full framerate" of the 
original source)

>   It makes a huge difference when encoding svcd (since you usually 
> just toss out the interlaced frames, too) if you don't mind losing a 
> little framerate.

So, applying inverse telecine on material that began as 24fps--movies, 
animated shows, etc.--provides significant benefits with no losses.

Mike