Subler subtitles generating very high data rate for iTunes stream to iPad

Hi,

I'm using Subler to mux subtitles from a .srt file into an .mp4 file that was 
output by VideoReDo (running in Parallels Desktop on my Mac). My aim is to put 
the Subler .m4v output file into my iTunes library and then stream it via Home 
Sharing to my iPad's Videos app.

The .mp4 video file input to Subler has 960x544 resolution and a data rate 
shown in QuickTime Player of 2.40 Mbit/s.

I have several questions/issues:

(1) If I don't modify the Visual Settings of the subtitle track that I see in 
Subler after I have dragged my .srt file into Subler's window (where it appears 
alongside the dragged-in video and sound tracks) I get a remuxed .m4v file that 
nominally works fine on my Mac but gives me trouble when streamed to my iPad. 
The issue seems to be that I get unwanted pauses during playback, but only when 
I am viewing the video with subtitles turned on. The playback buffer seems to 
run out of material to play. With subtitles turned off, there is not a problem.

I am able to monitor the data rate, i.e., the numbers of bytes being sent and 
received, by watching these statistics as reported by my router. With subs off, 
the streaming data rate is quite moderate. When I turn subs on, the streaming 
data rate skyrockets.

The problem persists if, in VideoReDo, I drastically lower the data rate of 
what is to become the Subler .mp4 input video, by restricting the average and 
maximum bitrates to the point where the video has a lot of macroblocking.

So, best I can tell, Subler is by default creating a Tx3g subtitle track that 
generates a lot of bitrate (or data rate) overhead.

Before Subler creates its remuxed .m4v output file, the Visual Settings 
parameters for the subtitles track are basically all zeroes, and Preserve 
Aspect Ratio is checked. (Alternate Group is 2, Forced is No, and Forced track 
is None.)

Afterward, I see:

Normal Size: 960 x 81 pixels
Scaled Size: 960 x 81 pixels
Preserve Aspect Ratio: checked
Aspect Ratio: 1 x 1
Offset: 0 x 462 pixels
Alternate Group: 2
Forced: No
Forced track: None

With subtitles off on the iPad, my network sees about 6 million bytes being 
streamed every 20 seconds. With subtitles on, it sees about 20 million bytes 
being streamed every 20 seconds.

So that's a sizable data rate overhead. My main question is, how can I lower it?

(2) I made the assumption that I could lower it by fiddling with the Visual 
Settings of the subtitles track before saving Subler's .m4v output file. But I 
find that changing the parameters I see there doesn't work as I expected.

If I start with these default/null values:

Normal Size: 0 x 0 pixels
Scaled Size: 0 x 0 pixels
Preserve Aspect Ratio: checked
Aspect Ratio: 0 x 0
Offset: 0 x 0 pixels
Alternate Group: 2
Forced: No
Forced track: None

I can change them to:

Normal Size: 0 x 0 pixels
Scaled Size: 960 x 10 pixels (i.e., 1/8 of the original 81 vertical pixels)
Preserve Aspect Ratio: not checked
Aspect Ratio: 0 x 0
Offset: 0 x 534 pixels
Alternate Group: 2
Forced: No
Forced track: None

After saving the .m4v file and closing the window in Subler, then reopening the 
resulting .m4v file in Subler, the subtitles' Visual Settings turn out to have 
become:

Normal Size: 960 x 10 pixels
Scaled Size: 960 x 10 pixels
Preserve Aspect Ratio: checked
Aspect Ratio: 1 x 1
Offset: 0 x 462 pixels
Alternate Group: 2
Forced: No
Forced track: None

So it looks like my settings were overridden for some reason. My second 
question is, why?

(3) That second .m4v file streams with, if anything, a higher data rate than 
the first one: on the order of over 40 million bytes every 20 seconds, when 
subtitles are on. Third question: Why?

(4) So I believe I need to get the elevated data rates down, when I am 
streaming these videos to the iPad with subtitles on. How can I go about doing 
that? Would I be able to do it with manually entered Visual Settings 
parameters? If so, how would I keep the parameters I enter from being 
overridden?

Thank you very much in advance for your advice on your quite useful remuxing 
utility.

Best,

Eric Stewart
Catonsville, MD
epstewart2004@yahoo.com
Original issue reported on code.google.com by epstewar...@gmail.com on 9 May 2014 at 7:04
XeresRazor / subler

Subler subtitles generating very high data rate for iTunes stream to iPad #754
