Adding subtitles almost doubles the size of mpeg file

[meleaca]

I searched this forum using ¡§srt¡¨ and ¡§subtitles¡¨ keywords and I didn¡¦t find any thread raising this issue.
I tried a couple of times to use VS11+ to add srt subtitles to a mpeg file and save the whole thing on my hard drive as mpeg file. Although each time I followed the same steps, the size of the resulting file is anywhere from 60k to 2.7G higher than the initial file:
video file 1: initial file size 2.32G, resulting file 2.38G
video file 2: initial file size 2.8G, resulting file 3.8G
video file 3: initial file size 3.2G, resulting file 5.9G (too big to put on a DVD)
I checked the properties of the initial and resulting mpeg files and in each case they looked identical to me. Could anybody please explain me what¡¦s going on? Thank you.

[sjj1805]

I haven't checked but I would have thought it unlikely to alter the file size very much. If you are using a constant bit rate I would have thought the file sizes would be almost if not exactly the same, If using a variable bit rate I wouldn't have expected subtitles to have altered things by quite that amount.

When you render the video have you tried the setting
"Same as First Video Clip"

[DVDDoug]

I checked the properties of the initial and resulting mpeg files and in each case they looked identical to me.

Something's different... probably the video bitrate, but it could be the audio format/bitrate that's changing.

Subtitles do add to the file size... I don't know how much to estimate, but is much less than the audio or video.

Bitrate is indicated as kbps = kilobits-per-second. This can be scaled-up to calculate megabytes-per-hour. From the bitrate and playing time, you can determine the file size.

Here is a handy online Bitrate Calculator. (You plug-in the playing-time and audio format, and it tells you what video-bitrate to use.)

Or, you can estimate file size with the folllowing formula:
File Size in MB = (Bitrate in kbps x Playing Time in minutes) / 140

The bitrate in the formula is the total combined audio, video, and subtitle bitrates. And of course if you are using variable bitrate, you need to use the average bitrate in the formula. (I don't know what the typical subtitle bitrate is, but it's not much...)

And of course, you can use the same formula for MP3 files.

P.S.
I found one reference on the Net that says each subtitle stream (i.e. each language) requires a bitrate of ~40kbps. (Assuming that's correct, the space for a subtitle track is about 1/10th to 1/20th of the space required for of a typical Dolby audio track.)

[meleaca]

Thank you very much for your replies. I tried to do the same thing again to one of my video files, but this time I used three different settings to render it: Same as First Video Clip, MPEG Optimizer and DVD/VCD/SVCD/MPEG (NTSC DVD (16:9)). The Properties for the initial file are:

File format: NTSC DVD
File size: 3,588,428 KB
Duration: 7473.473 seconds

Video type: MPEG-2 Video
Total frames 223,980 frames
Attributes: 24 bits, 720 x 480, 16:9
Frame rate: 29.970 frames/sec
Data rate: Variable bit rate (Max 6800 kbps)

Audio type: Dolby Digital Audio
Total samples: 358,726,727 Samples
Attributes: 48000Kz, 5.1 Channels
Layer: None
Bit rate: 448 kbps

The Properties for the resulting file in all three cases are identical with the initial file (I double checked) except for the File size which is about 5.3 MB in all three cases. I guess you are right, the issue seems to be the Variable bit rate. According to DVDDoug¡¦s formula the bit rate for the initial file is 3800 kbps while the bit rate for the three resulting files is somewhere around 5690 kbps. I don¡¦t think that any of the three settings I mentioned gives the user the opportunity to set a fixed bit rate. Am I missing something?
Thank you.