Is Losslesscut really lossless?

[msgrabau]

Hi There,
I have been using Losslesscut to trim MP4 videos. They are MPEG4 AAC files. It doesn't seem like a lossless process and I think my files are being recompressed.

For example I have a file that is 1:06:00 long that is about 700MB. After using Losslesscut to trim six minutes from the end of the file, the output file is only about 300MB. So how does cutting only 6 minutes of a one hour file result in that much loss in size?

Scott

[jagabo]

IIt's not impossible that trimming 6 minutes off the end of a 66 minute video would reduce the size by 60 percent -- a variable bitrate file could be heavily weighted at the end. But it's more likely other tracks were removed, or maybe there were errors in the file that cause problems for losslesscut. Have you tried watching the new file to see if there are any problems? Do all 60 minutes play properly?

[msgrabau]

Thanks for the response Jagabo. The file plays fine all the way through. Your post made me realise that the six minutes I deleted was off-air-static which I suspect is pretty data intensive.

The trimmed file shows as an average bit rate of 8566kbps. I don't know if that indicates it is variable or fixed. The source file just has typical tracks. One video and stereo audio.

[jagabo]

Yes, with a variable frame rate, off-air static would likely consume much more bitrate than the rest of the video. Try using Mpeg File Bitrate Viewer to view a graph of the bitrate through the video. It doesn't work with all codec and containers though.

An hour long video at 300 MB has an average bitrate of less than 700 Kb/s. So that bitrate report isn't correct. It's either the max bitrate or just an error.

300,000,000 bytes * 8 bits per byte = 2,400,000,000 bits
2,400,000,000 bits / 60 minutes per hour / 60 seconds per minute = 666,666 bits per second

[msgrabau]

Thanks again. I tried the bitrate tool that you suggested and the graph does confirm that the rate spikes dramatically at the end. So that does explain my bit change in file size.

Cheers.

[jagabo]

Glad to hear you got it sorted out.