Strange bit rate question.

[rdsii64]

When ever I want to keep something I have DVR'd the HD source material that gets imported into Final Cut Pro X has a bit rate of 13.5.(I know thats really low but HD content from my service provider is compressed at 14.0 anyway.) What usually happens next is I remove commercials and replace them with transitions then use compressor to burn to disc. I fiddle with bits rates until the finished product will fit on a double layer DVD. A bit rate of about 25 usually does the trick. The final product is visually equal to the original broadcast. Until this morning when I had a strange scratch your head moment, all has been right with my world.

Now comes the strange part. If I tell compressor to use the same bit rate as the original source material, the final product will be roughly half the size of what I get now. My question is with the lower bit rate (13.5 vs 25) degrade the final quality when the original source material was encoded at 13.5.
Sorry for the long post but thanks for the help.

 

[KeithPratt]

When you recompress, the bitrate of your source file is not directly relevant. In order to do that recompression, Compressor (or any other software you might be using) must decompress the source — at which point all existing compression ceases to exist.

Now, I said the source bitrate is not directly relevant, and what I mean by that is that the original compression will have left its mark. If the original compression smudged out a good bit of detail, the image you're trying to compress this time might be quite undemanding in terms of how many bits are needed to accurately render it. But that number won't often turn out to be the same as the source.

Does that make sense? If not, my direct answer is that you haven't had a eureka moment — matching bitrates does not equate to matching image quality. You'll have to try it and see.