AVCHD to Blu-ray – Bitrate?

[Dave Haynie]

Here’s how to think of it… when you edit in Vegas, Vegas turns your video into uncompressed. You never see this, since it only happens in RAM.

Some kinds of input video work with “smart-rendering”… if you render out to the same format your have coming in, and don’t make any changes to it, you’ll get the same video out without re-compressing. Otherwise, you need to re-compress.

So you drop an AVCHD video on the Vegas timeline. Vegas does processing to it… could be nothing, could be compositing with other video, but in short, Vegas sets up all the instructions for you edit. When you render, those instructions “run”. And part of that is what do with the series of uncompressed video frames created by Vegas.

When you select AVC output, you’re telling Vegas to compress to AVC. The thing that’s compressed are those in-memory frames… how they got in memory has nothing to do with the output format (except in the case of smart-rendering, which is a special-case bypass of the decompress-composite-recompress process).

Now, it can be pretty well shown that your re-rendered video at 20Mb/s is not going to look any better than your input video at 17Mb/s. In fact, it’s virtually guaranteed — some loss occurs with each re-rendering. That’s also not the right question to ask. Since you’re editing and re-rendering, the real question is whether your video looks better output at 20Mb/s than some other bitrate, and whether MainConcept AVC is better/worse for the same bitrate than Sony AVC.

-Dave

[Dale Mcclelland]

Ah, I think I see now. The higher bit rate Blu-ray file is less compressed than the original AVCHD. That makes sense and means that the render process isn’t creating bits that didn’t previously exist. Thanks for explaining – I hadn’t considered compression when trying to figure this out.

In case anyone is interested, here is a long-winded explanation of my reasoning for using MainConcept for rendering Blu-ray instead of Sony AVC, and for setting a higher target bit rate than the native clips:

1. The max bit rate in VMS for Sony AVC is 16 Mbs, which matches my camcorder, so should be fine. However,

2. I found in Movie Studio 9 Platinum, which I used before v10 came out, that the rendered file using Sony AVC had a somewhat reduced bit rate compared to the original clips – perhaps 500 to 750 bits/sec lower on average. My target media at that time was AVCHD on standard DVD discs, so I continued to use Sony AVC because it is the only one that had an AVCHD template.

3. With VMS 10, I found that the reduction in bit rate with Sony AVC is even greater than in version 9 (using the same clips with the same render settings). My native 16 Mbs clips, that almost always stay between 15 and 17 Mbs, end up at 14 to 16 in the rendered file and sometimes dips below 13, which never happens with the original clips. The reduction may or may not be visible when viewing the rendered video, but I just don’t like the idea that the bit rate ends up lower. The reason for that is:

I used to use a non-Vegas NLE and it reduced the bit rate even more, to the point that a difference in quality was clearly visible when viewing the rendered file, especially on panned footage where vertical edges had a stair-step effect that wasn’t present in the original clips. That not only led me look for a different NLE (ending up with VMS), it also caused me to associate a lower bit rate with lower picture quality.

My target media these days is a Blu-ray file for playing on a set top media player. The Main Concept Blu-ray settings allow for setting a higher target bit rate than Sony AVS, which limits Blu-ray to 16 Mbs (in Movie Studio, not sure about Pro). If I set it higher than the original clips, I’m hoping that the rendered file will have little or no reduction in quality that can be directly attributed to a reduction in bit rate. I’m not expecting that the rendered file will look better than the original clips, but want it to be as close as possible to the originals.