CBR vs VBR Digital Encoding and What It Means to Your Project

This past week I received four inquiries about digital encoding videotape to DVD where I ended up explaining the differences between constant bitrate encoding (CBR) and variable bitrate encoding (VBR)... and the related price differences.

What it boils down to is that there are different qualities of digital encoding... even when going to the same end product like a video DVD.

Let's say a client wants to transfer a digital betacam videotape to DVD. At Video Labs, we have three routes we can follow.

The least expensive option is to play back the videotape and encode the signal directly into our Pioneer DVD recorder.

We have three of these Pioneer DVD recorders. Even though they are not made anymore, they continue to be workhorses fo us. We like them for a number of reasons, including: they can take component and SDI in, can handle both NTSC and PAL signals, and offer some basic authoring capability. (And as a side note, an added bonus is that when encoding SD captioned tapes, these DVD recorders don't strip the line 21 captioning information in the vertical interval, thereby making for an easy way to carry over captions from a tape to a DVD. Read more about this in my article: "The Challenges of Subtitles and Captions in DVDs" )

A more expensive solution for encoding a video would be to digitize the footage into our Final Cut Pro and then use Compressor to create a MPEG2 video and AC-3 audio file, and then create a DVD in DVD Studio Pro, Adobe Encore or Sonic Scenarist.

Our top of the line solution would be to output the tape to a digital betacam dub, and then encode that dBeta into our Sonic SD-2000 encoder via SDI, and then take the resulting MPEG2 video and AC-3 audio files and import those either into Adobe Encore or Sonic Scenarist to create a DVD.

There are two types of digital encoding. One is constant bit rate encoding (CBR). This is what lower-end priced encoders employ. The operator sets the bit rate and that's the rate the video will be encoded throughout. One advantage is that encoding occurs faster.

But is that best use of the allotted bit budget? Think about it. Doesn't it make sense that a static series of images within a plain background would take less data (read bit rate) than a series of images with a lot of motion and action and/or a wide range of color and contrast?

Well, if that's true, then would it not be more efficient if the "bit rate budget" (remember, like a, you have only so much available digital space on a DVD) could be spent such that it used a lower bit rate for the static scenes in a program and then "poured it on" when really needed for those scenes with high activity and/or lighting/color?

That is the concept behind "variable bit rate encoding" (VBR). And higher-end encoders employ this more sophisticated approach.

(Our higher-end Sonic D-2000 encoder also employs "segment encoding" which accounts for its premiere pricing. But let's save that for another article.)

Now, technology is moving fast in our field, and I am getting new info from my various multimedia "gurus," who say that actually there are now very cost-effective software encoders on the market that employ CBR which are actually pretty darn good. As explained to me, these software encoding programs are kinda setting the ole "hardware is always better than software" philosophy on its head. The thinking is starting to be that since tech is changing so fast, the ease with which software can be updated vs. hardware offers a increasingly distinct advantage.



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