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Variable Bit Rate
Hey guys!
I’m trying to render a movie for a DVD, I’m using MPEG 2 and at Variable Bit Rate there’s an option for Two-Pass. What is two-pass??
Thanks guys!!
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4 Replies
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Thanks to John Cline for the following information.
Two pass encoding is only useful when you are using variable bit rate encoding (VBR) in order to fit more than about 72 minutes of video on a single-sided DVD and need to use a bitrate lower than a CBR of about 8Mbps.
Let’s say you were trying to fit 2 hours on a DVD (using 192kbps .AC3 for the audio), that would require a video bitrate of about 4,896,000 bps in order to make it fit on the disc. You could encode the whole video at that bitrate, but the more effective method is to use VBR encoding where the bits available are used more flexibly to encode the video data more accurately, with fewer bits used in less demanding passages and more bits used in difficult-to-encode passages. The average bitrate over the length of the file would stay at 4,896,000 for example, but could go as high as whatever maximum you have set (usually 8,000,000 bps) for those passages which need it. Of course, other less complex passages would get fewer than 4,896,000 bps in order to maintain the average.
By doing a 2-pass encode, the encoder analyzes the file on the first pass looking for scenes which need the extra bits and those that don’t, on the second pass it does the actual encode using the bitrate allocation strategy it calculated on the first pass. 2-pass encoding also ensures that the encoder more accurately hits the average bitrate target for which it was set. The only downside to 2-pass encoding is that it takes about twice as long to encode a file. I can easily see the difference.
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So it’s not really necessary to enable it when I’m just rendering 10 to 20 minutes of video right?
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That’s correct.
I personally use a CBR of 8,000,000 and Best mode for anything under 70 min. in length.
Stick to good name brand media like Taiyo-Yuden or Verbatim and you’ll be all set. -
Hey Mike,
I thought in prior posts I had read that the 2 pass always gave a better result even where bitrate and length were not factors. If the CBR is the proper format for what you stated then I will in the future do as you suggest. Thanks for advice as always.
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