|May 2, 2009, 11:13 PM||#1|
Difference between 256 kbps and 256 kbps (vbr) ?
Before the update of iTunes I was encoding at 256 kbps. Now it encodes everything in 256 kbps (VBR). What is the difference? The sound quality is the same so is there anything else about it?
|May 2, 2009, 11:14 PM||#2|
Variable bitrate (VBR) is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate (CBR), VBR files vary the amount of output data per time segment. VBR allows a higher bitrate (and therefore more storage space) to be allocated to the more complex segments of media files while less space is allocated to less complex segments. The average of these rates can be calculated to produce an average bitrate for the file.
MP3, WMA, Vorbis, and AAC audio files can optionally be encoded in VBR. Variable bit rate encoding is also commonly used on MPEG-2 video.
 Advantages and disadvantages of VBR
The advantages of VBR are that it produces a better quality-to-space ratio compared to a CBR file of the same data. The bits available are used more flexibly to encode the sound or video data more accurately, with fewer bits used in less demanding passages and more bits used in difficult-to-encode passages.
The disadvantages are that it may take more time to encode, as the process is more complex, and that some hardware might not be compatible with VBR files.
Perhaps confusingly, a newer acronym for VBR is also becoming prevalent: CVBR. This means Constrained Variable Bit Rate. In CVBR encoding, the maximum allowed bit rate and (usually) the average bit rate are requested up front by the user of the encoding tool. Since this ability only exists upon encoding, CVBR and VBR are essentially the same thing once they have been implemented. While CVBR introduces massive advantages in tweaking down file sizes, this tweaking can become a long process of trial and error. Specifying a higher maximum or average may simply make the file bigger with no discernible quality effect beyond the potential for stutter when streaming the file. However, reducing these criteria too low will eventually lead to quite drastic losses in quality. The effect on video is typically an increased blockiness, because the frames are no longer being fully detailed in their rendering.
There are no ideal "one-size-fits-all" settings for the CVBR in video encoding. For a low resolution (320 or 640 lines) video encoded with MPEG-1 or MPEG-2, a usable average bit rate can be as low as 1000 to 3000. For a high resolution video such as 1080, this average will typically need to shoot up to 6000 to 8000. Changing the encoding from MPEG-2 to MPEG-4 will help promote a lower bit rate. However, what tends to indicate the limit for lower rates per example is bouts of significant motion in the video. In the end, you have to watch through the entire video and decide upon the quality subjectively if what you are aiming to do is achieve a minimal file size.
As of December 2006, most newly produced portable music devices and software offer modern support for VBR encoded files. Devices that support only CBR encoded files are becoming increasingly difficult to find.
In the past, many hardware and software players could not decode variable bitrate files properly, partly because the various VBR encoders used were not well developed. This resulted in common use of CBR over VBR for the sake of compatibility.
Support for VBR in AAC and MP3 files is found in most modern digital audio players, including those released by Apple Inc., Creative Technology, and SanDisk. Early VBR algorithms occasionally introduced audible artifacts when encoding monotone or minimal tones (for example audiobooks and acoustic music). These artifacts often mimicked a "digital chirp" during the quiet portions of the song or when there was only speaking. As VBR encoding algorithms have improved, these problems have been resolved in subsequent generations of the VBR standard.
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|May 8, 2009, 11:40 PM||#5|
VBR trouble downloading
Every time I try to download a VBR linked file to put in my iTunes, it increases its estimate time to finish continuously. Before a few days ago, they would finish in 10 seconds tops and now its almost refusing to do it. Any reason for this or how to fix it? Thanks.
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