CBR - Constant BitRate
VBR - Variable BitRate
I like to say there are two ways to explain VBR vs CBR. There is the non-tech way to explain it and that is simply:
VBR = Bad, CBR = Good
And really that is all you need to know - some really smart people figured that out back in 2004 when podcasting started - and that is one thing that has held true over time.
Ok - I realize not all people accept the simple just because type answer - for those looking to be shown why - here we go:
VBR is an old tech / hack that was created to make MP3 music files smaller and was popular back in the heyday of file sharing. Today there is no need for it - available bandwidth and storage today is much different than 15 and 20 years ago. But more importantly ISO standards for MP3 do not require players support it.
According to the standard (ISO/IEC 11172-3:1993) Section 126.96.36.199
"In order to provide the smallest possible delay and complexity, the decoder is not required to support a continuously variable bitrate when in layer I or II. Layer III supports variable bitrate by switching the bitrate index. However, in free format, fixed bitrate is required."
"For Layer II, not all combinations of total bitrate and mode are allowed."
Hence, most Layer II coders would not have been written with VBR in mind, and Layer II VBR is a hack. It works for limited cases. Getting it to work to the same extent as MP3-style VBR will be a major hack.
In short VBR's day in the light and mass use is way way behind us - back in the late 1990's and pre-podcasting.
If you really want to make your music files sound better and take up less file size today - use .m4a format. But for podcasting especially for spoken word podcasts - I do not see any advantage to going with VBR vs going with CBR at 96 kbps or 64 kbps and MP3 format. And I only see potential issues with players that do not support VBR.
What we recommend at Libsyn is:
64 kbps - bit rate
44.1 khz - sample rate
mono or joint stereo.
Personally - I go with 96 kbps on the bit rate and mono.
Here is some more info:
"MP3 Format Caveats: Additionally, some very low and very high bitrate MP3s, and Variable Bitrate (VBR) MP3s may play either too quickly or too slowly ("the chipmunk problem"); if you are encountering this issue, try re-encoding at a different bitrate (between 64 kbps and 192 kbps, for example.) Using Constant Bitrate (CBR) encoding may also alleviate this problem."
This is for their HTML5 player. HTML5 players do not need to support VBR and many do not - which means the play back time displays wrong and in some cases the file stops playing before you get to the end or you get the aforementioned chipmunk effect.
iOS 6, iOS 5, iOS 3 - these are all devices we had reports on at libsyn where VBR was causing issues with play back of episodes. This is something that was put to bed a long time ago. But still we get reports of files not working at libsyn that are VBR in nature. Look at all the big shows out there you will not find a single big show that does VBR. Not one.
Now granted most articles you find about issues with VBR are a few years old or older - but I think that is because most people learned early on not to do VBR - all the books out there say don't use it - all the tutorials say don't use it - well all the credible tutorials. And guess what basically no one is using it - Hence you don't see any new posts about issues with it.
At libsyn we have seen many users that used VBR have issues. One of the first checks I do when I hear someone is reporting playback issues with a flash player or html5 player is check how they encode - if they encode with VBR I tell them to change it to CBR - and the issues almost always go away. There is a reason that This American Life, TWiT, Adam Carolla, Joe Rogan, NPR and all the other big shows use CBR - it is just the better solution for the end users and causes the least amount of issues.
To sum up - just remember - VBR = Bad / CBR = Good.