CBR vs VBR
 

...OK. this explains allot of what I've been seeing in my testing, I have consistently noticed a superior result in MPEG 2 software encoding using the VBR method versus the CBR (Constant Bit Rate) scheme. I think it's a very good idea for CD to employ VBR encoding in lower data rate settings, since they will benefit the most from this superior type of encoding method. This also explains my consistent test results with I-Frame encoding at 280 Mbps vs Long GOP @ 180 Mbps. I find the i-Frame very, very, very clean and noticeably superior to the Long GOP. (When viewing results on my properly calibrated Sony broadcast HD-SDI monitor). If I color my camera output slightly by setting the XL H1 to a preset with cine gamma 1 setting, then I get a much more detailed film look in I-Frame 280 versus even 220 Mbps ! It looks more film like. (I recognize this to be a very subjective point)

Mark,

Frankly, I don't see why a VBR codec should always be superior to a CBR one, when the maximum bitrate of the former equals that of the latter.

All other things equal, they should give identical quality - and the CBR only disadvantage being more media space requirements.

Vbr & cbr
 

....Hey Piotr: OK., but why would you conclude that CBR & VBR yield about the same results quality wise ? A variable bit rate encode yields a file which is usually superior and most importantly able to adapt the encoding process for sections which require more processing (Excessive movement for example) than other sections, thus yielding a smaller file size as well. I don't think bit rate is everything.

True, but if we assume VBR of 25 Mbps MAX vs. a CBR of 25 Mbps CONSTANT - there is no margin in the VBR for the better quality. It's just more efficient size-wise, that's all.

VBR Encode's Margin for Error @ Low Bitrates
 

...Hey Piotr:
I'm not sure this to be strictly true. (??) Usually, you see VBR encoding does the most good, when you have a low encoding bit rate. I believe the encoding margin in VBR is still present as long as the footage being encoded doe not require an increase in encoding variable bit rate above the maximum ceiling of the target bit rate.

Hi Mark,

I guess you may be right, but only when the VBR encoding uses more than a single pass.

If I'm wrong - I'm all ears :)

It's my understanding that if you set a CBR of say 25Mbps you are doing the same as setting a VBR of Max, targeted average and minimum bit rate of 25Mbps. Therefore simply setting a max in the VBR setting of 25Mbps will yield inferior quality to a CBR of 25Mbps. That is because at certain times the bitrate will drop below 25Mbps and will never be above it. However, if you have an average rate of 25Mbps in VBR, with multipass encoding, you should have better picture quality if say you set the max bitrate at 30Mbps and the min at 15Mbps. Then it really depends on the algorithm used to determine the chosen bitrates used for each frame.

Garrett

You've got it nailed, Garrett.

Perhaps I just wasn't clear enough in my messages that I was comparing VBR MAX bitrate with CBR of that same (constant) bitrate.

If you compare VBR with the target, or average, bitrate being equal to that of CBR, of course you have the potential for a better quality from VBR - especially if it's multi-pass.

Mark, was this what you meant?

What Did I Mean ?
 

...Hey Piotr: Yeah-sort of. :-) I've noticed VBR encoding scheme (Wether it be single or double pass) seems to have more of a positive effect on encoding footage when used in low data rates.

i.e. A 25 Mbps encoding rate encoded using VBR instead of CBR looks better than using CBR - even if the VBR is only single pass. This is not to say that CBR encoding cannot also look very clean. I think the VBR will have the edge.

Does anybody know what kind of encoder is in the Canon XL H1 ? (CBR or VBR ?)

Mark and Piotr, you are both right: When comparing VBR with the maximum rate same as the regular CBR, CBR will keep more information, so the result will look better.

When, on the other hand, comparing VBR with the average rate same as the regular CBR, then VBR will most likely look better than CBR. I say most likely because it will still have portions of the video compressed with a lower rate than CBR. But if it makes good choices as to which areas can get by with a lower rate and which require a higher rate, then it will look better overall.

Mark,

When specifying the Data Rate for VBR you need to say if the number you are stating is the minimum, average or maximum rate.

The XL H1 records HDV which is CBR.

Garrett

What VBR Data Rate ?
 

...Hi Garret: Oh yeah, sorry. 25 Mbps is the *Average.* The CBR encoder in this camera is second to *none !*

Mark,

The H1 is a HDV, tape based camera - therefore its codec is CBR 25 Mbps. With CBR, there is no "average", "max" or "min".

Hi Mark,

I use to have an XL H1a and XH A1 and will agree that Canon's HDV encoding is excellent. But, I have to say that my current setup of Sony EX3 with a nanoFlash cranked up above 100Mbps gives a truly stunning picture.

Garrett

Nano & Flash XDR Yield Superior Results
 

Hi Garrett: I agree ! No one's arguing this fact. Although I consider the Canon XL H1 camcorder to have the finest HDV encoding quality I have ever seen, I find the image quality of my Flash XDR recording from my camera's uncompressed HD-SDI output @ even Long GOP MXF 50 Mbps to be truly superior. I find I-Frame Recording with my Flash XDR @ 280 Mbps to be utterly stunning ! By far the results look like someone changed out the internal circuitry of my XL H1 and put in very high end Sony HD camera cirucits instead !