Bitrate Calculators are Lying to Me

[koberulz]

I have Premiere Pro CS6 project file, with a sequence that is 1hr, 15mins 22secs long. I'm trying to output it at a size that will fit on a single-layer DVD. The audio is 224kbps AC3.

According to VideoCalc, even with a 20MB margin for error, I should output at 7855kbps, which Google says is 7.6709mbps. According to BitrateCalc, I should output at 7.56mbps. According to Bitrate Calculator, I should output at 7982kbps (7.79492mbps).

None of these is right. I even rounded down to a flat 7.5mbps, and it was still too big.

What on earth is going on?

[Cornucopia]

Let's start from the very beginning. Use consistent nomenclature and unit sizes so everything is on the same page.

1. 01:15:22:00 = 4522 sec
2. SL DVD = 4.7GiB = 4.37GB = 4474.88MB or 35799.04Mbits
3. AC3 audio = 224kbps aka 0.21875Mbps. It is CBR (Constant bitrate)
4. Capacity will need to handle V+A+~5%muxing overhead

Remove the overhead first.
35799.04 - (0.05*35799.04) = 35799.04 - 1789.952 = 34009.088 Mbits needed for V + A

Next, minus the audio.
(Audio size = 4522 * 0.21875 = 989.1875Mbits)

34009.088 - 989.1875 = 33019.9005Mbits capacity available for V

for the duration, works out to
33019.9005 Mb / 4522 sec = 7.302056 Mbps for the Video.

That is AVERAGE bitrate if using VBR (where PEAK/MAX br would have to be <9.5Mbps and MIN would usually need not be specified), or ACTUAL bitrate if using CBR.

Those calcs aren't LYING to you, they are rounding, or they are making certain assumptions that YOU aren't making, or you are not on the same page with them WRT nomenclature & units.

Note that going down "even to flat 7.5Mbps" WOULD still be too big. Of course, I'm assuming what info you gave me (runtime, etc) is exactly correct, so YMMV in reality.

Many times people don't realize they're skimping on the conversions or missing a step, etc, unless you lay it all out like I just did.

Scott

[koberulz]

2. SL DVD = 4.7GiB = 4.37GB = 4474.88MB or 35799.04Mbits

VidCalc says 4.37GB, but the other two claim 4480MB. So that would presumably explain two-thirds of the issue, although I only started using the others once VidCalc started spitting out the wrong number.

Plugging the numbers into VidCalc gives an audio size of 123.65MB (989.2 Mb, pretty much exactly what it should be) and a video size of 4356.35MB (34850.8Mb, way bigger than what you've said).

So where is VidCalc getting that extra space from?

[Cornucopia]

#1 4.37GB vs. 4480MB is only a difference of 5.12MB! Certainly not enough to quibble about.

#2 4480 - 4356.35 - 123.65 = exactly 0MB. IOW, they are assuming NO overhead, just the exact size of the media payload. This isn't "lying" per se, it is unreasonably optimistic regarding how encoders can target their bitrates, and it is naively misunderstanding overhead. . There is ALWAYS some muxing overhead (VOB packets fitting to sector size boundaries, room for subtitles, room for IFO & BUP files, CC userdata packets, etc). Much depends, of course, on how you are creating your title (features, complexity, etc).

With that in mind, if you find you don't need a cushion of 5%, but instead only need say 2%, you've saved ~134MB which you can then apply toward some portion of audio and/or video. But you'll have to test to find out what works best for you. 5% is a good starting point, sometimes you need more, sometimes less.

Where did I get it? From the Spruce DVDMaestro & similar calculators, which happen to be VERY exhaustive (though I have since expanded upon them).

Scott

<edit>Notice above I said that about encoders: You can always set a specific target AVERAGE & PEAK bitrate, but because of the nature of the algorithms that the encoders use, and the GOP structure, they cannot ALWAYS hit those exact targets, sometimes lower, oftentimes a little higher. Another reason to have that "overhead cushion."

[koberulz]

VidCalc offers a "Space for extra files" option, which I normally set to 20MB. With that set, it offers up 4336.35MB (34690.8Mb) for video.

I've done dozens of DVDs this way. Usually I load it up into Encore and find around 100MB free. It's never overshot before. And this one is just the one timeline, I usually have a menu with background music at the very least.

[Cornucopia]

Not knowing how they actually set up the calculation, I couldn't say. As mentioned above, could be rounding, etc. Why I like using a spreadsheet: more transparent & user-expandable.

Are you saying it gave you different numbers (given same input) than previously? If true, that would be messed up.

Scott