Limitingand reducing out file size

[TJF]

As I have a 54" HD TV and sit relatively close, the advice is that I should use 1080p where possible. I'm currently testing that theory out.
I note from a guide from Baldrick advice that for 1080p, I should set file size at 2-3G for each hour - lets say 2.5G/hour.

I just converted a 2.5 hour movie at 1080 using mp2 and it came in at 16G! So I looked at both Ripbot and AVS to convert to 1080p but reduce to around 6G (on the basis of your guide). The problem is that neither of those apps allow you to limit higher than around 4.5G.

Any thoughts? I trialled Xilisoft which states that you can manually limit size but when you look to limit at above 5G (for example, for entering 6000MB), it states that I must "enter a value higher that 0" WTF? Anyway I will send a note to the Xilisoft support team.

I understand that I can also play with file size by varying bitrate? But I'm not not aware of any guides to go by when looking for a output in Gig terms etc. Are there any? To be honest, notwithstanding some critisism, I find AVS to be the most straightforward and would prefer to solve the issue using this app.

[jagabo]

file size = bitrate * running time

Use a bitrate calculator: https://www.videohelp.com/tools/sections/bitrate-calculators

But what are you converting from and what are you converting to? Just use constant quality encoding of you want a specific quality and don't need a particular file size.

[olyteddy]

Also, if your source isn't 1080p to begin with, nothing but wasted space is gained by converting it. Let your TV upscale and save the quality of the source material.

[TJF]

Jaygabo and Olyteddy, many thanks gents. The source will generally be Blu Ray so the 1080p standard may be worth maintaining. The bitrate calculator will be very useful. I appreciate it.

Cheers

[TJF]

Jagabo, re the formula of file size = bitrate * running time:

I have a 16G file with a bit rate of 15000kbps. The file has a running tme of 152 minutes (9,120 seconds). That gives a file size of 136,800,000kb or 136G. I have no doubt I'm missing something obvious?

Anyway, of the calculators, I tried using VideoCalc and Media Calculator inputing 15000kbs and 9,120 seconds and they gave me bit rates of 14,147kbps and 14,785 respectively - both pretty close.

So, using these (and being the simpleton that I am), do you think I can simply reduce my 1080p file from 16G to 6G by simply reducing the video bitrate from 15,000 to 5,200kps (using VideoCalc)? As I said, I came up with 6G because of Baldrick's recommendation of 2 to 3G per hour for a 1080p movie. Do I have to make any changes to the Audio bitrate?

Thanks.

[TJohns]

I have a 16G file with a bit rate of 15000kbps

Correct. Bitrate means BITS PER SECOND. You've got 15 million every second

The file has a running tme of 152 minutes (9,120 seconds). That gives a file size of 136,800,000kb

Correct. BITS PER SECOND times SECONDS gives BITS

or 136G

Correct. You've got 136 giga BITS. Divide by 8 and that's the 16 giga BYTES in your file.

Trev

[jagabo]

Jagabo, re the formula of file size = bitrate * running time...

TJohns has already addressed that. I would just add that 1 byte = 8 bits, hence the divide by 8 in his last step.

So, using these (and being the simpleton that I am), do you think I can simply reduce my 1080p file from 16G to 6G by simply reducing the video bitrate from 15,000 to 5,200kps (using VideoCalc)?

Yes. But...

As I said, I came up with 6G because of Baldrick's recommendation of 2 to 3G per hour for a 1080p movie.

That little bitrate is way too low for most 1080p movies. People who are shrinking Blu-ray sources that small will usually also reduce the frame size (smaller frames require less bitrate to maintain quality) to 1280x720.

Do I have to make any changes to the Audio bitrate?

In the equation "file size = bitrate * running time" the "bitrate" is the sum of the audio, video, and any other stream bitrates. There is also some overhead for the container. So the full equation is more like:

file size = (audio bitrate + video bitrate + other stream bitrate + container overhead bitrate) * running time + fixed container overhead.

But the video bitrate is usually much larger than the others and the container overhead is usually very small. So video bitrate * running time is a close approximation. If you are including several audio streams with very high bitrates, or are shooting for unusually small video bitrates, you have to consider the bitrates of all streams.