Quality Slider...what does it really do?

andywaddell · Post by **andywaddell** » Wed Mar 28, 2007 5:20 pm

All, I'm wondering about the difference between using a lower bit rate vs. moving the quality slider in trying to fit a bit more material on a DVD.

More specifically, I have a 1hr. 14min. project that I can jusssst fit on a single DVD using MPEG audio. If I bump that to LPCM audio, no go...it's too big to fit.

I burned a project using a bit rate of 6000, since I know that would definitely fit everything, but didn't like the quality degradation of the video. I could see it in the final product on DVD.

When I left the bit rate up at 8000 and moved the quality slider down to 70% from the 100% I typically use, I was able to fit it all on one DVD and don't THINK I was able to see any significant quality loss on the DVD.

OK, I've been pretty long-winded in getting to it, but what exactly does the quality slider adjust? What's going to degrade or improve when moving it? Bit Rate I understand, and can see the difference. But I've never heard a good explanation of what the Quality slider really adjusts. TIA.

ADW

etech6355 · Post by **etech6355** » Wed Mar 28, 2007 6:18 pm

As far as I can tell the quality slider changes the amount of motion search/depth when encoding. In other words how many pixels to search in the matrix/block. Mpeg encoding is complex.
I've found that Ulead's 70% setting may be the same amount of depth/motion searching in other programs set to 100% compression.
To much compression can also cause qualtiy loss although I use the 100% setting and my videos look good.

Maybe try 6000kbs & 2 pass encoding, or a high-bit rate and dual-layer media.

DVDDoug · Post by **DVDDoug** » Wed Mar 28, 2007 7:38 pm

Yeah, that's how I understand it too.

MPEG is lossy compression... Data is thrown-away during compression. But, it's also smart compression.... It trys to throw-away the least important data. Setting the slider to higher quality makes the compression smarter, but it takes more time to encode.

I don't know how the much difference the quality setting makes compared to the bitrate setting. (I've read that commercial DVDs are encoded with 20-pass encoding!!!)

etech6355 · Post by **etech6355** » Wed Mar 28, 2007 10:22 pm

I don't know how the much difference the quality setting makes compared to the bitrate setting. (I've read that commercial DVDs are encoded with 20-pass encoding!!!)

Yes, they must be working with extremely high quality video , not my videos

In VS if you enable the advanced=1 to it's ini config file when you look at the advanced encoder options the quality slider directly affects the setting in the encoder for motion search/depth 70%=8 and 100%=11 in the encoder.
I usually leave the advance=1 out of my config file. Doesn't always work correctly on my system.

daniel · Post by **daniel** » Thu Mar 29, 2007 1:19 pm

I half-expect Steve to come up with links about this, anyway what I think I understood here is that the quality setting affects indeed quality VS time taken for encoding, not final size, or incidentally and marginally.

The way to reduce the size without lowering bitrate is to use dual-pass variable encoding.

sjj1805 · Post by **sjj1805** » Thu Mar 29, 2007 7:14 pm

daniel wrote:I half-expect Steve to come up with links about this, anyway what I think I understood here is that the quality setting affects indeed quality VS time taken for encoding, not final size, or incidentally and marginally.

The way to reduce the size without lowering bitrate is to use dual-pass variable encoding.

Please view:
Estimating file size -mpeg - slider Speed-Quality

If you just need to reduce the file slightly to get it to fit you have the following options.

1. Use Dolby(AC3) Audio
2. Use Variable bit rates (For normal use stick to constant)
3. Reduce the Bit Rate.

A higher quality slider should not affect the overall size of the video but the processor is taking more time and care and (in theory) will create a better video. Essentially it is a balance between speed-v-quality.