Problems with MXF to DVD.(Pal)

[Michael Threadgold]

Hi, just recently started with MXF from Canon 300.Bluray is fine but when downscaling to DVD I'm getting spiraling on instrument strings and strange patterns on suit material. I've also noticed waviness on a symbol on a plain tie background. Never had this with HDV from tape.
Using the usual DVD widescreen DVD Architect template.
Tried reversing fields but no difference.
V.Pro 12 Build 770.
Thanks Mike.

[Adam Stanislav]

Have you tried downscaling in Vegas rather than in Architect? Downscaling requires a low-pass filter, and AFAIK Architect doesn’t apply one.

[Michael Threadgold]

Hi Adam, thank you for replying. I did use Vegas to downscale, using the usual template for Architect.
I don't know the terminology for what I'm seeing but it is strange that it's not there on Blu-ray.Mike

[Adam Stanislav]

It’s not on Blu-ray because it is not downscaled there. An example of a low-pass filter is mild blurring. See Low-Pass Filtering (Blurring) for a simple explanation.

You’ll need to experiment with it, for example to see if applying such a filter before downscaling gets a better result than applying it after downscaling. Or to determine which of the many possible blur filters works best for your particular footage. Unfortunately, low-pass filtering of video is not as simple as similar filtering for audio.

If nothing works, try HandBrake (http://handbrake.fr). It does a great job downscaling, though it will only give you an .mp4 output, so you may need to load the result into another Vegas project and export it to the format Architect uses for DVD.

[Seth Bloombaum]

Agreeing with Adam that the rescaling is likely the source of the visual artifacts you're seeing. Feel free to post samples!

"Best" rendering quality should always be chosen in the custom render settings dialog when rescaling. This is important to getting the best that Vegas can do, and you should definitely try that before anything else.

If you do need to go outside of Vegas to get the quality you want, Adam's recc of Handbrake is solid. HB's Lanczos scaler sets the standard. Lanczos is open-source, and available elsewhere, but HB makes it very accessible. Albeit with the limitations of MP4/h.264 output. But here too Handbrake has the best h.264 encoder...

More, you can also reduce this effect by applying a very small amount of gaussian blur. Somewhere I have some settings noted, but basically you look at picture at full resolution (this is important!), advance the filter setting until you see some effect, then back it off just until you don't.

[Adam Stanislav]

OK, when I posted my first two replies, I was about to leave home, so I didn’t have much time for a detailed explanation. Let me elaborate on what a low-pass filter is.

I will start by talking about sound waves, as it is more intuitive to understand. We’re all familiar with what digitized sound waves look like (since Vegas shows them to us). Sound consists of various frequencies. A low-pitch sound has low frequencies, so the wave changes slowly. High-pitch sound has high frequencies, so the wave changes rapidly. And most sound consists of a mixture of frequencies, so when looking at the image of digitized sound, we see a slow-changing wave overall (that is the lowest frequency in it) which, however, is not smooth, because the high frequencies are just added to the low frequencies.

Now, when digitizing sound, we can only record the frequencies that are less than half the sampling rate and have to cut off any frequencies above that half (called the Nyquist frequency). If we downsample the sound, say from 96k to 16k, we cannot just average several neighboring samples because that would cause aliasing. Any frequency above 8 kHz (the Nyquist frequency of a 16k sample) would be aliased, i.e., would result in a very low frequency wave and cause a hum that did not exist in the original. So we apply a low-pass filter, i.e., a filter that lets anything below 8 kHz pass into the output and removes anything above that. Audio software does it all for us without us having to think about it.

We don’t think of our digital images as waves because we tend to think of individual pixels with various levels of red, green and blue, and they are not in a line but in a 2D arrangement. But the same principle applies. When there is a slow change from one pixel to another, i.e., they are very similar, that is a low-frequency change. When there is a rapid change between neighboring pixels, that is a high frequency change.

In static photography we generally just combine several neighboring pixel and calculate their average. This causes some noise but our brains perceive the image as a whole and we often do not even notice the noise.

But in video we are watching one frame after another and when an object is moving, the aliased noise is also moving and then we notice it. So we need a low-pass filter to only pass the important visual information. By using certain blurs (such as the Gaussian blur) we remove the high-frequency changes and only keep the low-frequency changes, hence the name low-pass filter. When we then combine several neighboring pixels in the downsampling process, we do not see the blur anymore, but neither do we see the noise. There are methods of downsampling that include a low-pass filter (the Lanczos filter being the most popular), but AFAIK Vegas does not use the Lanczos filter for downsampling (though, I believe, HandBrake does).

Applying a Gaussian (or similar) blur before or after downsampling (but probably before is better than after) can help remove this aliasing. The trick is to blur it enough to remove the noise while still keeping the downsampled image sharp. That is why you need to try different levels and types of blurs (I would start with Gaussian) and use whatever is best with your specific footage. Footage with 1080 lines has roughly 16 times as many pixels as DVD. So downsampling throws out about 15/16 pixels. Hence the need for a low-pass filter.

I hope all this doesn’t sound too confusing.

EDIT: I see that Seth posted while I was typing this. That is why I have posted some of the same information. Sorry about that.

[Michael Threadgold]

Thnks guys for the information.I'll give it a go. I may be gone for some time! (see Scott expedition) Mike

[Michael Threadgold]

Hi, just to say thank you to Adam and Seth for their advice. Gauss. didn't work but handbrake did!
If I,d known I was looking at aliasing I could have read all five thousand posts. Sorry.
Why didn't I get this with HDV, was it Neo? Mike

[Graham Bernard]

HDV 1080 is 1440x1080 NON-Square Pixel. This WILL fill a 1920x1080 Preview.

Grazie

[Michael Threadgold]

Morning Graham,
Thanks for your response.I 'm sorry but I don't understand, I'm only an amateur and must have been lucky before with HDV.I just wondered if the conversion to Neo helped with the downscale and avoided what I now know to be aliasing. Michael.