It looks like combing but the bands are too big for 576 lines.

Yes, it is not interlace tearing, as each wave covers a number of scan lines. Below are the screenshots you asked for:

Project Properties - Deinterlace Method - None? Your source is interlaced. Try Interpolation.

Downscaling HD to SD is always very disappointing. But despite the upperfield of your HD, you should render your SD to lowerfield and not to upperfield. So do not change the original SD mpeg2 rendertemplate.

Also check your mediaplayer, I can't see those wobbling lines when I downscale 1080i to 576i at my place (WMP), also not with your settings.

Choosing a deinterlacing method should fix the problem.

Yes, one must select a deinterlace method when scaling.

Also, Render quality should be set to Best.

Thank you to all who helped, and in particular to ERICLNZ who had the right answer. 🙂

Did you change the deinterlace settings in the project settings to none, manually?

Seems that, in my opinion, an everlasting bug in Vegas is still present. Depending on how you start a new project, the deinterlace changes from default blend fields to none. When starting a new project by clicking the New empty project button, the deinterlace mode is blend fields. When starting a new project by selecting File/New the deinterlace method changes to none.

A Vegas booby trap, because this causes problems when rendering, as seen above, even when deinterlacing is not used, like rendering from interlaced to interlaced as Alderny did.

Did you change the deinterlace settings in the project settings to none, manually?

Thanks, and no I didn't change the deinterlace settings in the project to none, nor have I had to check this setting before. I believe I started the new project by launching Vegas and adding clips, then saving it with a name.

[...]even when deinterlacing is not used, like rendering from interlaced to interlaced as Alderny did.

If scaling is involved, the interlaced source has to be deinterlaced, then scaled, then again interlaced. Otherwise you get those "big wavy lines" @alderny mentioned. That effect is called aliasing and is the sign that no deinterlacing was applied prior to scaling like @EricLNZ noticed.

(Maybe a bit too picky, but perhaps this helps understanding...)

Did you change the deinterlace settings in the project settings to none, manually?

Thanks, and no I didn't change the deinterlace settings in the project to none, nor have I had to check this setting before. I belive I started the new project by launching Vegas and adding clips, then saving it with a name.

When launching Vegas, deinterlace method is also always default blend fields (when Vegas has not manually been forced to start new projects with other settings). It only suddenly changes to none when starting a new project by File/New.

I didn't know that for scaling first a deinterlace of the interlaced source is needed. This explains the unexpected bad result when Vegas deinterlace method is switched to none without having noticed.

As an aside I wonder why the Vegas default is Blend Fields and not Interpolate Fields. I've always interpolated and other consumer editors I've used or tried usually have this as the default. It preserves my original 50fps for smooth movement and avoids soft edges from the blending where there's slight movement.

I also preferred interpolate fields instead of blend fields. Since Vegas 14 or 15 a new deinterlace method option was introduced: smart adaptive, which I prefer now.

Since Vegas 14 or 15 a new deinterlace method option was introduced: smart adaptive, which I prefer now.

I tried to use the 'smart adaptive' deinterlace method, but on my PC it rendered very slowly then crashed Vegas. Perhaps my graphics card (NVIDIA GeForce GTX 750) is not up to the task. Would a more powerful graphics card make much difference to how Vegas runs? My PC is based on Intel Core i7 4790K @ 4.00GHz, with 8.00GB Dual-Channel DDR3 @ 799MHz. By the way, in the video preferences I too have had to set the 'dynamic RAM preview max' to zero, to prevent Vegas freezing during renders. Once this happens I have to use the Windows task manager to kill Vegas.