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aliasing and frame rate questions / editing with the LZX
MUFF WIGGLER Forum Index -> Video Synthesis  
Author aliasing and frame rate questions / editing with the LZX
jjr
So at times I notice "small notches" or jagged pixels with the LZX along both clean edges and also more "organic" curved patterns and shapes - these would all be coming out of VWF generators. What causes this ? also what are strategies to minimize it ? I know that with the sign wave out and the triangle out I see it rarely or not at all.

Also - once I was recording an LZX test on to a mini dv casette that I later digitized onto my laptop in Final Cut Pro. There was one portion of the image that when recorded - was pulsating intensely and ou could almost see feedback mixed in. When I imported this in to a NTSC 29.97 timeline the pulsing was totally missing. Finally I changed the frame rate of the timeline to 60 fps and it returned. but 60fps presents many challenges as a workable format. Any idea what might be going on here?

do you all de-interlace your footage when working with it in final cut?
what do you find give you the best results in terms of editing settings? strategies ?
lizlarsen
Hey Jimmy!
Great thread topic.

Everything you mention applies equally to the LZX system and other video synthesizers -- these are just byproducts of analog video and synthesizing it directly to NTSC/PAL interlaced video (in other words, they come with the territory and are arguably part of the appeal.) But yes there are things you can do -- it's a matter of understanding how the signals you are using to create your video are related to the resulting video signal itself.

Quote:
So at times I notice "small notches" or jagged pixels with the LZX along both clean edges and also more "organic" curved patterns and shapes - these would all be coming out of VWF generators. What causes this ?


This happens when you have a hard-edged waveform in the audio range, creating horizontal bars (either sync'ed to Field rate/Vsync or unsync'ed.) The reason why this happens is that the edge of the waveform occurs during the middle of the line somewhere as it is displayed (instead of at the beginning of the line). This is because the waveform is not an exact division of the line rate. The reason you don't see it with triangle and sine waveforms is because there is not an abrupt edge that happens so fast that the change is visible inside the line. If you want to avoid this effect, there are two immediate solutions:

1) Low pass filtering. Send the hard-edged waveform thru a low pass filter and slew it a little until the hard edge disappears before adding it to your video mix. There are some LZX filter modules coming out soon but for this purpose, any audio-range filter will work.

2) Sample & Hold. By sample & holding the waveform, with S&H triggered by line rate, it forces the voltage to be stable across the entire line. We have a 6-channel S&H that works for this and other purposes coming out soon.

It's also worth noting that this effect can be seen in play when using a square-wave LFO to modulate video too... but in a different frequency range. There's nothing to force the hard edge to occur at the top of the frame, so it happens at random times in the middle of the frame which can be undesirable if you want clean strobing. Using a clock divider to divide down frame (odd field) rate or using Sample & Hold triggered by frame rate on the incoming waveform are both ways to force a signal to switch cleanly at the top of the frame (the above mentioned 6-channel S&H module will work for this too.)

Quote:
There was one portion of the image that when recorded - was pulsating intensely and ou could almost see feedback mixed in. When I imported this in to a NTSC 29.97 timeline the pulsing was totally missing. Finally I changed the frame rate of the timeline to 60 fps and it returned. but 60fps presents many challenges as a workable format. Any idea what might be going on here?


This sounds like some issue of what's happening with the captured video and how the software is playing it back, in terms of interlacing. Remember that you are creating interlaced video inside the system -- yet the video waveform generators are linear (in other words "progressive"). So is any audio you send in as a modulation source. When dealing with some frequencies or modulation sources, what happens is you will end up with a double image sometimes. Or flashing that relates to interplay between the two fields. My guess is that at 29.97fps in FCP, there was some sort of deinterlacing playback going on, that needs to be turned off or adjusted (can you upload clips to show what you mean)? There can be a lot of variation in quality and how interlacing is handled between software programs and capture devices from what I've seen. Some LCD monitors may try to deinterlace the footage too, which can lead to an inaccurate look at the actual signal you're creating.

Quote:
do you all de-interlace your footage when working with it in final cut?

Typically I find a good deinterlacing algorithm helps a bit. But sometimes I use interlacing as part of the effect to purposefully create a doubled image (try FM'ing a VWG using the odd/even outputs of the VSG to see what I mean!) Here's an example of an image I created that purposefully uses interlacing to create a doubled image:
daverj
It all depends on what the final use of the video is. If you are putting it on the web you usually want to de-interlace because the frame rate from most web players can't keep up with interlaced video and usually create lines from skipped fields.

But if the video is to play on a monitor then it's usually better to keep it interlaced. Though, as Lars points out, some LCD screens attempt to deinterlace flicker on their own and that causes visual problems.
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