jmac698
2nd November 2011, 00:17
http://www.sendspace.com/file/4gymw3
This is an experiment to answer the questions, what would a perfectly calibrated VHS look like?
What are the effects of tape, VCR, and capture card on noise, jitter, linearity, and
frequency response?
To answer the questions, custom software was designed and test recordings carefully measured.
The resulting technologies advance calibration far beyond the test patterns on typical
test DVD's. In essence, practically every measurable aspect of analog capturing can be optimized
to the fullest. These experiments are just beginning, but included is one example of it's
use.
How this video was created
First, the video was separated into it's component values (Y, Pb, Pr). Each image was recorded separately as a frame. It was also anamorphically squeezed.
Next, a special alignment signal was added to the video.
Upon playback, custom software aligned the video to within .1 pixel. While theoretically
perfect, the processing was affected by noise and distortion in the video.
Then the component frames were merged into a color composite. The anamorphic video was
unsqueezed. Finally the levels were perfectly stetched between the resulting black/white points.
The resulting video has only these distortions remaining:
-noise
-frequency response
-levels non-linearity
-some banding was introduced by the levels adjustment
-various other minor effects
Commentary on the Video
The video is fairly blurry as can be expected from VHS. The image is quite stable, but one
can see a slight movement in the edges of the colorbars. This can probably be eliminated
by further work in the software. The noise level is quite good for VHS. Sometimes there
is line jitter as the video exceeded the range of the software (this can be corrected too).
Finally, the bottom lines of the video look normal. In fact in the orignal, it was
badly slanted and had head switching noise. This part is normally cutoff, but now we
don't need to!
Further possibilities
HD
With this technique, pixel-perfect video can be recorded. This brings the possibility of
recording HD resolution. To do so, we would separate the HD image into 3x2 tiles or more.
Then each tile is recorded as a separate image. The tiles are then stacked back together
upon playback, increasing resolution 6 times. The only downside, is that the video takes
6 times as much tape!
The perfect alignment enables this technique because otherwise, it would be difficult to
piece the tiles together, as the borders would be wiggling into each other (due to jitter) resulting in obvious artefacts.
Noise reduction
We could also record the video twice, and average the noise upon reconstruction. If this
were done without alignment, the result would get much blurrier, as the two copies
would be overlapping each other.
Perfect linearity
Currently when you record brightness levels of 128,129,130 etc. we get back 128, 130, 130
for example, as the mid-tones are a little brighter than they should be. Though these
levels can be adjusted, there will be some banding as half of the mid-tone values have
disappeared. There could be a scheme of HDR where the shadows and highlights are recorded
as separate frames.
Frequency Response
Another test signal (frequency sweep) could be recorded to determine frequency response.
This can easily be undone with an FFT.
Other methods of encoding video
The alignment would be critical for other methods of recording video. For example the
video could be recorded as it's 2d FFT. In this case, mis-alignment would ruin the
results, possibly resulting in severe ringing effects.
Measuring VHS Parameters
Further test signals could determine the ideal noise profile of each VCR/Tape combination,
the jitter performance of a VCR/TBC/Capture card, frequency response, linearity, etc.
Using the VCR model to improve normal captures
Once we have an ideal model of these parameters, it can be used to tune normal VHS
captures to the highest quality. For example, a plain grey image with ideal noise
can be provided for use with the well-known Neat Video noise reduction program.
The frequency response profile can be used for sharpening. The linearity profile
can be used, with debanding, to improve the brightness curve (although this is usually
adapted to user preference anyhow).
Further work
There are many aspects to VHS recordings. We can further examine tint/saturation errors,
contamination between luma/chroma levels, chroma placement,
and other various linear/non-linear shifts within and between signals.
Example of my ideal TBC performance
http://screenshotcomparison.com/comparison/91487
You'll see some aliasing, that's due to a simple resizer to fix the jitter. On VHS you'd never notice.
This is an experiment to answer the questions, what would a perfectly calibrated VHS look like?
What are the effects of tape, VCR, and capture card on noise, jitter, linearity, and
frequency response?
To answer the questions, custom software was designed and test recordings carefully measured.
The resulting technologies advance calibration far beyond the test patterns on typical
test DVD's. In essence, practically every measurable aspect of analog capturing can be optimized
to the fullest. These experiments are just beginning, but included is one example of it's
use.
How this video was created
First, the video was separated into it's component values (Y, Pb, Pr). Each image was recorded separately as a frame. It was also anamorphically squeezed.
Next, a special alignment signal was added to the video.
Upon playback, custom software aligned the video to within .1 pixel. While theoretically
perfect, the processing was affected by noise and distortion in the video.
Then the component frames were merged into a color composite. The anamorphic video was
unsqueezed. Finally the levels were perfectly stetched between the resulting black/white points.
The resulting video has only these distortions remaining:
-noise
-frequency response
-levels non-linearity
-some banding was introduced by the levels adjustment
-various other minor effects
Commentary on the Video
The video is fairly blurry as can be expected from VHS. The image is quite stable, but one
can see a slight movement in the edges of the colorbars. This can probably be eliminated
by further work in the software. The noise level is quite good for VHS. Sometimes there
is line jitter as the video exceeded the range of the software (this can be corrected too).
Finally, the bottom lines of the video look normal. In fact in the orignal, it was
badly slanted and had head switching noise. This part is normally cutoff, but now we
don't need to!
Further possibilities
HD
With this technique, pixel-perfect video can be recorded. This brings the possibility of
recording HD resolution. To do so, we would separate the HD image into 3x2 tiles or more.
Then each tile is recorded as a separate image. The tiles are then stacked back together
upon playback, increasing resolution 6 times. The only downside, is that the video takes
6 times as much tape!
The perfect alignment enables this technique because otherwise, it would be difficult to
piece the tiles together, as the borders would be wiggling into each other (due to jitter) resulting in obvious artefacts.
Noise reduction
We could also record the video twice, and average the noise upon reconstruction. If this
were done without alignment, the result would get much blurrier, as the two copies
would be overlapping each other.
Perfect linearity
Currently when you record brightness levels of 128,129,130 etc. we get back 128, 130, 130
for example, as the mid-tones are a little brighter than they should be. Though these
levels can be adjusted, there will be some banding as half of the mid-tone values have
disappeared. There could be a scheme of HDR where the shadows and highlights are recorded
as separate frames.
Frequency Response
Another test signal (frequency sweep) could be recorded to determine frequency response.
This can easily be undone with an FFT.
Other methods of encoding video
The alignment would be critical for other methods of recording video. For example the
video could be recorded as it's 2d FFT. In this case, mis-alignment would ruin the
results, possibly resulting in severe ringing effects.
Measuring VHS Parameters
Further test signals could determine the ideal noise profile of each VCR/Tape combination,
the jitter performance of a VCR/TBC/Capture card, frequency response, linearity, etc.
Using the VCR model to improve normal captures
Once we have an ideal model of these parameters, it can be used to tune normal VHS
captures to the highest quality. For example, a plain grey image with ideal noise
can be provided for use with the well-known Neat Video noise reduction program.
The frequency response profile can be used for sharpening. The linearity profile
can be used, with debanding, to improve the brightness curve (although this is usually
adapted to user preference anyhow).
Further work
There are many aspects to VHS recordings. We can further examine tint/saturation errors,
contamination between luma/chroma levels, chroma placement,
and other various linear/non-linear shifts within and between signals.
Example of my ideal TBC performance
http://screenshotcomparison.com/comparison/91487
You'll see some aliasing, that's due to a simple resizer to fix the jitter. On VHS you'd never notice.