Noise or aliasing problems on a 4K sensor?

Hi all again.

I have just been called a few minutes ago about a very well-know usage problem on 4K sensors, and especially in our FS700’s: customers use to claim that some artifacts or “moiré/aliasing” effects appear on their camcorders, especially after upgrading into v3.0 (that one that allowed to output 4K, amongst other new features).

Fortunately, those effects only appear when shooting in Full HD or 2K mode. Why do I say “fortunately”? Because it’s not a malfunction in the sensor, but it has a physical explanation.

If you have ever used a PMW-F55, you may have seen that there is an option called “CBK-55F2K”, which is actually an OLPF (optical low pass filter) of which cut-off “frequency” (actually, wavelength) is 2K.

F55body

As you know, cameras like FS700, FS7, F5 or F55 incorporate a native 4K sensor. In audio environment, if we want to record certain bandwidth (let’s say the typical 20 kHz), we need to filter it at its maximum frequency before sampling (and sampling frequency should be, at least, double that rate, according to Mr. Nyquist). That filtering process will avoid undesired frequencies to appear and cause artifacts on the final signal. Well, the same happens for optical signal: if we are shooting at 2K resolution, but our sensor is 4K (or, in a DSLR, much bigger), it is recommended to use a low-pass filter to avoid undesired artifacts/noise on that band that goes from 2K to 4K wavelength.

CBK-55F2K Filter - 3Q

So, for those models in which CBK-55F2K cannot be installed, it is recommended to use an EXTERNAL optical filter; CBK-55F2K is mounted between sensor and lens, and is easily detached or installed via screws. In FS700’s case, a 2K-cutoff wavelenght low pass filter should be installed before the lens, as if it was a regular filter.

NEX-FS700 3Q5

Finally, let me explain how I understand that the upgrade into 4K works on FS700. You may remember it was relased on its birth as a “4K-ready” camcorder. About 1 year later, we announced how it could be converted into a “real 4K” camcorder, allowing it to output 4K via its 3G-SDI output (you can see another article I wrote last week about how it works). That upgrade needs to be done via both software and hardware; that’s why it was usually charged (except in sporadic events in which we did it for free). In models NEX-FS700R and NEX-FS700RH, that upgrade was installed by default.

HXR-IFR5 with AXS-R5

[Image extracted from the V3.00 presentation when it was released]

Bear in mind this explanation is just “conceptual”, and is far away from being accurate and technical; just my understanding. This V3.00 firmware allowed to output RAW signal (2K or 4K) via a compressed dataflow (not actual baseband video) through FS700’s SDI output. Remember I’ve just said RAW. So, as the sensor outputs the signal, it is driven through 2 ways: first one, in RAW, into the encoder for outputting that dataflow through the BNC connector, and the second one, the regular one, to which we can apply signal modifications (picture profiles, including S-Log2, which is another improvement via V3.00 version, codec, HDMI output… all those typically “FullHD”). So, as you can see, sensor is “serving” signal to those two paths, but don’t forget it is a native 4K sensor, so, unless we work in 4K resolution (over AXS-R5 or Odyssey7Q), we are likely to find those artifacts due to the fact that, between 2K and 4K wavelengths, there is a gap in which such effects can be shown.

I just wanted to explain that those effects are actually not due to a malfunction on the camera, but a purely physical light behaviour. Also, sorry for being very “soft” about the explanation of how RAW path works in FS700; just wanted it to be comprehensible enough.

Thanks-a-lot for reading!

Alvaro Ortiz

Product Specialists Entry Level Camcorders and Monitors – Sony Professional Solutions Europe

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