8 bit or 10 bit. My thoughts.

[Alister Chapman]

Over the years there have been many, often heated debates over the differences between 8 bit and 10 bit codecs. This is my take on the situation, from the acquisition point of view.

The first thing to consider is that a 10 bit codec requires a 30% higher bitrate to achieve the same compression ratio as the equivalent 8 bit codec. So recording 10 bit needs bigger files for the same quality. The EBU recently evaluated several different 8 bit and 10 bit acquisition codecs and their conclusion was that for acquisition there was currently no advantage to be gained by using any of the commonly available 10 bit codecs over 8 bit because of the data overheads.

My experience in post production has been that what limits what you can do with your footage, more than anything else is noise. If you have a noisy image and you start to push and pull it, the noise in the image tends to limit what you can get away with. If you take two recordings, one at a nominal 100Mb/s and another at say 50Mb/s you will be able to do more with the 100Mb/s material because there will be less noise. Encoding and compressing material introduces noise often in the form of mosquito noise as well as general image blockiness. The more highly compressed the image the more noise and the more blockiness. It’s this noise and blockiness that will limit what you can do with your footage in post production, not whether it is 10 bit over 8 bit. If you have a 100Mb 10 bit HD compressed recording and comparable 100Mb 8 bit recording then you will be able to do more with the 8 bit recording because it will be in effect 30% less compressed which will give a significant reduction in noise.

Now if you have a 100Mb 8bit recording and a 130Mb 10 bit recording things are more evenly matched and possibly the 10 bit recording if it is from a very clean, noise free source will have a very small edge, but in reality all cameras produce some noise and it’s likely to be the camera noise that limits what you can do with the images so the 10 bit codec has little advantage, if any.

I often hear people complaining about the codec they are using, siting that they are seeing banding across gradients such a white walls or the sky. Very often this is nothing to do with the codec. Very often it is being caused by the display they are using. Computers seem to be the worst culprits. Often you are taking an 8 bit YUV codec, crudely converting that to 8 bit RGB and then further converting it to 24 bit VGA or DVI which then gets converted back down to 16 bit by the monitor. It’s very often all these conversions between YUV and RGB that cause banding on the monitor and not the fact that you have shot at 8 bit.

There is certainly an advantage to be had by using 10 bit in post production for any renders or effects. Once in the edit suite you can afford to use larger codecs running at higher bit rates. ProRes HQ or DNxHD at 185Mb/s or 220Mb/s are good choices but these often wouldn’t be practical as shooting codecs eating through memory cards at over 2Gb per minute. It should also be remembered that these are “I” frame only codecs so they are not as efficient as long GoP codecs. From my point of view I believe that to get something the equivalent of 8 bit Mpeg 2 at 50Mb/s you would need a 10 bit I frame codec running at over 160Mb/s. How do I work that out? Well if we consider that Mpeg 2 is 2.5x more efficient than I frame only then we get to 125Mb/s (50 x 2.5). Next we add the required 30% overhead for 10 bit (125 x 1.3) which gives 162.5Mb/s. This assumes the minimum long GoP efficiency of x2.5. Very often the long GoP advantage is closer to x3.

So I hope you can see that 8 bit still makes sense for acquisition. In the future as cameras get less noisy, storage gets cheaper and codecs get better the situation will change. If you are studio based and can record uncompressed 10 bit then why not? Do though consider how you are going to store your media in the long term and consider the overheads needed to throw large files over networks or even the extra time it takes to copy big files compared to small files. For field production I think the NanoFlash hits the sweet spot with compact, clean, low noise files that are easy to store and easy to work with.

[Perrone Ford]

Nicely done Alister. Although I would be interested in knowing what the EBU considers "commonly available". Based on what I am seeing from my work, I am trying to move away from DCT based compression codecs and more toward wavelet. There is an efficiency there and a quality there that I believe would allow us to compress a 10-bit signal into the space of our current 8-bit codecs and still receive a boost in quality and performance.

We are currently seeing that from the Cineform folks both in the edit suite and from Silicon Imaging's use of the Cineform RAW codec. Similarly, RED is doing amazing things with it's wavelet based codec. Essentially giving us 4K based recording (with 4 channels of audio) at ~145Mbps. My own testing shows that on HD material, 50Mbps Jpeg2k (Wavelet) is visually indistinguishable from 220Mbit DCT (DNxHD).

So while I agree that staying 8-bit makes sense using the codecs that are commonly used today, including mpeg2 and mpeg4 variants, once the concept of wavelet based recording becomes commonplace, we'll be able to rethink this and go to 10-bit or 12-bit acquisition without much penalty and potentially, a lot of gain.

[Alister Chapman]

I agree Perrone. In the future it is likely that we will be using 10 bit codecs.

Here is the full EBU document:

http://www.ebu.ch/en/technical/trev/...rod-Codecs.pdf

And the EBU's summary:

A 10-bit bit-depth in production is only significant for post-production with graphics and after
transmission encoding and decoding at the consumer end, if the content (e.g. graphics or
animation) has been generated using advanced colour grading, etc.
For normal moving pictures, an 8-bit bit-depth in production will not significantly degrade the HD
picture quality at the consumer’s premises.

[Perrone Ford]

This statement was particularly telling:

For acquisition applications an HDTV format with 4:2:2 sampling, no further horizontal
or vertical sub-sampling should be applied. The 8-bit bit-depth is sufficient
for mainstream programmes, but 10-bit bit-depth is preferred for high-end acquisition.
For production applications of mainstream HD, the tests of the EBU has found
no reason to relax the requirement placed on SDTV studio codecs that “Quasi-transparent
quality” must be maintained after 7 cycles of encoding and recoding with
horizontal and vertical pixel-shifts applied. All tested codecs have shown quasitransparent
quality up to at least 4 to 5 multi-generations, but have also shown few
impairments such as noise or loss of resolution with critical images at the 7th generation.
Thus EBU Members are required to carefully design the production workflow
and to avoid 7 multi-generation steps.

[Justin Benn]

Thanks for this explanation Alister.

[Mark Job]

Originally Posted by Alister Chapman

The EBU recently evaluated several different 8 bit and 10 bit acquisition codecs and their conclusion was that for acquisition there was currently no advantage to be gained by using any of the commonly available 10 bit codecs over 8 bit because of the data overheads.

....What an odd conclusion. I guess the EBU didn't try any Primary & Secondary Color Correction then ;-) In order to get a specific look I want get (The Super 16 MM Film look), I have to set Cine Gamma 1 in my XL H1's rather extensive image settings, then play with the Knee settings and tweak the skin tone settings (All in 8 bit) in camera. In post I immediately transcode after finishing the picture elements editing and go to DNxHD 220 X (The *X* stands for 10 bit) and go into Media Composer's Color Correction Mode. Sometimes it takes me days of fiddling looking at a warmed up monitor before I finally get the look I'm searching for. If I stayed in 8 bit I wouldn't get there. I've noticed a big difference in Avid between the performance of 8 bit CC and 10 bit CC. Episode 2 of my Internet Tv Series Please Stand By was shot in 25 Mbps HDV (4:2:0 Color space), so I need all the help I can get. I'm also now looking into Color in the Final Cut Pro Suite to see what it can do for me, since Media Composer's Color Correction leaves allot to be desired IMHO. Apple's Color correction suite looks quite interesting. Avid Symphony Color Correction is Awesome. There has been some talk of Avid rolling Symphony into Media Composer, as they did with Avid Xpress Pro.

.....What I also found immensely helpful was shooting with the XDR, which did three things:

A) Raise my HD Raster from 1440 x 1080 to 1920 x 1080.

B) Increase my color space from 4:2:0 to 4:2:2

C) Lower the amount of compression I was recording clips at. from 25 Mbps to 160 Mbps.

In conclusion the XDR will transform to 10 bit recording once uncompressed recording is enabled, since all HD-SDI is inherently a 10 bit log input and since uncompression will bypas the XDR's encoding engine - Voila ! Our digital cinema shooting will be amazing in 10 bit !

[Alister Chapman]

If you read the report you will see that they are quite specific about there being little difference between 8 bit and 10 bit for acquisition, however they do recommend 10 bit for any post production. Colour correcting from 8 bit to 8 bit will very often lead to banding and other possible artifacts while going from 8 bit to 10 bit will not.

[Steve Phillipps]

Originally Posted by Alister Chapman

From my point of view I believe that to get something the equivalent of 8 bit Mpeg 2 at 50Mb/s you would need a 10 bit I frame codec running at over 160Mb/s. How do I work that out? Well if we consider that Mpeg 2 is 2.5x more efficient than I frame only then we get to 125Mb/s (50 x 2.5). Next we add the required 30% overhead for 10 bit (125 x 1.3) which gives 162.5Mb/s. This assumes the minimum long GoP efficiency of x2.5. Very often the long GoP advantage is closer to x3.
.

Nice explanation Alister, thanks.
Am I right in thinking that the AVC-Intra from the HPX2700 for example should by your calculations be rated as top draw, as its 100 mb/s I frame codec is reckoned to be about twice as efficient as other I frame codecs, so equating to a 200 mb/s mastering codec in 10 bit?

Steve

[Mark Job]

Hi Alister:
This makes sense. I have to go read that EBU report.

[Perrone Ford]

Originally Posted by Steve Phillipps

Nice explanation Alister, thanks.
Am I right in thinking that the AVC-Intra from the HPX2700 for example should by your calculations be rated as top draw, as its 100 mb/s I frame codec is reckoned to be about twice as efficient as other I frame codecs, so equating to a 200 mb/s mastering codec in 10 bit?

Steve

I'd place it behind wavelet at the same bitrate and HDCamSR which is also i-frame mpeg4 but at 440 Mbps and not subsampled for color. So no, not top drawer, but better than most.

[Mark Job]

Hi Perrone:
It was because of the HDCAM SR MPEG 4 quality that we want to capture to MPERG 4 compression in our SD Card SSDR. If you folks like MPEG 2 Long GOP, then you're going to faint when you see how good MPEG 4 can look ! IMHO Mpeg 4 blows away Long GOP Mpeg 2, and we all know just how good Long GOP MPEG 2 can look.

[Alister Chapman]

I have not seen any information to corroborate Panasonics claim that their I frame only codec is 2x more efficient than Mpeg 2. Side by side comparisons that I have seen show 100Mb/s AVC-I to perform no better than Mpeg 2 at 50Mb/s. In addition AVC Intra's predictive coding has a real problem with certain types of in-frame motion, for example fine repeating patterns or areas of fine detail moving across the screen at slow speeds.

[Mark Job]

Hi Alister:
Yeah, that's Panasonic's MP4 not Sony's. MP4 can be *Far* superior to MPEG 2, but no one is bothering to experiment with high data rate hardware encoded MPEG 4 (Except us and Sony). BTW, you can cram so much more onto an SD card at 2 Gig or 32 Gig size (Not to mention SDXC 1 Terrabyte Card) in hardware encoded MPEG 4 versus MPEG 2. There's also one even better than MPEG 4 and it's called MPEG 7 friends !

[Alister Chapman]

The Panasonic implementation is interesting as it appears to gets less efficient the more detail there is in the picture, which obviously is undesirable. I regularly encode HD using Mpeg4 at 20Mb/s and it always looks very good, but it's a swine to then re edit because of the CPU overheads to decode. Perhaps in the future there will be hardware decoders cards that can work at high bit rates.

[Perrone Ford]

Originally Posted by Mark Job

Hi Perrone:
It was because of the HDCAM SR MPEG 4 quality that we want to capture to MPERG 4 compression in our SD Card SSDR. If you folks like MPEG 2 Long GOP, then you're going to faint when you see how good MPEG 4 can look ! IMHO Mpeg 4 blows away Long GOP Mpeg 2, and we all know just how good Long GOP MPEG 2 can look.

Is it REALLY so difficult to license Jpeg2k compression? I'd MUCH rather deal with that in post than ANY mpeg4.