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Getting SD from HD and the problems of oversampling.

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Ever since the release of the XDCAM EX cameras users have been having problems getting good looking SD pictures out of downconverted HD.?Why is this and what can be done about it? This is an issue that effects all high resolution HD cameras and is not unique to the EX’s. There are two key issues. The first is the way basic software converters handle fields in interlace material and the second is the amount of information in an HD image that must in effect be discarded to get a SD image.?At first glance you would think that starting off with lots of picture detail would be a good thing, but in this case it’s not. Let’s see if I can explain.?Imagine that you have something in you HD picture that over 4 pixels goes from light to dark, in Hd you get a gradual transition from light to dark and all looks good. Now what happens when you take those 4 pixels and convert them to SD. The 4 pixels become just 2 and instead of a stepped change from light to dark the picture now goes instantly from a light pixel to a dark pixel. If these pixels were the edge of a moving object, as it moved the pixels would be switching instantly from on to off and unless the object moved at exactly one pixel per frame you will get a flickering effect. Clearly our nice gradual transition from light to dark has been lost and if there is any motion we may now be seeing flickering edges. Niether of these look good.

Take a look at these images:

hd-sd-full-frame Getting SD from HD and the problems of oversampling.
Original Frame showing box with area of interest
hd-sd-original Getting SD from HD and the problems of oversampling.
Original HD Image
hd-sd-sd-no-blur Getting SD from HD and the problems of oversampling.
Same image, downconverted to SD

As you can see the down converted SD is very blocky and there is some strange patterning (aliasing) going on amongst the bricks of the houses in the background. This does not look good and if there was motion the brickwork would shimmer and flicker.

So what can be done?

Well the best way to improve the SD down conversion is to soften the HD image before it is down converted to prevent this single pixel light to dark switch from happening. You need to end up with an SD image where you go from full light to full dark over at least 3 pixels to prevent flicker (Twitter).

How much you will need to soften you HD by will depend on how sharp it is to start with. Simply turning down the cameras detail settings can be a big help, but even then the best results are often obtained by applying some kind of blur filter in post production. In FCP I find the flicker filter works quite well. As you can see from the frame grab below the difference in the quality of the downconvert is quite striking.

hd-sd-SD-blur Getting SD from HD and the problems of oversampling.
SD Image created by adding blur to HD before conversion.

I have also found that another problem is that the detail settings on an HD camera are not optimised for SD. The detail correction edges created in HD are very thin and when these are down converted to SD they all but disappear and can cause further aliasing. The solution is to make the detail correction edges thicker (on an EX turn detail frequency down to -60 to -99) but this then looks ugly in HD. The bottom line is that a camera optimised for HD works best in HD and SD will be a compromise.

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PMW-350 – Good for Indie Movies?

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I have been giving a lot of thought to the PMW-350 and whether it fits in with what I do. I’m really, really torn. While I love my PDW-700 and the optical disc workflow I also really liked the PMW-350 package. Now to bring my PDW-700 up to a similar package would mean purchasing a colour viewfinder (C35W £6,500) and a new ALAC compatible HD lens (£8k +). The cost of these options is around the same price as a PMW-350 kit with lens. The 350 has a wonderful colour VF as standard and is very nice to use. It has much lower power consumption than the 700 and weighs a lot less, which would be very nice for me with all the traveling I do.

350-nano-right-side-300x200 PMW-350 – Good for Indie Movies?
PMW-350 – With Convergent Designs NanoFlash

The downside to the PMW-350 is the 35 Mb/s data rate and the use of CMOS sensors. The 35 Mb/s issue is easy enough to get around as I have a couple of NanoFlashes which can record at higher bit rates, making the 350 suitable for HD broadcast without any issues. The CMOS sensors, for most people would not really be an issue, especially now Sony have incorporated flash band removal into the clip browser software, but for me it’s important because of the amount of lightning I shoot. Certainly I can get good results with my EX1 and EX3 but I really don’t know yet how the 350 will perform.

While thinking about all this, it occurred to me that the 350 would probably make a really good camera for Indie Films. The use of 2/3? sensors means that it’s easier to get a reasonably shallow depth of field compared to 1/3? or even 1/2? cameras, without having to resort to 35mm lens adapters. The very high resolution images with very, very low noise would certainly look good projected on to a big screen. If you took the HDSDi output and record it on a NanoFlash at 100Mb/s or higher then it will grade very well. The lack of camera noise (59db) means that you can really push and pull the picture very hard during grading before it will degrade. Even if you only record at 35 Mb/s this low noise floor is going to help with grading as less noise means less stress on the codec in the first place. On top of all that the supurb sensitivity means that you will be able to use very minimal lighting rigs, perhaps just using practicals to light a scene, which should really give you more scope in your composition.

Further PMW-350 advantages include the use of Sony’s Hypergammas, step gamma and multi-matrix. These settings are normally only found right at the high end of the product range and give you excellent control of the look of your images. Add to that 24P (23.98P) and the ability to overcrank and undercrank and it all adds up to an extremely capable camera.

So, will I be getting a PMW-350? Well if I didn’t have my PDW-700 I would certainly be getting a 350. At the moment I’m still undecided….. if only I could work out a way to afford both.

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Working with XDCAM EX (or other file based) material.

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The one big question of everyones lips is how do you archive your material? Well here are my thoughts and some ideas.

The most important thing is to think ahead and plan your end to end workflow. You also need to consider the fact that hard drives will almost certainly fail at some point (maybe not now but in the future) and the vast majority of problems are due to human error. Possibly simply forgetting to copy something or not fully understanding the workflow.

Some Golden Rules, no matter what format or workflow you are using are:

Copy EVERYTHING off the card, keeping the original file structure.

Ensure all copies incorporate some kind of error correction or error checking.

Don’t skimp on the quality of your backup system.

Check, check and double check your workflow before you start shooting.

So you go on a shoot and start filling up you expensive memory cards, at some point you will have to start off loading your material onto something else. In the field this is likely to be hard drives of some sort. Backing up to a single hard drive should only be done as a last resort or for media that you don’t mind loosing. You have several options here, you could use Shotput Pro to backup to single or multiple drives. I really like shotput as you can use it to eliminate a lot of user errors. For a start shotput can be set to backup to multiple locations simultaneously from the source media. Then once it has made the copies and verified the copies it can, if you wish, format the card, ready for re-use. Allowing Shotput to format the media helps prevent human error. How? Well if I ever put a card in my camera and find it has footage on it, it means that card has not been backed up and verified by Shotput. This is better than backing up yourself as there is always the risk of a mix up between backed up and not backed up cards. The other way to backup with a computer is to use the Sony XDCAM EX Clip Browser. You should never use the windows explorer or Mac finder to backup your valuable media as there is no form of error checking. Clip Browser has built in error checking which is enabled under the preferences tab.

A further option is to use a dedicated backup device such as the NextoDi products or soon to be released Sony PXU-MS240 backup device. These are easier to use than taking a laptop into the field. The NextoDi devices can backup to 2 drives at once (full review of the NVS2500 comming soon) and the Sony device backs up to removable esata drive cartridges.

So what sort of hard drives should you use? Well I am currently using pairs of USB Western Digital “Elements” hard drives. Where possible I use 3.5? drives as opposed to the smaller 2.5? laptop type drives. These are low cost yet so far have proven to be reliable and of good quality. The larger 3.3? drives should be more reliable, but they are bigger and bulkier and require mains power, so in the field I use the 2.5? drives. By storing these drives at separate locations, one at home and one in the office, I have a very safe system. If my office were to burn down or get flooded, I would have a spare copy at home. Over time however these drive will fail so every couple of years I move my footage on to new, larger hard drives. Another hard drive option is to use G-Tech G-Raid drives. These units contain two separate hard drives and can be used in raid 1 mode so should one of the drives fail your data should be safe. The cost is similar to using a pair of drives and it’s certainly less fiddly than using pairs of drives but it doesn’t give the security of separate storage locations. If you are doing corporate videos then you could consider selling drives to your clients. The client then keeps the drive and as a result you are no longer responsible for it’s storage or safety, just like if the client kept your rushes tapes.

For longer term storage, again there are many options. I backup a lot of my material to BluRay discs. This is not a fast process, use high quality discs and you should be good for 20+ years. Another option is to backup to Sony Professional discs using a Sony PMW-U1 drive. This is a lot faster than most current BluRay burners and the discs are protected in a rugged caddy. Sony claim a life of 50 years for the discs so it is a very good long term storage solution. The new Sony PMW-350 and EX1R as well as the Convergent Design NanoFlash (next firmware release)  have shooting modes that allow footage to be saved on XDCAM discs (Sony Professional Discs) as video clips and not just data files. Using these modes you can put the discs in a player and play back the material directly.

A further long term storage solution is LTO tape. It seems strange to be going back to tape, but LTO4 tape is very reliable and widely supported. It’s not suited to applications where you need quick access to your footage, but is very good for long term security. A good compromise may be one copy on a hard drive as a working copy along with a backup on LTO for archive.

Raid Arrays can be used for long term storage, but even Raid arrays can fail. If the lookup table becomes corrupted it can be next to impossible to recover the data off the discs, so do be careful. Do remember however you store your footage try and be organised. Store your material in a sensible folder structure that will help you find your rushes quickly and easily. If you are out shooting for a day you may be generating a hundred or more files, do that day in, day out and you will generate thousands and thousands of files. Make sure you work out you clip naming and clip prefixes in such a way that you won’t get duplicate names and can find your footage quickly and simply.

And just one more reminder, always save the full file structure. In the case of XDCAM EX keep the full BPAV folder and all it’s contents, also don’t rename the BPAV folder. Even if you edit on a Mac and use the Sony Transfer Tool to make .mov files you should keep the BPAV folders as trying to edit the  .movs on a PC or AVID is a nightmare. If you have the original material you can easily work with it on any platform.

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The 8 bit or 10 bit debate.

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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 little 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 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 for acquisition, 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, grading 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. Also 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.

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EX3 With NanoFlash Camera rig.

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I have been asked to show how I mount my NanoFlash on my EX3 so below are a couple of pictures of the full rig with some details of some of the various items that I use.

EX1-left-side-small EX3 With NanoFlash Camera rig.

1. This is my Petroff 4×4 matte box with bellows hood. I love the old fashioned style bellows lens shade as you can adjust it in and out very quickly to eliminate stray light. A french Flag can be added if desired, but I find in most cases that the bellows shade is fine. Stray light causes reflections both between filters and within the lens, this can reduce contrast in the image so a good lens hood is essential for getting the best pictures. The Matte Box has 2 rotating filter holders. I often use some very gentle blue or grey grads to help with bright skies.

2. The standard EX3 lens. This lens is a remarkably good lens. Keep it between F8 and F2.8 for best results. The sweet spot is F4. Never use the Iris at F16 or F11, your pictures will be soft due to diffraction limiting. This is not a lens fault but something that would happen with any lens and 1/2? sensors (it’s even worse with smaller sensors). The beauty of the standard EX lens is that it incorporates automatic chromatic aberration correction which means no nasty blue or purple fringes around areas of high contrast.

3. SxS Cards. Even though I was one of the first people to work out how to use low cost SD cards with the fabled Kennsington Adapter I still use SxS cards. The reason is simple: reliability. If you look around the forums you will find lots of people having issues with SD cards. For example a wedding videographer that lost a large part of a service he had shot. If he had used SxS then that just would not have happened. I am a professional and my reputation is vital. That reputation could easily be destroyed if I came back from a shoot with nothing but corrupt data because I had tried to cut corners.

4. Kata Camera Glove. Protects the camera from everyday knocks and bumps as well as unexpected rain or dust.

5. The Cheek Pad. I know many EX3 users don’t fit these. It looks flimsy, but in use it’s strong enough and it really helps to stop the cameras tendency to want to tilt to the left. It makes the camera much more stable and really is worth trying.

EX3-right-side-small EX3 With NanoFlash Camera rig.

6. Convergent Design NanoFlash. This incredible little box allows me to record from the HDSDi output at upto 160Mb/s long Gop. At 100 Mb/s you can’t tell the compressed from the uncompressed. By shooting with this I can grade and color correct my footage, make dubs, go multi generation without seeing any drop in quality. It also means my footage is accepted for HD broadcast by the BBC and the majority of other HD broadcasters. The EX 35Mb footage is good, don’t get me wrong but the 100Mb is sweet. It uses inexpensive Compact Flash cards and by recording to both SxS and the Nano at the same time I can be sure that even if I were to get a card failure I have a backup.

7. IDX V-Lock batteries. One of these will run this rig for around 5 hours. I am also looking at getting some of the Swit EX batteries with the Power-Con/D-Tap out to run the EX and NanoFlash, but by putting the battery out behind the camera the overall balance of the rig is improved. In fact without this larger battery the matte box tends to make it very front heavy.

8. Quick release shoulder pad. This is a home brew affair that incorporates a shoulder pad the V-lock battery adapter and NanoFlash or Radio Mic mount. It can be removed without tools in seconds. Perhaps one day I will get some Zacuto rails or similar, but they are rather expensive and this setup works very well for me.

9. Sony ECM-680S. This is a nice Stereo/Mono switchable gun mic. It is great for capturing nice stereo ambience and effects sound. Flick the switch and it becomes a useful interview mic.

10. DM accessories EX3 reinforcement plate. This is a MUST HAVE item. It provides you with a much stronger tripod fixing with both 1/4? and 1/2? threads as well as a host of other threaded holes for various applications. It makes the camera feel so much more solid on a tripod. I really can’t recommend it enough. below this is a cheap, Indian made Matte Box rail kit. It works, but again I could really do with some Zacuto rails or similar.

11. Chamois leather viewfinder cover. Much nicer against your skin than hard rubber. Also absorbs sweat and moisture which helps prevent the viewfinder from fogging. Another option is a small sweat band.

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Low Cost 2/3? HD Lenses.

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OK, here’s my take on the situation.

If money is no problem then the safest bet is to purchase a good quality HD lens, expect to spend at least £8k.

If you budget is restricted then the situation is much less clear. There are now several low cost 2/3? HD lenses designed for cameras such as the Panasonic HPX500. In my opinion these lenses are just not worth the money. They might be cheap (£4k ish) but the one’s I’ve played with have been pretty grim, suffering from lots of CA and soft corners.

If your on a tight budget the best thing you can do is take your camera to a good dealer and go through their second hand lenses, trying them on the camera. Check for resolution (use a chart), corner softness, CA and contrast. I did this and ended up with a Canon 16x8x2 IF lens. I found that lenses with lower zoom ratios tended to be better than those with higher ratios. I’m really pleased with my lens and when compared to the latest HD equivalents I can not tell the difference in real world use. It certainly outperforms all the budget HD lenses I’ve tried.

One interesting thing that I have discovered in my research into this subject is that Contrast is what makes the biggest difference in lens performance, not simply resolution as one might expect. Visually the next thing you notice is CA. This is a tough one as when you increase the resolution or sharpness of a lens you also tend to increase the CA.

Until lens manufacturers start to release MTF curves for their lenses the only thing we have as buyers to go on is the advertising blurb. It’s easy for a manufacturer to claim improved performance or new glass or other technology, but without accurate MTF curves it’s all pretty meaningless. You would only need the tiniest resolution improvement to be able to claim that you new HD lens range is sharper than your SD range, it could just be a fraction of a percent difference.

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Arizona Count Down

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I’m off to Arizona on Thursday for a long weekend to shoot the monsoon thunderstorms. I had postponed this trip from earlier in the month when the weather just wasn’t right. This tim around the forecast is really good (for storms that is). I will be shooting with my PDW-700 and my EX1. I will also be giving the Nanoflash a shake down making good use of the cache function to capture what I hope will be some great lightning storms.?Anyone want to join me? I have a couple of spare seats in the car and motel rooms are peanuts.

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Stetting up video cameras without charts.

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There is far too much emphasis on color charts and 100% one to one – set it up with a scope settings. Very often a 100% accurate one to one response won’t look right as the video gamut is smaller and lopsided than that of the human eye so a small amount of skewing of the color gamut can often help produce a picture that visually looks more natural. One of the very best ways to set up a camera is to use a high quality color photograph of a known scene. Shoot the photograph and look at the picture on a monitor and adjust until it looks right. This will give a more natural looking image than aligning with charts and scopes and is a technique that has been used since the very beginnings of color television. I have a scene that contains vibrant colored cars, green fields and trees, buildings and blue sky. I have a dozen large copies of this picture and use it whenever I am making camera adjustments to make sure my pictures still look natural. Of course scopes should still be used if you are making any extreme settings to ensure your images are still legal, but at the end of the day what you are after is an image that looks right too you (or the producer) and whoever else will view your material, not what looks right according to a chart and a scope.