Stop Worrying About Compression With an On-Camera Video Recorder
Full Raster High Definition Video has six times the number of pixels as SD video. Theoretically it should take up six times as much space as SD. However, AVCHD has a slightly lower bitrate (24 Mb/s) than the DV codec used on most of the SD tape cameras of yore (25 Mb/s). How is this possible? Compression, and lots of it.
In most situations, compression is your friend, but in some situations it can lead to frustration. Unfortunately, (with the exception of the Panasonic HPX Line) all cameras below $15,000 record heavily compressed footage. Because of this limitation a number of external video recorders have arisen to free videographers from the shackles of compression. But the feature set, price and workflow of these recorders vary wildly, so choosing one can be tough.
This article will explain what advantages there are to using an external recorder and how to choose the best one for a particular camera. It will also briefly go into the capabilities of the various on-camera recorders B&H sells. It is important to note that we are only going to look at recorders that are designed to record in a higher-quality format than what is recorded in camera. There is another subset of on-camera recorders that record the same video codec as is recorded in camera to a disk or card in order to bypass recording to tape or to record for much longer.
Before diving into the capabilities of each recorder, it is important to understand what methods are used to compress video, and what detrimental effects they can have on the image.
One of the oldest ways to cut down on the amount of information in a frame is through chroma sub-sampling, which has been around since the analog days. It reduces the bandwidth of video by skipping the color information on some pixels, depending on the level of sub-sampling. A luminance value (brightness/darkness ratio) is still recorded for every pixel. A 4:2:0 image, which is the chroma sampling method used in AVCHD and XD CAM EX, only changes color every fourth pixel. Where 4:2:2 sub-sampling changes color every other pixel. 4:4:4 color sampling retains all color information. While only having color information for every fourth pixel may sound extreme, it is actually not as noticeable as you may think, as is demonstrated in these images:
|500% blowup of this image in two colorspaces:|
|4:2:2 Color (Canon XF Codec)||4:2:0 Color (AVCHD)|
The eye is much more susceptible to differences in luminance, so the loss of color information is very hard to spot, but it is there. This is why almost everything is distributed in 4:2:0. However, if you plan on doing heavy post work such as green screening and color grading, then it is much better to have footage that is 4:2:2 or higher, because the more you push footage the more sub-sampled color shows.
If you have a camera that shoots AVCHD, XDCAM EX or HDV then your camera records internally at 4:2:0. However, if your camera outputs uncompressed video from its HDMI or SDI connector, then it will output in 4:2:2. Also, all of the recorders in this article record in at least 4:2:2, so if 4:2:2 is what you are after then don’t let that narrow down your search. If you are looking for 4:4:4 then either the Convergent Design Gemini Recorder or the Sony SR-R1 is what you are after, but make sure your camera can output it.
Color Bit Depth
Color Bit Depth is another element that influences bandwidth. Most cameras record internally with 8-bit color. This means that each variation in color is defined as an 8-bit value, or 8 numbers in binary. This leads to 256 possible variations in color hue. 10-bit color, as you might expect, uses a 10-bit value which leads to 1,024 possible variations in color hue with just a 20% increase in bandwidth. Similar to 4:2:0 color sampling, 8-bit is common for distribution and doesn’t look much different from 10-bit color, to the eye. The large amount of extra color information in 10-bit color can make a huge difference in color correction. However, a camera’s image sensor has to be able to resolve the difference in color hue or the extra color bit depth will just show an increased amount of noise. It is for this reason that most cameras do not record at or output 10-bit color, with a few exceptions. All of Sony’s XDCAM EX line output 10-Bit Color through their SDI ports. The 2/3”-chip models and the F3 will benefit the most since the large sensors will produce less noise. It is also worth noting that all of Panasonics P2 Card cameras record 10-bit color internally. As far as recorders go, every recorder except for the Convergent Design NanoFlash record in 10-Bit Color. But currently there aren’t many cameras that output 10-bit color. Here’s looking to the future, though.
A 1920 x 1080 video signal with 4:2:2 sub-sampling and 10-bit color depth will take up a huge amount of space, about 10GB per minute. And there are a few recorders available, such as the Blackmagic Design Hyperdeck Shuttle 2 that will record fully uncompressed video. But if you are looking for something that takes up less space than 600GB an hour, then you probably want some form of compression applied to your image. The question is, what type and how aggressive do you want it to be?
Intraframe Compression was the first type of digital compression to appear. It compresses each video frame independently. DV, DVCPRO, AVC Inta, ProRes, DNxHD and MJPEG are all intraframe compression codecs. Modern intraframe codecs can achieve virtually lossless results at relatively low bitrates (compared to uncompressed video). For example, ProRes 4:2:2 is a 10-bit 4:2:2 codec that records at 220Mb/s which equates to about 1.65GB per minute. This is considerable in size, but still way less than the 10GB needed to record a minute of uncompressed 10-bit 4:2:2 video. The problem arises when you’re recording at a much lower bitrate. Intraframe doesn’t hold up well, which is why another compression type exists.
What do AVCHD, XDCAM, H.264, Canon’s MXF codec and HDV all have in common? Interframe, or Long-GOP compression. Interframe takes all the benefits of intraframe compression and takes it even further by combining several frames together into a group and recording only the differences between the frames. This leads to a huge gain in quality at lower bitrates, compared to intraframe codecs. Interframe codecs have a few disadvantages, though. They take a lot more computer muscle to edit, which is why many editors choose to transcode them into a high quality intraframe codec such as ProRes to edit with, which takes time. The second reason is that interframe codecs can degrade in quality when there is a high amount of variability between different frames. In some instances, such as shaky handheld footage or scenes with large crowds, interframe codecs will “break” and become pixelated. The effect can easily be seen on sites such as YouTube or Vimeo. Luckily, the codecs used in professional video cameras are much less susceptible to breaking than what you usually see online, and when they do it isn’t as noticeable. For an example, look at these frame grabs from a shot with very fast motion:
|Frame Grab Comparison|
|Interframe (AVC-Intra)||Interframe (AVCHD)|
As you can see, the AVCHD footage begins to block up into macro-blocks because the speed at which this scene is changing between frames exceeds the capability of AVCHD’s bitrate; the intraframe footage is frame independent so it doesn’t matter how much motion there is. Using an external recorder will allow you to always record in an intraframe codec so you don’t have to worry about your codec breaking. Also, you can skip transcoding your footage to a more editing-friendly codec to save time.
What Recorder Should I Get?
Before picking out an external recorder it is important to consider how your camera outputs uncompressed video, if it does at all. If your camera outputs 10-bit color then you probably want to steer away from any 8-bit recorders, but if it only outputs 8-bit color then that isn’t an issue. If your camera only outputs 4:2:2, then getting a 4:4:4 recorder will just make your recording files larger. If you plan on shooting progressive footage, it is important to check how your camera outputs progressive footage, especially if you plan on recording through HDMI.
Many cameras output progressive footage in a 60i video signal. Over SDI there is a standard for alerting the recorder or monitor that the incoming footage is really progressive and that pulldown removal should be done to the footage. Therefore, if you are recording via SDI the chances of a pulldown mishap are vastly lower (though in some instances you may still run into an issue). Over HDMI there is no standard. Sony has started implementing its own way of flagging progressive footage in its NXCAM cameras, such as the FS100, and there are now a few recorders that work with them. But in most cases progressive footage will be recorded to an external recorder as 60i over HDMI, and so you must go through an extra step in post to de-interlace the footage.
Also consider what editing platform you are using. The two most common codecs you will see on recorders are ProRes and DNxHD. Both are 4:2:2, 10-bit, Intraframe codecs that can record up to 220 Mb/s. They are very similar codecs, except ProRes is owned by Apple and DNxHD is owned by Avid. If you are a Final Cut editor, then ProRes will lead to an easier workflow and if you are an Avid Editor, DNxHD will. Keep these things in mind as you choose which recorder will work best for you.
AJA Ki-PRO Mini Compact Field Recorder
The AJA Ki-PRO Mini is a small external recorder that records to CF cards. It is important to use approved CF cards or your footage may have dropped frames. It records variants of both ProRes and DNxHD via either HDMI or HD-SDI inputs. You can add additional audio tracks via its XLR or RCA audio inputs. The Ki-PRO Mini is powered via 4-pin XLR, and a number of powering options are available. AJA has updated the firmware of the Ki-PRO Mini to work with various new cameras, such as the Sony FS100U, so it’s a good purchase if you plan on keeping the recorder longer than your camera.
AJA Ki-PRO Portable ProRes File Recorder
The AJA Ki-PRO Portable stretches the definition of camera mountable, but if you are willing to deal with the extra heft, the original Ki-PRO can double as a VTR for studio use as well. It has just about every form of video input you can think of, including analog. The Ki-PRO records ProsRes variants to disk drives, SSDs or SxS cards. It is powered via a 4-pin XLR input.
Atomos ATOMNJA001 Ninja Video Hard Disk Recorder
The Atomos Ninja is an HDMI-only recorder that records ProRes to user supplied 2.5-inch hard drives or SSD drives. For handheld use, SSD drives are recommended to eliminate the chance of dropping frames due to vibration. The Ninja is powered by Sony L-style batteries and comes with two. It is controlled via touch screen on its built-in confidence monitor. The Ninja makes a good companion to the Nikon D800 with its most recent firmware update. One thing to remember when using the Ninja is that if you plan on recording progressive footage you will need to manually do pulldown removal in post because the Ninja is unable to recognize progressive footage that is delivered in a 60i stream. If you do not do the correct type of pulldown removal, the image resolution will be severely affected.
|Atomis Ninja||Atomis Samurai|
Atomos Samurai HD-SDI Hard Disk Recorder
The Atomos Samurai is a newer addition to the Atomos lineup. Similar to the Ninja, it records ProRes to user supplied 2.5-inch drives. A paid upgrade to allow recording to DNxHD has also been announced. The Samurai records via HD-SDI only and has a brighter and higher resolution screen than the Ninja. It is also able to do pull-down removal as long the camera flags it correctly.
Blackmagic Design Hyperdeck Shuttle 2
The Hyperdeck Shuttle 2 is the least expensive recorder and is able to record fully uncompressed video as well as DNxHD. The Hyperdeck records to conventional SSD drives but be sure to get one of the approved drives if you plan on recording uncompressed footage. The Hyperdeck accepts both HD-SDI and HDMI inputs. It is powered via a built-in battery, but other powering options are available. It is also important to note that if you plan on recording progressive footage via an HDMI input, you will have to manually do pull-down removal in post.
Convergent Design Gemini 4:4:4 Uncompressed Video Recorder
The Convergent Design Gemini also records uncompressed video, but can do so at 10-bit 4:4:4. It is a great match for the Sony F3 with S-Log. The Gemini is great for recording Log footage from any camera because it is able to output footage to a monitor with a LUT applied for easier monitoring while it records the flat Log footage internally. The Gemini records to high speed 1.8-inch SSD drives and with a paid upgrade, is able to record from two cameras simultaneously, which is great for 3D work.
|Convergent Design Gemini||Convergent Design nanoFlash|
Convergent Design nanoFlash
The Convergent Design nanoFlash is a unique recorder. It offers the flexibility of recording either interframe or intraframe compression onto CF cards. An interframe codec at 100 Mb/s is much less susceptible to motion artifacts than the 24 Mb/s of AVCHD. The nanoFlash is also able to record intraframe at up to 160 Mb/s. All of the nanoFlash’s codecs are variants of Sony’s XDCAM. The nanoFlash is also very power efficient and comes with a battery. The one downside of the nanoFlash is that it only records 8-bit color. However, if your camera only outputs 8-bit color, this isn’t an issue.
Sound Devices Pix 220 and Pix 240
Sound Devices has a history of making professional recorders for the audio industry and has just entered the video industry with two on-camera video recorders, the Pix 220 and Pix 240. Both recorders record either ProRes or DNxHD to either 2.5-inch SSD drives or high-speed CF cards. They are powered by two Sony L-style batteries or an external AC or DC source. Two additional audio tracks can be added to the embedded audio from the camera via two XLR input jacks. The Pix 220 is HDMI only, while the Pix 240 is HDMI or HD-SDI. Both recorders are able to do pulldown removal for recording progressive footage.
The Sony SR-R1 is a high-end recorder that records Sony’s HDCAM-SR codec to SR memory cards. HDCAM SR has previously been a tape-only codec because of the massive amount of bandwidth (up to 880 Mb/s) it takes up. But with memory cards now up to 1 Terabyte in size, users of Sony’s high end cinema cameras can now skip recording to tape. HDCAM-SR is 10-bit, 4:4:4, and intraframe. The SR-R1 is also able to record two cameras simultaneously to one card, for easily organized 3D post work.