StreamingandRecordingUncompressedHDMI
Video
Supplement to Mastering the Nikon D7200
This supplemental information is intended for readers of Mastering the Nikon
D7200 only. Please do not distribute this extra material to anyone other than a
legitimate owner of the book. Mastering the Nikon D7200 is available at the
following website: http://www/.pictureandpen.com/NikonBooks.asp
Copyright © 2016 Darrell Young, All Rights Reserved
The Nikon D7200 is a multi-format video camera, in addition to creating
beautiful still images. What do I mean by multi-format? The camera can record
compressed H.264/MPEG-4 Advanced Video Coding (AVC) video to either of its
two internal memory cards for personal video creation, or, it can stream
uncompressed, broadcast quality, 1080p, 4:2:2 video out of its HDMI port for
commercial video needs.
This document is a little more technical than Mastering the Nikon D7200,
due to discussing deeper things like video standards, formats, and
compression. However, I will try to keep things on an easy-to-understand level,
while providing as much detail as possible. There are entire books devoted to
the subject of video and they can be quite deep and hard to understand.
We will cover the important video information in this document, plus
provide information on one of the best external video recorders available for
your camera, the Atomos Ninja Blade:
http://www.atomos.com/ninja-blade/
Let’s consider some of the basic information a person wanting to record
streaming HDMI video needs to know.
Understanding D7200 Video Standards
The Nikon D7200 is capable of producing two types of video output:
H.264/MPEG-4 AVC 4:2:0 8-bit Compressed Video – This is the mode that
records video to the memory cards in the camera. It is normally used for
those times when you want to make a simple home video and are
unconcerned about commercial video quality. The H.264 output is stored
in a .MOV container (see the next main section Containers and Their
Formats). This Apple format allows pretty much any computer that can use
Apple QuickTime to play your videos, which is basically all home
computers. A video frame in this format ends up looking like a very
compressed JPEG still image, which means it has sufficient quality for
home use and for displaying on websites like YouTube and Facebook, but
may not be the best for serious commercial work. All video output from
this type is recorded to the camera’s memory cards for later transfer to your
computer. It is best to use at least Class 6 SDHC/SDXC cards to record the
video. Slower cards may not be able to keep up with the video stream.
4:2:2 10-bit 1080p Uncompressed Video – Although the video stream
coming out of the HDMI port on the D7200 is called “uncompressed” by
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Nikon, it does in fact use a form of chroma subsampling, reducing the total
colors output from the better 4:4:4 format to 4:2:2. This is not so much a
compression of the video stream as it is a compression of the number of
colors in the video stream. The human eye is much more sensitive to
luminance (brightness) than it is to chrominance (color gradiants). That
simply means the camera can reduce the range of color flowing out of the
HDMI port without your being able to detect the change, as long as the
same levels of brightness are maintained. This chrominance compression
is similar to how a JPEG image compresses the color range and is quite
effective, even for serious commercial use. The container format is
controlled by the recording device for this video type. The Atomos Ninja
Blade can wrap the 4:2:2 video coming out of the camera into an Apple
ProRes or Avid DNxHD format (see the next main section Containers and
Their Formats). The ideal output level for hardcore commerical use is 4:4:4.
However, that level creates huge files that most people will have no
interest in storing. The 4:2:2 output level produces smaller files that are
visually equivalent in quality to 4:4:4.
Containers and Their Formats
Container formats are well known by you, although you may have not realized
that you were using a container. The ending of the file name allows you to
know which container you are using. For instance, I am sure you’ve heard of
WAV, AVI, MOV, and the various MPEG formats (e.g., MP4). Well, those are
container formats that are commonly played on your computer, smart phone,
or tablet.
Once you have recorded compressed video to your camera’s memory card
or uncompressed video to an external video recorder—through the HDMI
port—the video signal must be placed into a specific container format, which
will hold not only the video, but also the audio (sound) signal. The Nikon D7200
records audio as uncompressed, 2-channel, 16-bit, linear pulse-code
modulation (PCM) sound at 48Khz. This sound file is directly synchronized with
the video signal and is bundled into the container.
Here is how the camera deals with container formats, according to the type
of video you are recording:
H.264/MPEG-4 (AVC) compressed video, as mentioned previously, is
recorded to one of the camera’s memory cards, as configured in the
Shooting Menu > Movie Settings > Destination setting. The Nikon D7200
places the video and audio into a QuickTime .MOV file. As we’ve discussed
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previously, you can play a MOV file on any computer that can run an Apple
QuickTime compatible player.
4:2:2 8-bit 1080p Uncompressed Video, as previously discussed, is
streamed out of the HDMI port and is captured by an external recorder.
The external recorder takes the video and audio signals and places them
into a container it supports. If you are using an Atomos Ninja Blade, you
can choose to record into Apple ProRes or Avid DNxHD container formats
to removable 2.5 inch solid state drives (SSD) or 2.5 inch spinning hard
drives. Other external recorders may support additional commercial
container formats. You can download the Apple ProRes QuickTime Decoder
1.0 at these websites:
Windows computers:
http://support.apple.com/kb/DL2
Macintosh computers:
http://support.apple.com/kb/DL1
With this decoder installed you can play your Apples ProRes videos on your
computer, through Apple QuickTime.
For using the Avid DNxHD format, you will need to download a decoder
from this website (Windows and Mac):
http://avid.force.com/pkb/articles/en_US/download/en423319
The Avid decoder also uses Apple QuickTime to provide playing services.
You will need to have QuickTime installed on your computer to play any
version of the video from your camera. Download Apple QuickTime here:
http://www.apple.com/quicktime/download/
With this information you should have a better understanding of how the
camera writes video to the memory card or streams it out of the HDMI port to
your external video recorder or other HDMI device.
External HDMI Output
Sending video through the HDMI port removes the maximum recording time
and bit rate limitations found while recording video to the camera’s internal
memory cards. Also, the live video stream coming out of the camera’s HDMI
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port is not compressed and is 4:2:2 broadcast quality for saving in the video
container of your choice (e.g., Apple ProRes, Avid DNxHD).
Rolling Shutter
The D7200 uses a CMOS sensor to record video. This type of sensor uses a
“rolling shutter” and has three potential issues that we need to discuss: skew,
wobble, and partial exposure.
How the Rolling Shutter Works in Movie Mode
Since video is captured at 24 to 60 frames per second (fps), the D7200 has an
electronic shutter in addition to the normal mechanical shutter.
Have you ever used your D7200 in Continuous release (CL or CH) mode
when you are capturing up to 6.5 still images per second? The mechanical
shutter activation combines with mirror movement to make the cool chickachicka-chicka sound that causes passersby—with their little point-and-shoot
cameras—to look at you in awe.
You don’t hear that sound in Movie mode because your D7200 does not
use the mechanical shutter when shooting movies. If it did you would wear the
shutter out with only an hour or two of video capture. Remember, the camera
captures video at a minimum of 24 fps. An hour of video requires 86,400 frames
at that speed. You would quickly exceed the tested lifetime capacity—150,000
images—of the mechanical shutter.
Instead, the camera uses an electronic shutter and turns the sensor’s rows
of pixels on and off, as needed, in a scan from top to bottom. In other words,
the camera records each video frame by scanning it—one line at a time—from
top to bottom. This is called a rolling shutter. Not all parts of the image are
recorded at exactly the same time! It can produce a skewed or wobbly video
when you film rapidly moving subjects, like a race car or a flying bird. Rolling
shutters are used by video cameras that have CMOS sensors, like the Nikon
D7200. Most dedicated video cameras have CCD sensors, with global shutters
that do not scan the image. They are mostly immune to the effects we are
about to discuss.
Here is a list of the three video effects that can be experienced in extreme
circumstances with a rolling shutter:
Skew – The image leans in one direction as the camera or subject moves.
This is often seen at the edges of buildings and other static objects.
Wobble – This effect is harder to describe. The whole image wobbles in a
strange way. It looks like the top of the image is out of sync with the middle
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and bottom of the image. Since video is a moving picture, the whole video
can wobble back and forth in a very unnatural and dizzying way.
Partial exposure – If another camera’s flash goes off during the shot, the
burst of light may be present for only some of the rows of pixels in a
particular frame. The top part of the frame may be brightly lit by the flash,
while the bottom part appears dark. The partial exposure appears as a
bright band in one or a few frames, depending on the duration of the
bright light. Some older fluorescent light bulbs have slow ballasts and can
cause a video to have a series of moving bands as the light flickers. Our
eyes can’t see it, but the fast video captures it well. If you are shooting
images of an ambulance with its lights flashing, it can also cause banding.
Anything that has intense bursts of light for short periods may cause
partially exposed bands to appear in a video.
To understand skew and wobble better, let’s compare the D7200 to a
desktop scanner. It works in a similar way. If you place a paper document on a
scanner and press the scan button, you’ll see a band of light under the glass
travel from the top of the document to the bottom as it records one line at a
time. At the end of the scan, there is a copy of the document in your computer’s
memory that can be saved to the hard drive. You usually put the scanner’s lid
down on a paper document to hold it flat and keep it from moving. Imagine
that you’re scanning a paper document on your desktop scanner, and halfway
through the scan you move the paper a little. The top part of the scan would
look normal, since it was already captured by the scanner’s sensor, but the
bottom part of the scan would be at a different angle than the top part. You
could say that it is skewed from the original angle. This is an example of the
skew that can result with a rolling shutter by moving the camera during a
recording.
Now, what if you grab the paper during a scan and rotate it back and forth
all the way through the entire scan? The final scanned document would look
like a series of zigzags, with some parts at one angle and other parts at a
different angle. This is an example of the wobble that can result with a rolling
shutter while moving the camera back and forth during a recording.
The D7200 records video in a similar manner, except it is much faster than
a scanner. It records a frame of video in 1/24 second, or 24 fps, and can go as
high as 60 fps. Since the Nikon D7200 scans the image at this speed, there’s not
a problem in most cases. Most movement is too slow to be zigzagged
(wobbled) or angled (skewed).
Skew and wobble become especially evident when a person is walking and
recording a video at the same time. These combined movements can be
enough to cause wobble in the video. I call this the “jellywobble effect.” Like a
bowl of Jell-O, your video looks like it is wobbling. What can you do to prevent
it?
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Avoiding the Jellywobble Effect Primarily, you have to be careful not to
allow too much camera movement. It truly is best to use the D7200 on a nice
fluid-head video tripod, or a stabilizing frame, if you want great results. I’ve
found that Nikon’s vibration reduction (VR) lenses help when you don’t want
to use a tripod, since they stabilize the camera a little. VR won’t help much if
you’re walking while recording a video, since the camera movements are often
too great for the VR system to overcome. If you’re standing quietly and doing
your best to hold the camera still, it will help overcome small movements
caused by your heartbeat and breathing. This is one of the main differences
between a dedicated video camera and a hybrid like the D7200. Dedicated
video cameras use a CCD sensor, which has a global shutter instead of a rolling
shutter. A global shutter does not scan the image one line at a time. It uses the
whole sensor at once to grab the image. Many of the newer, low-cost video
cameras on the market use a rolling shutter, but the expensive video cameras
generally use a global shutter.
This is probably the worst problem with D7200 video. True videophiles will
turn up their noses at a rolling shutter. They’ll buy a dedicated video camera
with three separate CCD sensors—one for each RGB color—and a nice global
shutter to avoid jellywobbles. And they’ll pay several thousand dollars for the
privilege of owning that equipment. You, however, realize that the D7200 is
primarily a very high-quality still camera with added video capabilities. You
might be standing in a superstore parking lot one day when an alien spacecraft
lands. You’ll get both still images and cool video from the same camera. So
what if, in your excitement while running from the alien’s heat ray, you get a
few jellywobbles in your video? One of the rules for getting great video is
having a video camera with you. With the D7200, you have one at all times—
with no extra effort. You do keep your camera with you just in case, right?
Try to hold your camera still to greatly reduce any jellywobble effects. Use
a tripod when you can, or even a VR lens. Anything that helps stabilize the
camera will give you much higher-quality video.
Creating Commercial Video
One important caution that I want to give you is concerning licensing you may
need to acquire of you use H.264/MPEG-4 video for commercial uses. The best
advice is, don’t do it. If you are shooting a video you may sell or that will be
used for advertising or other commercial purposes, you should use the 4:2:2
10-bit uncompressed video from the HDMI port only.
The reason I mention this is that Nikon licenses the H.264 compression
from the MPEG LA group. Their license states that you can use the compressed
video for only “personal and non-commercial use.” Therefore, if you are
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shooting Aunt Minnie’s baby shower for viewing by family and friends, you are
safe in using H.264 video from the camera’s memory card.
However, if you plan to shoot the next blockbuster movie with your D7200,
or even to advertise your products online, you may want to use only the 4:2:2
10-bit uncompressed video from the HDMI port.
I am not a lawyer so consult your attorney before attempting to sell your
video masterpiece.
Information on Atomos External Recording Devices
You can contact Atomos.com to find out more about the exciting Ninja HDMI
recorders, at the following:
Website:
Enquiries:
Support:
Telephone:
http://www.Atomos.com
[email protected]
[email protected]
+61 3-9999-5908
Atomos Product Range Brochure
http://www.atomos.com/downloads/support/Atomos-Brochure-Sept-2014LowRez.pdf
Above is a picture of several Atomos video recorder units. Following is a picture
of my Atomos Ninja Blade HDMI Recorder kit in its water-resistant case.
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If you have a Nikon D7200, and are into video recording, you need one of
these! It has everything you need to connect your camera to the Ninja Blade
recorder. The only additional purchase you must make is one or two 2.5” SSD
or spinning hard drives (I recommend SSDs).
Once you have this Ninja Blade (or other Atomos) recording equipment,
you can shoot endless uncompressed video at the highest possible 1080p
quality. I highly recommend Atomos recorders due to their high quality and
ease of use. I’ve been using their products for years and they are the best!
Keep on capturing time…
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