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Here’s the basic problem: an uncompressed 12-bit
1080p/25 video clip requires about 155 MB/second for
playback! That’s faster than any single spinning media
hard disk. This format takes more than 560 GB to store
an hour of this media! Unless we are recording directly
to a hard disk, we must compress the file in order to
store it anywhere.
Many cameras record a version of AVCHD or H.264.
This is a very compressed format specifically designed
to be recorded on SDHX or SDXC cards that slip into
your camera. (SDHX stands for SecureDigital - High
Capacity, while SDXC stands for SecureDigital -
The good news about these cards is that they are
small, light and ubiquitous; lots of companies make
them. The bad news is that they have a very limited
bandwidth, between 6 and 10 MB/second. (There is
a UHS Class 3 card which supports speeds up to 30
MB/second, which is designed for 2K and 4K media
recording.) Still, 30 MB/second is a far cry from the 155 MB/s
source file. Hence, the AVCHD codec was invented
to compress the file in the camera so it can be safely
stored in real-time to the SDHC card.
THE BATHTUB VS. THE BUCKET
However, just because we use AVCHD for recording
media does not make it a good choice for editing.
AVCHD compresses video in clumps, called a “Group
of Pictures” (GOP). GOP compression looks at a seven
frame block and just notes the pixels that changed. This
block format is very efficient, but hard to decompress
with all the back-and-forth that video editing entails.
For editing, we need something much more efficient
where each image is compressed individually, not
as part of a group. This type of individual frame
compression is called “I-frame” compression and
provides for much more responsive editing, rendering
and exporting than GOP compression.
Also, the 8-bit color depth of AVCHD and H.264 is
constrained. While 8-bit is acceptable for black-and-
white images, it is not sufficient for accurate color. Color
ideally requires a 10-bit-depth or more for smooth
gradients and color grading.
This is where formats such as ProRes, DNxHD, AVC-
Intra or Cineform shine. All are 10-bit and use I-frame
compression. They provide outstanding image and
sound quality, but at the expense of creating larger
files; generally 2-4 times larger than the camera native
media. The process of converting from camera native to a
mezzanine format (we call it “mezzanine” because
this codec is in the middle between shooting and
distribution) is called “transcoding.”
Here’s a good analogy. Imagine that shooting video is
like pouring water into a five-gallon bucket. When the
bucket is full, we dump it into a bathtub. The bucket is
the AVCHD codec. The bathtub is ProRes (or any other
mezzanine codec). Because the bathtub is so much
larger than than the bucket, we don’t lose any quality
when we convert the media.
After editing is complete, the finished file is way too
large to post to the web or burn to a DVD; though it is
fine for distribution via broadcast or cable. If the web is
our goal, we need to compress the file from the master
file created by our editing software into something small
enough for people to download.
Again, we are back to a bandwidth issue. Most web
videos are compressed for playback at less than 1 MB/
second. This is even more compressed than the original
camera master, and a far cry from the uncompressed
data rate. This is where H.264 comes in. It creates very
small files, with pretty high image and audio quality.
Back to our bathtub analogy, we are now pouring that
bathtub filled with water into a 1 cup measuring cup.
Almost all the water in the tub will be lost - permanently
destroyed in the compression process.(This is why you
don’t want to recompress already compressed material
- there’s nothing there to compress a second time.)
SUMMARY Codecs exist to convert reality into ones and zeros
for the computer. Different codecs exist to meet the
requirements of different jobs: production, editing
or distribution. Understanding the strengths and
weaknesses of different codecs will allow you to
optimize quality and performance throughout
the entire production process.
KITPLUS - THE TV-BAY MAGAZINE: ISSUE 99 MARCH 2015 | 43