Storage Requirements

Storage Requirements

Next we'll list considerations for comparing storage options, as well as the data rates needed to handle many common formats.

Storage Considerations

Various video formats have different bit rates, or the amount of data that needs to be processed to capture, store, playback and output these video formats from a computer editing system. We need to have hard drives fast enough to handle the bit rates of the video. If we don't have drives with sufficient bandwidth, we will end up with "glitches" during capture, playback and output, often referred to as "dropped frames". In other words, the hard drives can't keep up with the video- a bad thing to be sure.

Sticking with the DV example, you may think that we only need a hard drive that can handle a data rate (bit rate) of about 3.5 MByte/sec, which sounds pretty low compared to the speeds of today's hard drives. But in reality, there are a few more things to consider.

1. Avoid capturing or playing back from your computer's main internal boot drive. This drive is occupied accessing information about the operating system, applications and open documents. Dedicated hard drives for media storage are essential for problem-free operation.

2. When purchasing or comparing storage, the data rate performance listed for drives are usually given in "peak" speeds. In other words, the highest speeds that the drive is capable of. That's not a good figure to work with. We need to know the "sustained" data rates- for both reading and writing to the drive, the minimum data rate that the drive will not go below, because we need drives that never fall below the bit rate of the video. To measure the speed of a hard drive or RAID, there are various third party utilities.

3. Allow for overhead. Although the bit rate of the video may seem low compared to what a drive is capable of, there are several things that affect performance.

• Drive communication: allow about 25% overhead for the "chit-chat" that happens back and forth between the drives and computer. This means you need higher data drive performance than just the video bit rate itself.
• Allow for overhead for other elements playing back at the same time- like multiple tracks of audio. Although audio has lower data rates, it should be taken into account.
• Don't fill up the hard drives too full. As a general rule of thumb, don't fill up any video storage beyond 80-90% of its usable capacity. The more you fill the drives, the higher the chances for dropped frames and an overall slowdown in performance. For FireWire hard drives, try to keep them to no more than 80% capacity.
• Real Time effects. This is often overlooked. In the next part of this article we list the data rates for playing back a single stream of video of multiple different formats. When you superimpose two images together and play back the effect without rendering, just using real time effects playback, then the drive now needs to handle 2 streams of video. This raises the bandwidth needs to play back the video. In general, this translates to twice the speed needed to handle the 2 streams of video being played back. The more streams, the more overhead is needed.

4. Allow for additional considerations with FireWire drives.

• If possible, use the FireWire 800 port over a 400 port for connection for better performance. Any 400 connected devices to a FireWire 800 connected device (called "daisy-chaining") will lower the overall performance to that of the slowest component on the bus.
• Avoid multiple drives on the same FireWire bus. Most computers will have any number of configuration of different FireWire ports. You may have FireWire 400 and 800 ports, ports on the front and back of the computer, ports in the computer monitor (like Apple Cinema displays) or drives connected to one another (referred to as daisy-chaining). Unfortunately, all these devices are probably playing off of only one FireWire bus in the computer. Therefore, even when connecting devices into different ports, they are all affecting the overall performance. One solution highly recommended is to add a separate FireWire bus to the computer. For a desktop computer, an extra bus can be added with the purchase of a PCI card that inserts into one of the internal slots in the computer.

Storage For Specific Formats

If you know the bit rate of the video format, then you can pretty much figure out what kind of data rates you need for storage. Below we'll list some common formats and the data rates needed for them. These rates are just a base comparison for video, and do not include audio or any overhead for RT effects in the calculations. After the video charts, there is a chart for data rates of uncompressed audio.

There is also a handful of formats listed that are specific editing formats by Avid and Final Cut Pro- for Avid, called DNxHD and for Apple, called ProRes. There are many more codecs available on both the Avid and Final Cut platforms, so this is not a comprehensive list, but is helpful for seeing how these various formats impact our needs for video storage.

Standard Definition Video

High Definition Video

   
Audio

The chart below lists the bit rates for the most common audio format that we work with in video editing- uncompressed audio, 48 kHz sample rate, 16-bit. In a Mac, the most common file format is AIFF. in Windows, the most common format would be WAV. Most of the rates below are listed in Kilobytes per second (KByte/sec).