If you are trying to estimate the amount of storage you will need for a given project, you will need to understand the bit rates. Bit rates can help you understand the amount of storage (or transfer bandwidth) needed for a second of audio or video footage.
Breaking it down
According to Canon’s website, the Canon 90D camera captures 1080p 30fps video at a bit rate of approximately 30 Mb/sec or “thirty megabits per second”. But what does this mean?
“Mega” is a metric prefix meaning “million”.
A “bit” is a single digital value containing either a 1 or a 0.
Thus, “30 Mb/sec” means that we are storing 30 million bits per second.
This means that to store an hour of footage recorded at a bit rate of 30 Mb/sec, we would need a drive that had a capacity of 108 billion bits! Now where are those 108 billion-bit drives?
Bits vs. Bytes
You may have noticed that we are working with “bits” rather than “bytes”. Most computer drives are rated in “Gigabytes” or “Terabytes” rather than “Gigabits” or “Terabits”. Thus, we will need to know how to convert bits to bytes.
A bit is a single binary digit that can either have a value of 0 or 1. Computers store bits in groups of 8 known as bytes.
To convert our bit rate into a byte rate, simply divide it by 8. So in our example, 30 Mb/sec (megabits) is equal to 3.75 MB/sec (megabytes). Notice that when abbreviating bits, we use an lowercase “b” (Mb) and for bytes we use an uppercase “B” (MB).
Making A Useful Estimate
Since our sessions tend to generate hours of footage rather than seconds, we should convert our bit rate to be MB/hour to get a better idea of how much storage we will need.
We can multiply by 60 to get the MB/minute and then 60 again to get the MB/hour.
So 3.75 MB/sec * 60 sec/min * 60 min/hour = 13,500 MB/hour = 13.5 GB/hour.
As you can see, video CHEWS through storage.
Conclusion
Obviously this figure doesn’t account for any extra metadata the camera may add to the footage which would slightly increase the amount of storage needed. But with these sorts of calculations, you can quickly make ballpark estimates for how much storage you will need for a shoot.
Note that you can do these same calculations for both audio and video formats.
Whenever you are capturing audio or video, bring more storage than you think you will need. This is helpful if you end up capturing more than you thought you would. It can allow you to be more flexible on the job, particularly when a last minute decision is make to shoot at a higher resolution or quality.
In general, it’s always better to have too much storage than not enough.
This page was written by Mason Kline for the University of Oregon Cinema Studies Program and is published under Creative Commons license (CC BY NC SA 3.0)