Tech Tips:
Understanding Bits and Bytes
By Andy DunnUnfortunately, these two words are constantly mixed up. Did you say “Megabits per second or Megabytes per second”? For a long time, it never affected most of us so we didn’t care. Occasionally, we used one of the words in a sentence and someone else may have needed to correct it. Today, however, the words are used more and more and understanding them is critical to plotting your digital strategy.
Both bits and bytes are units of measure. A byte is made up of 8 bits. By definition then, a bit is smaller than a byte. We use a small b when referring to bits and a capitol B to represent bytes when referring to a measurement. To confuse the issue just a little more, bits and bytes are not measurements in decimal (base 10) but rather in binary (base 2). This means that 1 kilobyte is not really 1000 Bytes (10 to the 3rd power), rather, it is 1024 Bytes. (or 2 to the 10th power). Furthermore, 25MB stands for 25 Megabytes or slightly more than 25 million bytes while 25Mb/sec is the same as saying 25 Megabits per second. In the latter example, we are saying that a stream of data may be transferred from one point to another at a rate of 25 Megabits per second.
So why is this so important for us to understand this today? Well, given today’s many different formats and rates, this knowledge will help in determining if one piece of equipment will work with another. Also, simply comparing different pieces of equipment will require a basic understanding of the terms used to describe them. For example, DVCAM records information in a 25Mb/sec (Megabits per second) stream. The SDX900 camcorder can record either at 25Mb/sec or 50Mb/sec DVCPro. The PDW530 XDCAM allows you to record in DVCAM 25Mb/sec, or IMX 30, 40, or 50Mb/sec while the professional disc they record onto can hold 23GB (Gigabytes) of data. Finally, the Firestore FS-3 has removable hard drives of 40GB and 80GB storage capacity.
Starting to get the picture? One way to remember the difference is to understand that most of the time (in our world) bits is about the speed or transfer rates while bytes is about capacity. This is because, in most cases, data storage has become incredibly large. Measuring them in how many bits of information you can store on them would seem pointless especially as their sizes continue to increase. In contrast, the pipe on which a data stream travels is more often relatively small limiting the size of the data stream. Measuring this in bits per second is more appropriate.
For example, we say a hard drive holds 50MB of data not 400 Mb (50 * 8) of data. This may be text files, video files, metadata etc’. A modem, on the other hand, may transfer data at 54Kb/sec or 54 kilobits of data every second rather than 6.75KB/sec (54 / 8). Also, if you are recording DVCAM onto the Firestore at 25Mb/sec, then you are recording at a speed of (or at a rate of) 25 Megabits of information for every second. So, in a 10-second clip at this rate, you will have recorded 250 Megabits (25×10) of information or just over 31MB (250 / 8) of data. You knew the math was going to come back, didn’t you?
We should mention that there are times when the transfer pipe is large enough to warrant bytes as the obvious choice of measurement. A perfect example of this is with today’s SCSI (Small Computer System Interface). The Ultra 2 Wide SCSI, for example, transfers data at 80MB/sec. Fiber Channel may be an even better example of this with full duplex allowing transfers at an incredible 100MB/sec.
Now, when comparing quality of one camera or format to another, more information usually equates to better quality. 50Mb/sec often represents a higher quality signal or format than 25Mb/sec because you have twice as much information in every second. I have to say “usually” and “often” because technology is constantly changing and new compression methods are allowing high quality images to be transferred at lower rates. For instance, the Sony HDR-FX1 and HDR-Z1U use an MPEG-2 compression method to transfer and record high definition captured footage at a rate of 25Mb/sec. But we will save that for another time.
