Bit Rate

Bit rate is a measurement of how much information a connection can send per second. As discussed in the File & Storage Size article, digital data stores information with bits, 8 of which equal 1 byte.

There are two major ways to measure bit rate. This is because bits can be sent in groups or one at a time. One way is to measure the bit rate in hertz (Hz), which means times per second. Another way is to measure the bit rate in bits per second (bps). If the bits are sent one at a time, these measurements will be the same number. However, if the bits are sent in groups, the measurements will be two different numbers. Ultimately, bps is the more "complete" way to measure bit rate. The abbreviated measurements for Hz and bps are shown below:

1,000	kHz = kilohertz	kbps = kilobits per second
1,000,000	MHz = megahertz	Mbps = megabits per second
1,000,000,000	GHz = gigahertz	Gbps = gigabits per second

Probably the best way to gain an understanding of how bit rate works is to look at how video is sent, which is what most of the rest of this article focuses on. More specifically, most of the rest of this article looks at how video is sent over the Video Graphics Array (VGA), Digital Visual Interface (DVI), High-Definition Multimedia Interface (HDMI), and DisplayPort connections.

Color Depth

The first step to understandig how video bit rate is sent is to establish just how many bits one pixel is. Basically, the size in bits of each pixel is equal to the color depth. Color depth determines how many colors each pixel is capable of showing. In images or video with 1-bit color depth, for instance, each pixel would be represented by a 0 or a 1, with one number representing white and the other number representing black.

Images and videos today have moved beyond black-and-white, and thus have color depths beyond 1-bit. Today, 24-bit color depth is typically used, which means that the color for each pixel is determined by a series of 24 0s and 1s. This allows for 16,777,216 possible colors for each pixel (2^24 = 16,777,216).

These 24 0s and 1s are typically subdivided into 0s and 1s for red, 0s and 1s for green, and 0s and 1s for blue. This is known as the RGB (red green blue) color system, and 24-bit color leaves 8 bits for each color (24/3 = 8). This is why 24-bit color may also be referred to as 8-bit color. Other color depths sometimes used are 30-bit (10 bits per color), 36-bit (12 bits per color), and 48-bit (16 bits per color) [1].

Finally, it is important to point out that the so-called "32-bit" color depth that computers use is actually just 24-bit color depth (notice that 32 is not divisible by 3).

Sending Video Over VGA

So long as you are using 24-bit color, VGA will send the video one pixel at a time. So, in terms of Hz, this would mean 1920x1080 video at 60fps would be sent out as a 124,416,000 Hz, or 124.416 MHz signal. This is because 1920 * 1080 * 60 = 124,416,000. Since each pixel is 24 bits, the total bit rate in bps would be 2,985,984,000 bps, or 2.985984 Gbps. This is because 1920 * 1080 * 60 * 24 = 2,985,984,000. Since VGA sends 24 bits a time, the formulas for converting between Hz and bps over VGA are shown below:

bps = Hz * 24
Gbps = MHz * .024
Hz = bps / 24
MHz = Gbps / .024

However, there is a complication to sending video over VGA, and that complication is blanking. Basically, in order for the video signal to work properly with CRT monitors, video sent over VGA has to send "extra" resolution, which is what the blanking is [2 page 19]. The form of blanking that VGA uses is known as Generalized Timing Formula (GTF). 1920x1080 video sent with GTF timing would actually be sent as 2576x1118 video, creating a bit rate of 172,798,080 Hz [3], or 4,147,153,920 bps. That is quite an increase!

Finally, for non-24-bit video, finding the Hz of such video would require taking the new color depth and dividing it by 24, and then multiplying that number by the resolution and frame rate. For bps, just substitute the new number for the original 24 in the resolution * frame rate * color-depth formula.

Sending Video Over DVI & HDMI

When video is sent over DVI, it is sent as a group of 30 or 60 bits depending on whether you are talking about single-link or dual-link DVI. However, in terms of Hz calculations, it is the convention to always treat DVI as if it is sending groups of 30 bits [4 page 7]. HDMI, meanwhile, also carries groups of 30 bits [2 page 11].

Given the 30-bit groups, one might assume that using 30-bit color results in one pixel being sent at a time. However, this is NOT the case. The key thing to understand here is that both DVI and HDMI use 8b/10b encoding, which is another way of saying that every 8 bits of data is sent as 10 bits. Although this ensures correct transmission, it also ends up creating 20% overhead [2 page 13]. Since 80% of 30 is 24, this means that, like VGA, 24-bit color results in one pixel (or two for dual-link DVI) being sent out at a time.

Given the above information, we can now discuss Hz and bps calculations. Hz for DVI and HDMI is calculated the same way as it for VGA - for 24-bit video, just take the resolution times the frame rate, and if the color depth is something other than 24-bit, take that number, divide it by 24, and multiply that number by the resolution and frame rate [2 page 18]. For bps, it depends whether or not you want to count the 8b/10b overhead. If you don't want to count the 8b/10b overhead, it would simply be resolution * frame rate * color depth. However, if you do want to count the 8b/10b overhead, it would be resolution * frame rate * color depth * 1.25. In terms of conversions, taking into account the 8b/10b overhead would require the following formulas:

Hz = bps / 30
MHz = Gbps / .03
bps = Hz * 30
Gbps = MHz * .03

Like VGA, DVI and HDMI also use blanking. However, DVI and HDMI have the option to use less blanking. In terms of blanking types, the two most commonly used are CEA-861 (the CEA stands for Consumer Electronics Association) and Coordinated Video Timing (CVT). CEA-861 is typically used for 720x480i/p, 720x576i/p, 1280x720p, and 1920x1080i/p [5 pages v-vi], while CVT is typically used for other resolutions.

First, let's look at CEA-861, the current iteration of which is CEA-861-E. CEA-861-E uses the following blanking amounts [5 page 16]:

640x480@59.94fps = (640 + 160) * (480 + 45) * 59.94 = 800 * 525 * 59.94 = 25,174,800 Hz
720x480@59.94fps = (720 + 138) * (480 + 45) * 59.94 = 858 * 525 * 59.94 = 26,999,973 Hz
720x576@50fps = (720 + 144) * (576 + 49) * 50 = 864 * 625 * 50 = 27,000,000 Hz
1280x720@50fps = (1280 + 700) * (720 + 30) * 50 = 1980 * 750 * 50 = 74,250,000 Hz
1280x720@60fps = (1280 + 370) * (720 + 30) * 60 = 1650 * 750 * 60 = 74,250,000 Hz
1920x1080@50fps = (1920 + 720) * (1080 + 45) * 50 = 2640 * 1125 * 50 = 148,500,000 Hz
1920x1080@60fps = (1920 + 280) * (1080 + 45) * 60 = 2200 * 1125 * 60 = 148,500,000 Hz

One thing to notice about CEA-861 blanking is that it is designed so that 50Hz and 59.94/60Hz video rates match up. Additionally, the 74.25 MHz rate used for 720p is also used for 1080i since interlaced video reduces uncompressed video bit rate by half and 148.5/2 = 74.25 [5 page 16]. This is all designed to keep necessary supported rates to a minimum and thus save costs - however, it also makes the sent video bigger than it needs to be. It should also be noted that audio for CEA-861 video is carried within the blanking data [6].

Next up is CVT. CVT with full blanking gives similar bit rates to GTF but does the math a little differently - the biggest change being that it slightly adjusts the frame rate. 1920x1080@60fps with CVT, for instance, has a bit rate of 173,000,450.56 Hz (2576 * 1120 * 59.963). However, if the reduced blanking option is used, then 1920x1080@60fps with CVT only has a bit rate of 138,500,281.92 Hz (2080 * 1111 * 59.934) [7]. CVT with reduced blanking is often referred to as CVT-RB, with the RB standing for reduced blanking.

One important thing to be aware of is that CVT-RB is designed to work best with a frame rate of 60fps [7]. Using CVT-RB with a frame rate other than 60 has limited usefulness since reduced blanking can't be used at all with CRT displays [2 page 19] and non-CRT displays don't flicker above refresh rates of 60Hz.

Sending Video Over DisplayPort

When video is sent over DisplayPort, it is sent in groups of 40 bits. However, DisplayPort also uses 8b/10b encoding [8 page 16], which means that only 32 of those bits are actually used for data. Since 32-bit color is not really used (remember, 32-bit color on the computer is really 24-bit color), video rates over DisplayPort typically use Gbps. Still, if you want to convert between Hz and bps for DisplayPort, just take the conversion formulas in the VGA and DVI/HDMI sections and substitute a 32 or 40 depending on whether or not you want to take into account the 8b/10b encoding. If you want to convert between DisplayPort MHz and VGA/DVI/HDMI MHz, divide the DisplayPort MHz by 0.75 or multiply the VGA/DVI/HDMI MHz by 0.75.

Half-Duplex vs. Full-Duplex

When discussing the bit rate of connections, two terms you might find used are half-duplex and full-duplex. Basically, half-duplex refers to a connection in which information can only be sent one direction at a time, whereas in full-duplex connections data can be sent from both directions at the same time.

Important Max Video Rates Today

Although HDTVs typically have a max native resolution of 1920x1080, computer monitors go beyond this. The calculations below show the rates of video at 1920x1080@60fps as well as higher rates used by computer monitors. Also, the calculations below use 24-bit color. Here they are (CVT with full blanking is not listed since it gives results that are within 1-2 MHz of GTF and is pointless to use with digital displays since CVT-RB can be used instead):

	no 8b/10b or blanking	GTF	CEA-861	CVT-RB
1920x1080@60fps	2.985984 Gbps	172.798 MHz	148.5 MHz	138.5 MHz
1920x1200@60fps	3.31776 Gbps	193.156 MHz	NA	154 MHz
2048x1152@60fps	3.39738624 Gbps	197.967 MHz	NA	156.75 MHz
2048x1536@60fps	4.52984832 Gbps	266.952 MHz	NA	209.25 MHz
2560x1440@60fps	5.308416 Gbps	311.827 MHz	NA	241.5 MHz
2560x1600@60fps	5.89824 Gbps	348.157 MHz	NA	268.5 MHz
1920x1080@120fps	5.971968 Gbps	368.759 MHz	297 MHz	285.5 MHz

Sources

3. Andy Morrish. THE VESA GENERALIZED TIMING FORMULA (GTF). Jan. 5, 1997.

5. A DTV Profile for Uncompressed High Speed Digital Interfaces: CEA-861-E. Mar. 2008. ©CONSUMER ELECTRONICS ASSOCIATION 2008.

7. Graham Loveridge. VESA COORDINATED VIDEO TIMING GENERATOR. Apr. 9, 2003.

8. Craig Wiley. DisplayPort Technical Overview. Jan. 10, 2011.