Since my posting on Video Resolution, I’ve had requests for similar information on Video Bitrates. Your wish (up to a reasonable point) is my command. This is for beginners, and many of you may already have all this in your heads. Lucky you!
Bitrate – What is it?
Bitrate = the number of bits per second put into the video/audio information, therefore the number of bits that will need to run through the interwebs while your video is playing.
The bitrate is commonly expressed in Kbps (Kilo-bits per second). Video and audio files are commonly labeled “client_content_300K.mp4” or just “300.mp4.” The bitrate is one of the four “defining characteristics” of online video: length, format, resolution and bitrate. If you know those four, you usually know as much as you need to use the video online.
bits, Bytes and Filesize
Note the small “b” for bits. It is not “B” for bytes. Bitrate and bandwidth have been expressed in bits and Kbits since the earliest days of the internet, mostly because of the tiny bandwidth available. It was much easier to talk about a 14.4K modem than to discuss a .01406KB modem.
Filesize is defined in Bytes: KB, MB and GB. A byte is eight bits. You can work out the file size from the length and bitrate of a video. Our 300K bitrate above would produce a file that was 37.5KB per second. If our file were 1.1MB, we would know that it was ~30 seconds long. I’ll let you play with the math form there.
A Word from the Eminence Grise
Back when I started in video we only worried about two bitrates, because we were only worried about two device types. There were 28.8K modems and there were 56K modems. We had to leave out some overhead, so we encoded to 22Kbs and 45Kbs. This was the time of “Postage Stamp” video at 96×72 and 128×96 pixels.
With the arrival of T-1 and DSL we could stretch out a bit, but the growth of bandwidth never was fast enough for us, and it still isn’t fast enough today. Clients and encoders are ready to stream 1080p, but the pipes just aren’t big enough.
Which brings us to the golden file of the Golden Age of Web Video: 320×240 at 300kbs.
320×240 @ 300K and Beyond
This was the perfect ratio of Resolution and Bitrate back in the year 2000. From this ratio, we’ve been figuring out bitrates for larger and larger files for the last eight years. So, here we answer the main question that I hear every week: what bitrate should I use for this video.
First we drop out the audio. It doesn’t count for much and it really comes down to 64K is good and 128k is better. In this case we’ll assume 64K. That leaves 236K of encoding power for the video. If you encode at the above specs, and the video looks good, you can use that as a formula for larger resolutions.
“So, what bitrate should I use for my 640×360 video?” Start with 320×240 pixels = 76,800 pixels. That number of pixels looks good when using 236Kbs. Your desired resolution is 640×360* or 230,400 pixels. Time for some math:
76,800/230,400 = 236/x or 236*230,400/76,800 = 708Kbs
*note that this is resolution independent – it doesn’t matter if your video is 4:3 or 16:9
So ~708Kbs for encoding video at 640×360. Add the audio, still at 64K and we get 772Kbs total bitrate for the file. You’ll probably want to round down or up so your files can be nicely labeled as “client_content_640x360_750.mp4” or something similar.
But Codecs Have Improved
They certainly have, and sometimes formulas from long ago give very big numbers. It would be hard to suggest to a client that they go much over 1500k, but they want to stream 720p (1280×720) video. Our formula puts that at ~3000K! At that point, you’re going to have to bring it down. That means create a file at 720p and 3000K, using the best settings you can achieve for your codec; determine that the file looks great, then start testing down. Most modern codecs will give great results as you go down to 2000K and go under 2000K. Find your sweet spot.
- Bitrate is the number of bits used to encode or used to stream a video
- A preliminary bitrate can be determined using a ratio of pixels to bitrate
- Using modern codecs you can take the bitrate down as the resolution goes higher
I hope this is helpful to someone out there. Enjoy!
I am available to consult on any Web Video Encoding issues: firstname.lastname@example.org
Free to Metro Encoding clients