Bitrate vs. Resolution – Which one do you think is more important for video streaming? This is a question that most streaming services need to think about while building out their bitrate ladders and often has people confused. In this article, we show how bitrate and resolution have a huge influence on your video streaming service’s QoE and QoS, why it is important to know the difference between the two, and how they interact with each other.
Sometime back, we wrote about compressing videos using HandBrake, where, you need to choose which combination of bitrate and resolution you’ll use to compress your video. Take a look at all the options that HandBrake provides – it is mind-boggling!
What would you do?
- choose 1080p or 720p?
- choose 5mbps or 3mbps?
- choose 1080p @ 5mbps or 1080p @ 7mbps?
Confused which combination to choose? That’s why this article has been written – to demystify the relationship between bitrate and resolution and help you make the right encoding setting choices.
Let’s start by understanding what bitrate is.
What is Bitrate or Video Bitrate?
Video Bitrate or (bitrate) is the number of bits used to represent one second of video. The units of bitrate are typically kbps (or kilobits per second), or mbps (or megabits per second).
Another way to look at it, is bitrate defines how much data is transmitted to the user or video player. If your video has a bitrate of 3 mbps, then, we can assume that, every second, 3 megabits of video will be transmitted to the player.
A higher bitrate generally results in better video quality, but also requires more bandwidth to transmit or more storage space to save the video file. Conversely, a low bitrate video suffers from poor video quality, but requires less CDN bandwidth or storage space. We’ll discuss more about this in the later sections.
Bitrate and Rate Control Algorithms
A transcoder uses a specialized algorithm called a “rate control algorithm” to decide how the bit-budget (or bitrate) is distributed between the frames of a video. If your video has a frame rate of 60 fps, and a bitrate of 3 mbps, then a Rate Control Algorithm in the transcoder decides how to distribute the 3 megabits between 60 frames of video.
Popular rate control algorithms are –
- CBR or Constant Bitrate: bitrate is kept constant while sacrificing video quality
- VBR or Variable Bitrate: video quality is kept constant while allowing the bitrate to fluctuate
- Capped VBR or Capped Variable Bitrate: the video quality is kept constant while allowing the bitrate to fluctuate within a limit or cap.
A detailed discussion of these rate control methods can be found here, here, and here (using FFmpeg) on OTTVerse. If you are new to video transcoding, then please read this article to understand the basics of video transcoding.
If you want to learn more about video codecs, here is a list of transcoding-related articles on OTTVerse.
- Explanation of the Block-based Transforms in VVC (Versatile Video Coding)
- What is Essential Video Coding (EVC) MPEG-5 Part-1?
- Discrete Cosine Transform in Video Compression – Explain Like I’m 5
- VVC, EVC, LCEVC – MPEG’s New Video Codecs
How does Bitrate Affect Video Quality?
To understand how bitrate affects video quality, it is important to understand how video compression or transcoding works. A video transcoding or compression software does two important things –
- It converts the video from the “pixel domain” to the “frequency domain” using the Discrete Cosine Transform or DCT. If you don’t know how the DCT works, check out our explanation to a 5 year old.
- It discards a lot of the frequencies using a process called “Quantization” in a way that the human eye cannot perceive this loss of information.
So, when you compress a video, you throw away some information at the expense of visual quality.
- If you compress a video heavily, you lose a lot of information, and the visual quality reduces.
- If you compress lightly, then the file size is big, but the video quality is excellent.
This is called the rate-distortion tradeoff in video compression.
What does the “rate-distortion tradeoff” mean?
Let’s see with an example. We’ve taken a sample video (CrowdRun) and compressed it using FFmpeg at 1.5 mbps and 5 mbps. From the screenshots of the videos below, can you tell me which is taken from the higher bitrate video and the lower bitrate video?
Remember, both are 1080p.
It is obvious that the upper one is the lower bitrate and the second image is the higher bitrate. When we use a bitrate of 5 mbps @ 1080p, the video quality is excellent and for the same video, when we use a bitrate of 1.5 mbps, the quality is quite bad for the CrowdRun video.
But, does a low bitrate always produce a poor quality video? Let’s run a quick experiment to answer this question!
For the next experiment, we take the popular Simpsons Trailer (1080p) and compress it at 2.5 mbps and 1 mbps using H.264/AVC (and ensuring all encoder configurations remain the same). Let’s look at the output for both bitrates.
We can clearly see that the images look quite similar to each other. This is because the motion is very low in cartoon video, and flat surfaces are easy to compress. This allows encoders to compress cartoons very efficiently, and produce great quality at low bitrates.
So, it is important to know your video before compressing it.
Note: Check out our detailed guide to Video Compression to learn more about video compression.
Wrapping up, when we talk about bitrate, there are a few of things to be aware of –
- A higher bitrate video is *generally* of higher quality than a lower-quality video. Remember that, video quality will not improve beyond a certain point (however high the bitrate).
- It’s “bits” and not “bytes.”
- Bitrate *alone* doesn’t determine the quality of a video. There are other important factors such as video codec, resolution, frame-rate, display-size, etc.
- The video codec used for video compression (H.264/AVC, HEVC, VP9, AV1) will greatly influence the output quality.
Now that we have understood what bitrate means, let’s move on to understanding what “video resolution” means.
What is Resolution or Video Resolution?
Video Resolution or Resolution of a Video is width by the height of a video. The unit of measurement of video resolution is Pixels.
Video Resolution is generally indicated in a couple of ways –
- using the height of the video like 1080p or 720p.
- or, by mentioning the values of width and height – 1920×1080, or 1280×720.
In the table below, we list some of the most popular video resolutions in OTT Video Streaming.
|1080p||1920 pixels||1080 pixels|
|720p||1280 pixels||720 pixels|
|480p||854 pixels||480 pixels|
|360p||480 pixels||360 pixels|
Video Resolution is very important because it affects what the user is seeing. Most video players adjust themselves to the viewing window or the device’s screen resolution.
If your user has a 48″ TV and you send him a 360p video, then his TV’s video player will upscale the video to fit the TV’s display window. And, if you upscale 360p to 1080p, the quality is not going to be good.
On the flip side, if your user has a smartphone and you send him UHD video, then he is unlikely to appreciate the video quality because his phone cannot show UHD video. The app will change the resolution to fit the device’s screen. Resolution plays a huge role along with bitrate in creating a smooth viewing experience, as we shall see next.
Secondly, when the resolution of a video is small (e.g. 480p), if you provide the encoder with a big bitrate budget (maybe 5mbps), use a good video codec (like HEVC or AVC), and choose the most complex setting the encoder can handle, then the quality will be great.
Did you see what we just did there?
We didn’t say “use a high bitrate” or “use a higher resolution”! We mentioned a bunch of factors and said that all of them put together will help get you a high quality video!
And this brings us back to our first question – which is more important? Bitrate or Resolution?
Bitrate or Resolution – Which Is More Important?
I hope the previous sections have convinced you that both bitrate and resolution are important for video streaming (Live or VOD).
It is important to understand a few things before you compress your video –
- Understand your audience and your content.
- Bandwidth that your audience has access to – High-speed connections or 2G/3G?
- Do they use a fixed connection? Or, are they always on the move and are using their mobile data plans?
- Devices your end-users are using: Are they typically watching on large screen SmartTVs, or are they primarily using hand-held smartphones?
- Nature of the content you’re streaming: Is it high-action movies, sports or low-action content like news, or simple-to-compress content like cartoons? Know your content.
- How much time do you have for compressing the video?
- If you have more time, then you can use complex algorithms to enhance video quality.
- If you have very little time, the video quality will suffer a bit.
- Which video codec should you use? Will your target audience’s devices be able to decode H.264/AVC, HEVC, AV1, or VP9?
Asking and answering these fundamental questions will help you to decide your bitrates and resolutions.
Now, let’s look a few practical examples where you need to choose a bitrate-resolution combination.
- The target audience watches video only on their smartphones: In this case, you should consider lower bitrates and perhaps, one 1080p, and concentrate on creating more combinations of 720p and bitrates around 2 – 3 mbps. This will allow your users to drop down to bitrates that might be more suitable to their mobile lifestyle and the possibility of a poor data connection.
- The target audience is urban and watches video only on SmartTVs or Roku: In this situation, you can easily create two or three variations of 1080p such as 1080p 8mbps, 1080p 6pmbps, 1080p 4 mbps, and keep your bitrate ladder loaded on the heavier side because you are almost assured that your users have access to high-speed internet and 1080p will look good on large TVs.
- Streaming news and talk shows, 24×7: In this case, it is easy for an encoder to compress your videos very efficiently, and so, you can choose 1080p and 3-4 mbps and end up with really high quality. To learn how to assess video quality, check out our guide to computing VMAF, PSNR, and SSIM on OTTVerse.com.
In this way, the first thing you should do is get a deep understanding of your target audience and their streaming habits. You can easily decide the various bitrates and resolutions that you need to use to compress your video by analyzing this information.
Wrapping Up – Bitrate vs. Resolution
That brings us to the end of our discussion on bitrate, resolutions, and understanding which of the two is more important for video compression.
I hope you understood the difference between both and how you need to use both bitrate and resolution (amongst a host of other factors) to achieve great video quality!
Until next time, take care and happy streaming!
Krishna Rao Vijayanagar
I’m Dr. Krishna Rao Vijayanagar, founder of OTTVerse. I have a Ph.D. in Video Compression from the Illinois Institute of Technology, and I have worked on Video Compression (AVC, HEVC, MultiView Plus Depth), ABR streaming, and Video Analytics (QoE, Content & Audience, and Ad) for several years.
I hope to use my experience and love for video streaming to bring you information and insights into the OTT universe.