Bitrate vs. Resolution – Which is more important for video streaming? This is a question that most streaming services need to think about while designing their bitrate ladders and often has people confused. This article shows how bitrate and resolution greatly influence your video streaming service’s QoE and QoS, why it is important to know the difference between them, and how they interact.
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. Look at all the options 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 about which combination to choose? 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.
Table of Contents
What is Bitrate or Video Bitrate?
Video Bitrate or (bitrate) is the number of bits used to deliver 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 in one second. If your video has a bitrate of 3 mbps, we can assume that three megabits of video will be transmitted to the player every second.
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: the bitrate is kept constant while sacrificing video quality
- VBR or Variable Bitrate: the 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, read this article to understand the basics of video transcoding.
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
- AVC vs. HEVC: Detailed Results Showing 50% Gains For Varying Content
How does Bitrate Affect Video Quality?
Understanding how bitrate affects video quality is important to understand how video compression or transcoding works. Video transcoding or compression does two important things –
- It converts blocks of video from the “pixel domain” to the “frequency domain” using the Discrete Cosine Transform or DCT. Several other transforms can be used, but for the sake of this article, let’s stick to the DCT. So, a block of video is converted to a block of frequency coefficients. Click here to read an intuitive explanation of the DCT.
- Next, the transcoder discards many coefficients using a process called “Quantization.”
- Throwing away coefficients reduces the amount of data needed to carry the video.
- But throwing away coefficients results in a loss of information and video quality – so you must carefully decide how many coefficients you want to discard.
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 a video lightly, the file size is big, but the quality is great.
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 compressed a sample video (CrowdRun) 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 and lower bitrate videos?
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 quality is excellent; for the same video, when we use a bitrate of 1.5 mbps, the quality is quite bad for CrowdRun video.
But does a low bitrate always produce a poor-quality video? Let’s run a quick experiment to answer this question!
We take the popular Simpsons Trailer (1080p) for the next experiment and compress it at 2.5 mbps and 1 mbps using H.264/AVC (in FFmpeg) without changing the rest of the encoder configurations. Let’s look at the output for both bitrates.
We can see that the images look quite similar to each other. This is because the motion is very low in cartoon videos, and flat surfaces are easy to compress. This allows encoders to compress cartoons efficiently and produce great quality at low bitrates.
So these two experiments tell us something very important – it’s not just the bitrate or resolution that is important; it is also the nature of the video or content in the video that is important and plays a role in determining the file size and quality!
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 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. Other important factors include 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. Check out this article demonstrating a 50% reduction in file size when you switch from H.264/AVC to HEVC.
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.
The table below lists some 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 sees. 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, his TV’s player will upscale the video to fit the TV’s display window. And, if you upscale 360p to 1080p, the quality will not be good.
On the flip side, if your user has a smartphone and you send him a UHD video, 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. As we shall see next, resolution plays a huge role along with bitrate in creating a smooth viewing experience.
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 many factors and said that all of them would 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) and other factors!
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 using their mobile data plans?
- Devices your end-users use: Do they watch on large-screen SmartTVs or primarily 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 to compress the video?
- You can use complex algorithms to enhance video quality if you have more time.
- If you have little time, the video quality will suffer, as most algorithms will be turned off.
- 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.
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 other factors) to achieve great video quality!
Until next time, take care, and happy streaming! In the meantime, let us know your opinions on bitrates, resolutions, and important factors for video streaming.
Krishna Rao Vijayanagar
Krishna Rao Vijayanagar, Ph.D., is the Editor-in-Chief of OTTVerse, a news portal covering tech and business news in the OTT industry.
With extensive experience in video encoding, streaming, analytics, monetization, end-to-end streaming, and more, Krishna has held multiple leadership roles in R&D, Engineering, and Product at companies such as Harmonic Inc., MediaMelon, and Airtel Digital. Krishna has published numerous articles and research papers and speaks at industry events to share his insights and perspectives on the fundamentals and the future of OTT streaming.