In this article, we discuss a fundamental question in video compression and processing for OTT – “Bitrate vs. Resolution – Which Is More Important For Video Streaming?”. Many people have this confusion because both factors have a very profound influence on your encoder’s output.
Sometime back, we wrote about compressing videos using HandBrake, a top-rated open-source software. In HandBrake, too, you need to decide which combination of bitrate and resolution you want to use for compressing your video. Just look at all the options that HandBrake provides – it is mind-boggling!
What do you think you should do?
- choose 1080p or 720p?
- choose 5mbps or 3mbps?
- choose 1080p @ 5mbps or 1080p @ 7mbps?
If you are confused, it’s perfectly fine. 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 simply, Bitrate is the number of bits of video information transmitted per second. The units of bitrate are typically
- kbps or kilobits per second
- mbps or megabits per second
When you compress a video, you typically have to tell the video compression software that it should not exceed a certain bitrate.
For example, if you tell your software to compress at 3mbps, the software will compress each second of the video such that only 3mb of data (on average) is sent to the decoder or the player. That is what bitrate means!
When you talk about bitrate, you are essentially talking about how much the encoder is expected to compress the video, while sacrificing video quality. The more you compress, the lower is the video quality (not always, as we’ll see later).
There are several algorithms for deciding and using a bitrate value for compressing video and they 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 kep constant while allowing the bitrate to fluctuate within a limit or cap.
A detailed discussion of these rate control methods is too much for this article and we’ll dive into this in a dedicated write-up.
So, when we talk about bitrate, there are a couple of things to be careful about –
- A higher bitrate video is *generally* of higher quality than a lower-quality video. I said “generally” because video quality will not improve beyond a certain point (how much ever you increase the bitrate).
- It’s “bits” and not “bytes.”
- Bitrate *alone* doesn’t determine the quality of a video, and you have to consider other factors such as resolution, frame-rate, display-size, etc.
- The codec used for video compression: H.264/AVC, HEVC, VP9, AV1? The same bitrate will provide wildly different video qualities if you change the video codec and its settings/tools.
How does Bitrate Affect Video Quality?
To understand how bitrate affects video quality, it is important to understand how video compression works. When you compress video, the compression algorithm does two things –
- 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.
- Discards a lot of the frequencies using Quantization in a way that the human eye cannot perceive this loss of information.
In essence, when you compress a video, you are throwing away some information so that the video quality is not lost. If you compress the video heavily, you lose a lot of information, and the effect of compression is visually perceivable. If you don’t compress the video a lot, then the file size is big, but the video quality is excellent.
This is called the rate-distortion tradeoff in video compression. The smaller the rate, the poorer the video quality – assuming that the resolution is fixed.
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.
Well, it’s quite obvious that the upper one is the lower bitrate and the second screenshot 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, is this always true? Does a low bitrate always produce a poor quality video? I dunno – let’s run an experiment, shall we?
Let’s test our assumption using cartoon video. For the next experiment, we take the popular Simpsons Trailer (1080p) and compress it at 2.5 mbps and 750 kbps. Quite different from the previous experiment, right?
Let’s look at the output, now.
Now, can you tell me which one looks better? They look quite similar to each other, right?
Unlike the previous experiment using CrowdRun, where the bitrates played a huge role, we see that we can get pretty good quality using low bitrates in a cartoon video. 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 give 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.
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 in video streaming 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 display UHD resolution. 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.
Bitrate or Resolution – Which Is More Important?
So, when you sit down to compress a video, and you ask yourself – “Which is more important? Bitrate or Resolution?” I hope the previous sections have convinced you that both are important – bitrate and resolution are both very important for live video streaming.
It is important to understand a few things before you compress your video –
- First and foremost, understand your audience. I can’t stress this enough!
- What is the typical bandwidth that your audience has? Are you streaming to an urban population with high-speed Internet? Or to a rural population with low Internet speeds?
- Do they use a fixed connection (cable internet)? Or, are they always on the move and are using their mobile data plans?
- What devices are your end-users using? Are they typically watching on large screen SmartTVs, or are they primarily using hand-held smartphones?
- What is the 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 on your hands for compressing the video? If you spend 15 mins vs. 3 hours to compress a video at 3mbps, it is quite likely that the video compressed using 3 hours will look better because it has had more time to apply complex algorithms and enhance video quality. If you have very little time, you might consider reducing the video’s resolution to compress it quickly and use the available bitrate better.
- 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!