When converting videos, most people use presets for 'iPhone' or simply 'MP4'. But using them might result in poor video quality, or you might end up with files that are too big, and use up a lot of your phone's storage. If you want to be more precise about things such as resolution, bitrate and aspect ratio, you need to understand what the jargon means. We explain what you need to know.

Resolution

If you're converting video from an 4K source, such as a GoPro, you'll start with a big, high-quality file. Let's say you want to watch that video on your smartphone. There's little point in retaining the video's full resolution of 3096x2160, even if your phone can play it.

Manual video settings

In your video converter, enter the resolution of your phone's screen. This will reduce the size of the resulting video file, and it should also make it look better on your phone, as well as saving the phone from having to downscale the video to fit.

Aspect ratio

This brings us neatly to aspect ratio. 1080p video has an aspect ratio of 16:9. Divide 16 by 9 and you get 1.77, which is the same as dividing 1920 by 1080. To find your phone's aspect ratio, divide the resolution of its longest size by its shorter side.

Some of the newest phones have 18:9 screens, or 2:1 - they're twice as wide as their height.

When your converted video has a different aspect ratio to the original, you either lose information (chopping off the sides or top and bottom) or gain black bars at the sides or top and bottom.

Each video converter is different, but it should give you the option of how to handle an aspect ratio change. Here's how Freemake handles it: 

Aspect ratio of video

Framerate

This is the number of video frames captured or displayed per second. In the UK, we have a standard (PAL) which is 25fps (frames per second). Movies use video with 24fps. We'd advise leaving the framerate alone, unless your device can't play the video at its original frame rate. The original Apple TV, for example, is limited to playing 720p video (1280x720 resolution) at 24fps.

Container

Codec and container are two frequently confused terms. One reason for this is that the codec and container can share the same name, such as MP4 (MPEG-4).

Put simply, the container is a way of packaging the audio and video together in one file. You can think of it like a zip file which can hold several files of different formats. Examples of containers include: .MP4, .MKV, .AVI, .MOV, .OGG, .ASF.

Codec

Within a video container is an audio file and a video file. A codec describes the method by which each has been compressed.

For example, an .AVI container might contain DivX video and MP3 audio files. An .MP4 file might contain MPEG-4 video and AAC audio. One of the most popular containers for HD video is .MKV, which can include H.264 video and AAC audio. But the latest is H.265 which is used for most 4K video as it's much more efficient that H.264 and results in smaller files without a loss in quality. It's also used to broadcast 4K video and virtually all 4K TVs support it.

For the vast majority of uses, including for Android and iOS devices, the MPEG-4 video codec is best. To be specific, H.264 (also known as MPEG-4 AVC) is the most efficient rather than plain MPEG-4, so you'll get the best image quality for the smallest file size by choosing H.264. Video converters sometimes call it x264. 

Bitrate

The video and audio within a container file have their own bitrates. Bitrate is a measurement of the number of bits that are transmitted over a set period of time. The bottom line is that the more bits in your video, the higher quality the image.

For video, bitrate is usually expressed in Mbps - megabits per second. Blu-ray discs typically have a bitrate of between 20 and 30Mb/s. That's way too high for a smartphone or tablet, where you can get away with perhaps only 2-5Mb/s.

Bitrate is related to file size - you can calculate the expected file size by multiplying the bitrate by the video's duration. Don't forget to add the bitrate and duration of the audio as well. Audio bitrate is much less than video, and is typically less than 320Kb/s (kilobits per second). 1Mb/s = 1,000Kb/s.

You'll have to experiment with bitrate to find the optimum setting for your device. Set it too low and you'll end up with a nice small file, but poor quality - especially when there's a lot of movement on screen.

Constant vs. variable bitrate

Making matters more complicated, you can opt for a fixed or variable bitrate. With variable bitrate, you may (or may not, depending on your video converter) be able to set a minimum and maximum bitrate. The encoder then tries to compress the video as efficiently as possible, using the minimum bitrate when there's little or no movement, and the maximum when there's movement in the entire frame.

The advantage is that you should get better quality video with a smaller file size, but variable bit rate isn't usually the best option if you're streaming the video across the internet.

Fixed bit rate means the bitrate is constant throughout the video. This usually gives smoother playback, at the expense of a larger file.

Again, you may have to experiment to see what works best on the device on which you're playing the video.

If you're unsure about the video specifications for your device, look in the user manual or search online to find them.