Bit(Rate)s & Pieces

 

BIT(RATE)S AND PIECES

During the recent Top Ten competition Jim Morton-Robertson’s film wouldn’t play on the club’s Blu-ray player. The picture would stutter every few frames as it struggled to process the footage.

I noticed that the file size was 1.42Gb which was ludicrously large for a 3 minute film so I asked Jim to check the bitrate of his clip, which was 64582 kbps. Jim changed his export settings, this time producing a file with a much more suitable bitrate of 16055 kbps and a far more appropriate file size of 365Mb.

 

But what does this all mean and why does it matter?

To start with a brief explanation of video bitrates.

Digital video is made up of bits and bitrate refers to the number of bits processed in a set amount of time, listed in kilobits per second (kbit/s). This usually determines the file size as 1 byte of data is made up of 8 bits, so 1Mb of data = 8 megabits (Mbit/s) or 8000kbit/s. The ultimate file size is therefore decided by the sum of the formula Bitrate x Duration.

Jim’s film might have a resolution of 1080p but with a bitrate of 64582kbps (64Mbitp/s) crammed into just 3 minutes, it contained the same data as a 4K file, which our Blu-ray player clearly isn’t capable of handling. As this was a setting oversight that was easily corrected, it is worth sharing this with everyone to be a bit more diligent when exporting your files in the future.

 

So why do we need high bitrates?

As cameras become increasingly capable of capturing ultra HD images (6K is already here) this needs to be reflected and replicated in the films we produce, so higher bitrates will ensure better quality film clips.

However this is more for the benefit of online streaming. Many of these sites, like YouTube, are notorious for compressing the hell out of clips uploaded to their platforms, so users have to export them at a higher bitrate (eg: a 1080p file encoded with a 4K bitrate) to trick them into thinking they are compressing a bigger file, ensuring decent quality post-compression.

YouTube suggest the following bitrate settings for uploading to their platform:

 

SD (DVD quality) 480p = 2.5 Mbit/s / 2500kbit/s

HD (Blu-ray quality) 720p = 5 Mbit/s / 5000kbit/s

HD 1080p = 8 Mbit/s / 8000kbit/s

HD 1440p (2K) = 16Mbit/s / 16000kbit/s

HD 2160 (4K) = 35-45 MBit/s / 35000kbit/s / 45000 kbit/s

 

N.B – These targets are based on SDR (Standard Dynamic range) whilst HDR (High Dynamic Range) aka Ultra HD, would require higher target values.

 

What about for playback at home on TV or PC?

The truth is you can get a perfectly fine looking HD clip for playback on your TV and PC without needing extremely high bitrates because the files aren’t going to be compressed. The only stumbling block will be the capabilities of the device you play them on, whether they have are able to accommodate current codecs – preferably H.264 for MP4 files.

As we mainly use 25fps SD/50fps HD, the following suggested basic bitrate targets are based on these parameters, again based on SDR bitrates:

 

SD (DVD quality) 480p = 1.2 Mbit/s / 1200kbit/s

HD (Blu-ray quality) 720p = 2.5 Mbit/s / 2500kbit/s

HD 1080p = 5 Mbit/s / 5000kbit/s

HD 2160 (4K) = 10 Mbit/s / 10000kbps

 

N.B – The following illustrations are from Adobe Premiere Pro CC 2018. Your editor may have different settings and options.

 

Your editor/encoder should offer a range of different settings and options for exporting your files, to cover all file formats, codecs, devices and streaming platforms. To wit:

One thing which is standard is suggesting a “target” and “maximum” bitrate depending on which settings you choose. For 1080p HD this will be around 20 Mbit/s (target) and 25 Mbit/s (max) though they will usually come in around 15-18 mbps.

 

Here you can see there is a preset which will encode the video suitable for uploading to Vimeo, with the following settings:

 

However, you could still get a good quality video with bitrates half these sizes. For those unsure of what to select, you can trust the  presets of your editor/exporter and use the “Match Source” option to replicate the project/timeline settings.

These values are easily adjustable, either manually or by changing the output resolution in your project/timeline, for a higher setting if you want a better quality film. Even with a high bitrate, the output result is still in line with the YouTube suggestions yet won’t produce and excessive bitrate or file size.

This example is the automatic setting from the Match Source – High Bitrate option:

To complicate things further, there are two kinds of bitrates – Variable (VBR), a dynamic setting which only peaks when the video detail requires it to, resulting in a smaller file size, and Constant (CBR), which stays at one rate throughout, but means a larger file size. Both have their benefits and setbacks, but most editors will default to VBR so the choice is made easier for you.

You’ll notice that there are two VBR options Pass 1 and Pass 2. Pass 1 analyses how much bitrate is required and therefore is a quicker encode, Pass 2 will render the entire video compression thus yields a better quality images but takes considerably longer (many hours!) to encode and export, so most people tend to stick to Pass 1.

 

How do I check the bitrates of my files?

Very easily. If you are using a Windows PC simply right click on the file and a new window will appear. Scroll to the bottom and select “Properties”:

 

In the window that opens, select the “Details” tab:

 

The main information is the “Video Data Rate” and “Total Bitrate” , the latter being the more crucial one.

 

For the more advanced user, I can recommend a tool you can download for free called MEDIAINFO which will tell you everything and more in great detail.

Once installed, as above, simply right click on the file and select “MediaInfo” from the option window:

 

Every detail about the file from the video encode to the audio and more is there for your perusal:

 

Hint – Make sure to go to “Options” then “Preferences” to set the “Tree” view in this illustration as default as this is easiest one to read, but not the default setting, otherwise select View from the top menu then “Tree”.

 

I hope you found some of the information here useful. I appreciate the technical side regarding bitrates might not seem easy to comprehend at first (I don’t understand half of it either!) but the message here is more about alerting you to this issue and hopefully avoid any future situations where the club Blu-ray player can’t play your files.

 

Thanks for reading.

2 comments on Bit(Rate)s & Pieces

  1. Another interesting article which helps me to understand bitrates, something I’d never got my head round. Thank you Lee

    Jane

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