Video resolution vs bit-rate
Ask iLounge offers readers the opportunity to get answers to their iPod-, iPhone-, iPad-, iTunes-, or Apple TV-related questions from a member of the iLounge editorial team. We'll answer several questions here each week, and of course, you can always get help with more immediate concerns from the iLounge Discussion Forums. Submit your questions for consideration using our Ask iLounge Submit Form. We reserve the right to edit questions for grammar, spelling, and length.
Q: I have a new iPod classic 160 GB that I’m loading with videos. I’ve read that a 640x480 file is the same MB size as a 320x176 file if encoded at the same kbps (bit-rate). Is this correct? Is there an advantage to the bigger size? Is there a “definitive” guide you can point me to that explains the relation between resolution and kbps clearly. Thanks in advance, and keep up the great work.
A: As odd as it may sound, it is actually correct that a given video file’s size is based on the bit-rate of that file, expressed in kbps or kilobits per second, rather than on the resolution.
Much like audio files, the bit-rate defines how much data is stored for each second of playback. This means that a 1500 kbps video file has 1,500,000 bits of data stored for each second of playback time. In that sense, basic math will let you determine the size of a file relative to the bit-rate.
This may seem counterintuitive, since you expect a higher resolution file to occupy more space, in the same way that a higher resolution picture occupies more space. While this may be true in the case of an uncompressed video, the reality is that modern video compression algorithms are “lossy” by nature, and do not actually store a full image for each frame of video—instead, “reference” frames are stored, and then additional data is used to represent the changes between reference frames. These are interpreted during playback to produce relatively smooth motion video.
The catch is that there actually is a direct relationship between the bit-rate of a video and the resolution—or perhaps more accurately, between the resolution and an acceptable bit-rate. The larger the image, the more data is going to be required to accurately represent the motion within that image. For instance, although you may technically be able to encode a 640x480 and a 320x172 video file at the same bit-rate, you will either be creating an unnecessarily large 320x172 video (if the bit-rate is too high), or creating a very poor quality 640x480 video (if the bit-rate is too low). Generally, the bit-rate needs to be appropriate for the video resolution in order to produce an acceptable playback experience with reasonable quality. Too low of a bit-rate will produce a movie file that with visual problems such as “artifacting” and “macroblocking” and possibly even choppy playback (skipped frames).
As a rule, for acceptable playback when encoding video for the iPod using the H.264 codec, a 320x240 video file (encoded for the iPod screen), can generally be done with a bit-rate of approximately 750kbps, whereas a 640x480 file should use a bit-rate of approximately 1500kbps. On the other hand, videos encoded into standard MPEG-4 require slightly higher bit-rates, as the standard MPEG-4 codec is not as efficient.
You can find a much more detailed discussion about this, as well the suggested formats and display resolutions in our iPod 101 tutorial The Complete Guide to iPod, Apple TV and iPhone Video Formats
- Will removing a credit card from Safari also remove it from Apple Pay?
- Can I mute Handoff calls coming into my Mac from my iPhone?
- How do I keep my iPhone calls from ringing on my Mac?
- Why doesn’t Traffic show up on my Today Notifications Screen?
- Why doesn’t my iPhone reconnect to Wi-Fi after I turn it on?
- Why can’t I see the iPad-style landscape view on my iPhone 6 Plus?
- Spotify penalizing artists who release Apple Music exclusives [Update: Spotify responds]
- Apple releases new round of iOS 10 and tvOS 10 betas
- Report: 2017 iPhone to eliminate home button
- Apple to add payment technology to iPhone for transit passes, Apple Pay in Japan
- Apple releases iOS 9.3.5 ‘security update’
- Report: Apple developing its own Snapchat-style social video editing iOS app
- Apple announces Apple Music Festival lineup including Alicia Keys, Britney Spears + more
- Universal calls an end to exclusives amid criticisms that Apple Music is hurting the industry
- Apple reveals some of its upcoming AI advancements for the iPhone
- Apple Music’s royalty rates complicate Spotify’s contract negotiations
- Western Digital My Cloud (OS 3)
- Distil Union Stanley Stand
- Philips Sonicare FlexCare Platinum Connected Bluetooth Toothbrush
- Audeze EL-8 Titanium Over-Ear Headphones
- Defined Corp Dome Stand for Apple Watch and iPhone
- Speck StyleFolio Pencil for 9.7” iPad Pro
- Audeze Sine On-Ear Headphone
- First Alert Onelink Wi-Fi Smoke + Carbon Monoxide Alarm
- Logitech Create 9.7” iPad Pro Keyboard Case
- iDevices Outdoor Switch Power Outlet
- Inside the betas: iOS 10 Photos gets Advanced Computer Vision
- Inside the betas: iOS 10 Music app delivers ‘clarity and simplicity’
- Inside the betas: iOS 10 Maps gets a major redesign
- Inside the betas: iOS 10 shakes up the user experience
- Inside the betas: watchOS 3 promises a real speed boost
- Inside the betas: A sneak peek at what’s new in tvOS 10
- Filling the Gap: A look at third-party HomeKit apps
- Instant Expert: Secrets & Features of tvOS 9.2
- Instant Expert: Secrets & Features of iOS 9.3
- Opinion: Why Apple needs a dedicated HomeKit app