Video resolution vs bit-rate

Article

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.

View the complete Ask iLounge archives...

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.

- Alex

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

The most asked question at iLounge.com:

How do I copy content from my iPod/iPhone onto my computer?

Converting DVDs, web video, TV shows and more:

The Complete Guide to iPod, Apple TV and iPhone Video Formats

Converting DVDs, web video, TV shows and more:

The Complete Guide to iPod, iPhone and Apple TV Video Conversion (Mac)

Converting DVDs, web video, TV shows and more:

The Complete Guide to iPod, iPhone and Apple TV Video Conversion (Win)

Best Speakers?

Best Headphones?

Best Case?

Best In-Car Solution?

Best Battery Extender?

Find more answers in the latest publications in our Library. Free downloads developed by the editors of iLounge!

Ask iLounge Archives:

6-7-13: Inline Remote + Mic adapters for iPhone

6-6-13: Filtering content by iTunes Store account

6-5-13: Deleting backups made under another iCloud ID

6-4-13: Blocking data access on an iPhone

6-3-13: Changing iPhone number for iMessage

View the complete Ask iLounge archives...

« Videos with no sound

Transferring videos to iPod »

Comments

If you have a comment, news tip, advertising inquiry, or coverage request, a question about iPods/iPhones/iPad or accessories, or if you sell or market iPod/iPhone/iPad products or services, read iLounge's Comments + Questions policies before posting, and fully identify yourself if you do. We will delete comments containing advertising, astroturfing, trolling, personal attacks, offensive language, or other objectionable content, then ban and/or publicly identify violators.

Recent Reviews

New Deals

Shop for Accessories: Cases, speakers, chargers, etc.

iLounge

Article