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Bitrate

lastmod 2022-07-13created 2015-02-2525 edits

In telecommunications and computing, bitrate is the number of bits processed per unit of time (for example, per second).

Bitrate is calculated using bits per second (bit/s or bps). SI prefixes such as kilo, mega, giga, and tera are attached to it.

Bitrate formulas

The relationship between size and bitrate is:

  • Size (byte) = Bitrate (bps) * playback time (seconds) / 8 (convert to bytes)
  • Bitrate (bps) = Size (byte) / playback time (seconds) * 8 (convert to bits)

The Bit Rate of Video and Audio is

  • Video bit rate = (width x height) * fps * bits per pixel
  • Audio bit rate = sample rate * bit depth * audio channel count

Examples

The Bitrate of a 30fps Full HD video sampled at 8bit is: 

(1920 * 1080) * 30 * 8 = 497,664,000 bps = 497,664 k/bps

WARNING

Note that "bits per pixel" for video does not mean something like the 24bit of RGB888, but quantized bits (Quantized bits). The example above used 8bit to aid understanding, but you should use quantized bits (Quantized bits). Just look at the #Video (MPEG1 or MPEG2) section: Blu-ray Disc quality is 50Mbps, while the value calculated above is a whopping 497Mbps. Far too high.

The Bitrate of 2ch stereo, 16bit, 48KHz audio is: 

48,000 * 16 * 2 = 1,536,000 bps = 1,536 k/bps

Making the video bitrate example more realistic and easier to understand

A single 26-minute anime episode from an Ohys release takes up 250MByte. You can tell just from the file name. 

[Ohys-Raws] Overlord IV - 01 (AT-X 1280x720 x264 AAC).mp4

Let's check the Audio/Audio information.

  • Duration: 26 minutes 1 second
  • Video
    • Dimension: 1280 × 720
    • Codec: H.264 (High Profile)
    • Frame rate: 23.98 frames per second
    • Bit rate: 1110 kbps
  • Audio
    • Codec: MPEG-4 AAC
    • Channels: Stereo
    • Sample rate: 48000 Hz
    • Bit rate: 173 kbps

The audio size is: 

173000 bps * (26*60) / 8 = 33,735,000 byte

Subtracting the audio size from the file size (ignoring header size and the like) gives the video size: 

(250*1024*1024) byte - 33,735,000 byte = 228,409,000 byte

Let's work back to the video bitrate: 

Bitrate (bps) = 용량 (byte) / 재생 시간 (초) * 8 (bit로 변환)
Bitrate (bps) = 228,409,000 / (25*60) * 8
Bitrate (bps) = 약 1,218,181 bps

Note that since it is 24fps, 

Video bit rate = (width x height) * fps * bits per pixel

bits per pixel = Video bit rate / (width x height) / fps
bits per pixel = Video bit rate / (width x height) / fps
bits per pixel = 1,218,181 bps / (1280 x 720) / 24fps
bits per pixel = 0.05507

In short, this means an Ohys release has a "bits per pixel" of about 0.05507. Looking at the #Quantized bits table, it falls (when rounded) under the "lowest quality allowed by twitch" item.

As an FFmpeg command: 

ffmpeg -i input.mp4 -acodec aac -ac 2 -ab 192k -vcodec libx264 -profile:v high -b:v 1150k output.mp4

Quantized bits

Sample "bits per pixel" values used when calculating video Bitrate:

  • 0.06 (lowest quality allowed by twitch)
  • 0.08 (the bitratecalc "low" quality approximately)
  • 0.1 (recommended by twitch - the sweet spot)
  • 0.15 (the bitratecalc "high" quality exactly)
  • 0.16 (highest quality allowed by twitch)

Video (MPEG1 or MPEG2)

  • 16 k/bps - video call quality
  • 128~384 k/bps - business-oriented video conferencing system quality
  • 1150 k/bps - VCD MPEG1, Video CD Standard quality
    • e.g. Ohys release
  • 2520 k/bps - SVCD MPEG2, Super Video CD quality
  • 9200 k/bps - DVD MPEG2, DVD Standard quality
  • 13~19 k/bps - HDTV, what is commonly called HD quality; typical quality of *.ts, *.tp
  • 15 m/bps - HD DVD quality
  • 50 m/bps - Blu-ray Disc quality

Audio

  • 32 k/bps - AM quality
  • 96 k/bps - FM quality
  • 192 k/bps - DAB (digital audio broadcasting) quality
    • e.g. Ohys release
  • 224~320 k/bps - CD quality
  • 96~640 k/bps - Dolby Digital (AC3), lossy data compression
  • 1,536 k/bps - DTS, CD PCM audio
  • 6,000 k/bps - lossy data compression of the DTS-HD High Resolution AUDIO standard
  • 18 m/bps - lossless data compression of the Dolby TRUE HD standard (VBR)
  • 24 m/bps - lossless data compression of the DTS-HD MASTER AUDIO standard (VBR)

Trivia

To give a practical sense of how fast these units are:

  • When downloading a file, it is hard to exceed 10 MB/s no matter how fast the server is.
  • When encoding a video file, the file size is bitrate * playback time. In other words, the higher the bitrate, the larger the file.
  • For audio files, mono is usually set to 128Kbps.
  • To upload a 1080p video to YouTube, you need to set the bitrate to around 8Mbps to 15Mbps.#
  • For a 1080p, 60fps internet broadcast to go out at proper quality, a bitrate of at least 5000Kbps (5Mbps) must be guaranteed. High-quality broadcasts usually need at least 7000~8000 Kbps (7~8 Mbps) to stream properly without the picture breaking up into blocks (the so-called blocking artifacts). However, as of 2018, few broadcasts can stream at this quality, and broadcasting platforms only provide such a streaming environment to some broadcasts with many viewers. If an ordinary internet broadcaster streams with these settings, it will very likely buffer. In that case, giving up either 1080p or 60 frames and lowering the bitrate to around 5000~6000 Kbps (5~6 Mbps) lets you stream reasonably smoothly.

See YouTube#Recommended upload encoding settings.

See also

Favorite site

References

  1. Blog.naver.com_-limsaid-_calculate_BitRate.pdf ↩

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