Bitrate = Data per Time

Mbps / 8 = MB / second
MB/s * 60 = MB / minute
MB/min * 60 = MB / hour
MB / 1024 = GB / 1024 = TB

If you know the bitrate, you can calculate the size of your files, by multiplying the bitrate by the runtime duration (time).

A fixed bitrate however is only available for:

constant bitrate (lossy) compression.
uncompressed.

For lossy compression, bitrate is an encoding parameter, but for uncompressed we’ll show later in this session how to calculate the size.

For lossless or variable bitrate, the exact size cannot be pre-calculated - only estimated. The actual size can still vary, since the size of these kind of encodings greatly depends on the content being encoded. Rule of thumb: Less motion = smaller size and vice versa. Noise make files larger.

There’s also the choice with some formats between constant bitrate and variable bitrate. Constant bitrate is simpler, more robust, straight to calculate/estimate. Simpler=preserves better. Variable bitrate is another way to compress space, but makes decoding/interpreting the data more complicated and may lead to different behavior in different tools/setups.

Bitrate: Constant vs Variable

CBR: Constant BitRate
Fixed value of data-per-time. Great to calculate/estimate size/bandwidth. Not optimized for saving space.
VBR: Variable BitRate Encoder increases/decreases bitrate, depending on complexity of AV content. Common to define min/max boundaries. Smaller files, but hard(er) to estimate size/bandwidth.

Size

Bitrate = Size vs Quality
(bitrate as parameter exists only for lossy encoding)
Uncompressed > lossless > lossy
Larger = less processing power, but: more disk/net I/O

Performance

Often a tradeoff between:

Processing power (CPU/RAM)
(format/algorithm complexity, smaller files)

I/O bandwidth (disk/network)
(data size)

Format Examples

	Video	Audio	in Container
Preservation:	V210 FFV1, J2K High-bitrate lossy …	PCM FLAC	MOV MKV MXF
Mezzanine:	ProRes H.264 DVCPRO50 …
Access:	H.264 VP9 DVD BluRay …	MP3 AAC Opus …	MP4 ^(M4V) MKV ^(WebM) MPG

Quality & Compression

Lossy, Lossless, Uncompressed?

“AV compression is commonly lossy, but not necessarily irreversible.”
“If you can define bitrate as encoding parameter, it is definitely a lossy format.”
“Uncompressed is the most dead simple to reverse engineer in the future, but not the most robust format.”
“Lossless allows fully-auto-validated format migrations. bit-proof.”

Visually lossless = lossy 😇

“Lossy compression” literally means: some parts of the content are “lost” (=thrown away) in order to gain more compression (=smaller files). This process is irreversible and accumulates (=generation loss).

Of course the above image is exaggerated, but it shows pretty well what lossy compression is and what artefacts a typical MPEG-like compression algorithm produces.

(btw: This is a snapshot image of the highest-quality version of the video on the original website (around 2009)).

Due to the vast data sizes of AV, lossy encoding is the current de-facto default in most environments that handle digital AV (except film). High quality lossy codecs may sometimes be called “visually lossless” (e.g. ProRes). Which only means that they’re pretty good at hiding their artefacts from your eyes/ears. High-bitrate lossy formats are standard in broadcast and production.

For audio this is different, merely because the data size is so “trivial” for modern computer systems that the studio standard is uncompressed (PCM) in WAV. Suggesting lossy encoding to audio engineers as production source format would probably get you kicked out of their studio ;)

Generation Loss

Here’s an example where one can see the difference between each encoded version in popular broadcast codecs.

Since it’s almost certain that there’s not “evergreen format”, any format will sooner or later have to be migrated to another one. The longer an item is preserved, the more often it will encour such a format change (transcoding). Therefore, generation loss should be considered.

Also: Usage of archive material: What happens if you hand out your material to an editor, say on DVD or a mezzanine format? The editor then edits and exports to another format, the broadcaster or cinema then to another? …and in the end this work is then also converted to an access format.

And theeeen, years later that access copy is used as a source for a documentary or online edit.

How many generation losses accumulate here? How could they be reduced?

Generally: please try to avoid unnecessary generation losses. Why not? :)

Lossless

“It’s like ZIP for film!”

No generation loss
Way larger than lossy
Smaller than uncompressed

Uncompressed

No generation loss
Dead simple (=preserves well)
The largest possible version
Uncompressed != Uncompressed?
There’s more than just 1 “uncompressed”
(Ex: RGB, BGR, UYVY, YUY2, V210, etc)

Uncompressed - Think of it as:

First pixels of RGB Lines/Frame (8bpc)

RGB RGB RGB RGB ...       (4 pixel * 3 Byte)  = 12 Bytes)
= 24 bit/px

Same with alpha (transparency) channel

RGBA RGBA RGBA RGBA ...       (4 pixel * 4 Byte)  = 16 Bytes)
= 32 bit/px

Quality Tips

Avoid generation loss. ^{(if possible)}
Avoid resizing. ^(=rescaling)
Don’t invent more bits. ^{(e.g. DV as v210)}
Preserve colorspace / bit-depth, etc.
More headroom for lower quality. ^{(e.g. 24bit/96kHz for Shellack)}
Select high enough bitrate. ^(lossy)
(Or proper “Constant Rate Factor (CRF)”)

Calculating Size

Filesize

Uncompressed Image

Width(px) x Height(px)
x Bits-Per-Pixel(bpp)
x FPS
/ 8 = 1 second (in Byte)

Filesize

Uncompressed Audio

Samplerate x Bit-depth
x channels (even if silence!)
/ 8 = 1 second (in Byte)

Exercise:

How large is an 8bpc SD PAL minute (720x576px, @25fps) minute?
with 4 channels audio at SDI standard (48kHz/16bit)?
Or a 2k 16bpc(!) scan (including 6 channels audio at 48kHz/24bit)?
Or 74min. of audio CD (red-book standard: 44.1kHz/16bit, stereo)?
60min 50MBit video (in GB)?

Default Formats

Video:	Image: Default = lossy encoding Audio: production = PCM, consumer = AAC Metadata: Often embedded. Sometimes sidecar.
Film:	Image: DPX / TIFF files Audio: PCM in WAV Metadata: Mostly sidecar, some MD in image files.

File formats: Quality & Size

Bitrate = Data per Time

Bitrate: Constant vs Variable

Size

Performance

Format Examples

Quality & Compression

Lossy, Lossless, Uncompressed?

Visually lossless = lossy 😇

Generation Loss

Lossless

Uncompressed

Uncompressed - Think of it as:

Quality Tips

Calculating Size

Filesize

Uncompressed Image

Filesize

Uncompressed Audio

Exercise:

Default Formats

File formats:
Quality & Size