Choosing CRF value for encoder

Context

Bandwidth usage by the Jami application is not optimal for all types of connections. Indeed, in some cases, the user experience is not good (satellite connection, …) despite the bandwidth management algorithm.

Observation

It is not necessary to aim for an optimal quality (CRF < 20) because beyond that, the visual perception is almost similar while the data flow (bitrate) required is much higher.

Objective

The purpose of this document is to verify the impact of a change in video quality with the CRF parameter of the encoder.

Test

These tests were performed by comparing :

The first one encoded with Jami’s current parameters
The second encoded with a lower quality

Each of these tests were performed for the following resolutions: 1080p, 720p and 436p.

For each of these resolutions several bitrates have been used:

300 Kbit/s (Jami low value)
1.5 Mbit/s (Intermediate value)
6 Mbit/s (High value)

The graphs show the evolution of the bitrate with the file being tested (resolution and specific set bitrate).

A visual comparison (side by side) was made for each test.

Thanks to this test we can estimate the bitrate that will be emitted in Jami according to the chosen parameters. We also have an overview of the visual quality.

1080p / 300 kbit/s / CRF28 1080p / 300 kbit/s / CRF38