2020 iMac 5K Display Stream Compression (DSC) and Its Role in Making Dual 6K Display Support Possible
The dual-6K display support of the 2020 iMac 5K puzzled me because of the huge bandwidth required. Bottom line is that Display Stream Compression (DSC) offers “visually lossless” bandwidth savings by compressing video data by up to 3:1. This is what accounts for the dual-6K display support.
Note that DSC is still poorly supported and even high-end video cards in the 2019 Mac Pro lack support for DSC.
Whether DSC is really visually lossless in the sense of professional photography and video work seems dubious*, so until and unless I can A/B compare, I will remain a skeptic that dual-6K support is suitable for professional use, even setting aside the bandwidth hit.
I didn’t really understand Display Stream Compression (DSC), so I did a little research.
* Tests show I have exceptional color discrimination, way out on the curve versus most people’s vision.
Emphasis added to excerpts below.
Display Stream Compression (DSC) is a VESA-developed low-latency compression algorithm to overcome the limitations posed by sending high-resolution video over physical media of limited bandwidth. It is a visually lossless low-latency algorithm based on delta PCM coding and YCoCg-R color space; it allows increased resolutions and color depths and reduced power consumption.
DSC has been tested to meet the requirements of ISO/IEC 29170-2 Evaluation procedure for nearly lossless coding using various test patterns, noise, subpixel-rendered text (ClearType), UI captures, and photo and video images.
... DSC supports up to 3:1 compression ratio with constant or variable bit rate...
DSC compression works on a horizontal line of pixels encoded using groups of three consecutive pixels for native 4:4:4 and simple 4:2:2 formats, or six pixels (three compressed containers) for native 4:2:2 and 4:2:0 formats...
Bit rate control algorithm tracks color flatness and buffer fullness to adjust the quantization bit depth for a pixel group in a way that minimizes compression artifacts while staying within the bitrate limits...
Note the “minimizes compression artifacts” part—great technology, but is it really suitable for professional work in color grading or fine-art photography?
There is additional information at “Tom’s Hardware: DisplayPort vs. HDMI: Which Is Better For Gaming?” that both clarifies and confuses: the table of data rates there does not mention the dedicated ~8 Gbps DisplayPort bandwidth that is available, which is over and above the Thunderbolt 3 data bandwidth. See Understanding Thunderbolt 3 Bandwidth and the Intel white paper referenced there for details.
Usable bandwidth using dual 6K displays vs data
Apple is lax at documenting 2020 iMac 5K technical details like the version of DisplayPort (“DisplayPort” with no version number), but it surely is Display Port 1.4 with 8b/10b encoding.
Assume a baseline dedicated 8 Gbps DisplayPort bandwidth along with a usable 25.92 Gbps for any general data (regular data, or video overflow data).
Dual 6K displays require bandwidth of 70.3 Gbps for 30-bit RGB color. Assuming 3:1 compression, the bandwidth requirement drops to ~23.4 Gbps—well within the bandwidth of a single Thunderbolt 3 bus but it does limit maximum performance for devices like fast SSDs.
Breaking it down, ~8 Gbps can be utilized from the dedicated DisplayPort bandwidth, and (23.4-8) = 15.4 Gbps of data bandwidth would also be consumed, leaving (32.4-15.4) = 17 Gbps write bandwidth for general purpose data. But with 8b/10/b encoding that really means (17*8/10) = 13.6 Gbps ~= 1.7 GB/sec usable bandwidth.
In other words, a fast SSD like a OWC Thunderblade would have a full gigabyte per second shaved off its performance (for writes).