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Apple 2020 iMac 5K: Might it have Dual Thunderbolt 3 Busses when Configured with Radeon Pro 5700 or Radeon Pro 5700 XT GPU?

Get 2020 iMac 5K at B&H Photo.

More about the 2020 iMac 5K and see yesterday’s overview of the 2020 iMac 5K.

See also:

Jeremi M writes:

I am a video editor that currently works off a maxed out 2019 iMac 5K.

My biggest question about the 2020 iMac update before it came out was if it would add a 2nd Thunderbolt 3 bus as I edit with LG Ultrafine 5K display on top of using a lot of high speed external SSDs via Thunderbolt 3. I'm probably maxing out the current single Thunderbolt 3 bus in my 2019 iMac 5K.

I got excited when I saw that the top 2020 iMac can support "Two 6K external Displays" thinking that they might have added a 2nd Thunderbolt 3 bus. However I couldn't find that specific info anywhere except on your blog.

Was just wondering where you got your info from and how one Thunderbolt 3 bus could support two 6K displays.

Do you think it's possible that by adding the Radeon Pro 5700 card it adds a 2nd Thunderbolt 3 bus? This would be similar to how Mac GPUs that get added into the Mac Pro tower add extra Thunderbolt 3 busses.

MPG: excellent question!

Dual 6K displays exceed the bandwidth of a single Thunderbolt 3 bus. Surely this means that each Thunderbolt 3 ports is on its own Thunderbolt 3 bus?

While never before been implemented on any Mac, dual busses seems the only possible explanation.

Video Support and Camera

1080p FaceTime HD camera
Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

One 6016-by-3384 (6K) external display at 60Hz with support for 1 billion colors, or
Two 6016-by-3384 (6K) external displays at 60Hz with support for 1 billion colors (requires Radeon Pro 5700 or Radeon Pro 5700 XT), or
One 5120‑by‑2880 (5K) external display at 60Hz with support for 1 billion colors, or
Two 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colors, or
Two 4096‑by‑2304 (4K) external displays at 60Hz with support for millions of colors

Thunderbolt 3 digital video output

Native DisplayPort output over USB‑C
Thunderbolt 2, HDMI, DVI, and VGA output supported using adapters (sold separately)

Reader comments

UPDATE Aug 19: DSC can compression in a claimed “visually lossless” way of up to 3X. That drops the dual-6K scenario bandwidth requirements from 70.3 Gbps scenario down to ~23.4 Gbps. See Display Stream Compression (DSC) and Its Role in Making Dual 6K Display Support Possible.

Michael C writes:

Some more clarification on the new iMac 5K and how the 5700XT model WONT have extra TB3 ports. See this AMD white paper (page 19):

Basically the new GPU architecture takes credit for the improved decoding and output improvements:

“The RDNA display engine was majorly re-designed and optimized for 4K and 8K displays and high dynamic range (HDR). The HMDI® output delivers the full 18 Gbps speed of HDMI® 2.0b to enable 4K at 60 Hz or1080P at 240 Hz.
The DisplayPortTM 1.4a connection is the same raw bandwidth, up to 32.4 Gbps over existing cables, but adds Display Stream Compression (DSC), which enables future support of displays up to 8K HDR at 60 Hz and 4K HDR at 240 Hz.”

MPG: DSC video compression might not be acceptable for pro work*, but set that aside.

Also, HDMI is inferior for pro color calibration, at least for still photography.

  • 4K at 240 Hz is 8.8 megapixels is equivalent to 4 X 8.8 = 35.2 megapixels. and requires ~63.6 Gbps for 30-bit RGB color (35.2*3 * 10 *60). -
  • 8K at 7360 X 5120 - 36.7 megapixels and requires 66 Gbps for 30-bit color (36.7 * 3 * 10 * 60).
  • Dual 6K at 6016 * 3384 * 2 is 40.7 megapixels and requires 70.3 Gbps for 30-bit RGB color (40.7 * 3 * 10 * 60)

How DSC can turn 70.3 Gbps into the 32.4 Gpbs or even 40 Gbps remains a mystery to me. Moreover, there would be no bandwidth left for anything else.

Page 19 of the AMD PDF states:

DSC can compress video without chroma sub-sampling (4:4:4) to as low as 8 bits/pixel for both HDR and SDR.

But... 40.7 megapixels (dual 6K) * 3 * 8 * 60 = 58.6 Gbps, which is still far in excess of Thunderbolt 3 bandwidth.

Unless I am reading it wrong, and it really means “a 3 X 10-bit pixel (30 bits) can be compressed to 8 bits in total”. I have a very hard time believing that is “visually lossless”. But perhaps that is what is going on.

* I have color vision (tested) better than 99.9% of the population (at least when I was 10 years younger), so I don’t buy “visually lossless” but I'll accept it as a "maybe".

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