Analyzing the iMac 5K Retina display: How do you squeeze 5K out of a last-gen GPU? (updated)
Last week, Apple announced a new 27-inch iMac $2,494.00 at Amazon that packs an impressive 5K Retina display. As we’ve already detailed, these new 5K displays are thinner, cheaper, draw less power thanks to a more-efficient LED backlight, and, perhaps most importantly, Apple is selling the whole 27-inch iMac system at a mind-blowing price of $2500. That’s the same price tag on Dell’s 5K monitor.
If this panel indeed uses an overclocked DisplayPort 1.2 signal, as we discuss below, it could be 60Hz up to 4K (3840×2160). DisplayPort 1.2 is capable of providing an SST link of 60Hz at that resolution. It’s still a beautiful display regardless, and many may argue (with justification) that the 30Hz limit is inconsequential because the GPU inside the system — a fairly weak mobile AMD GPU — can’t drive gaming above 30 fps at 5K anyway.
Nonetheless, if you edit video above 30 fps, and wanted to create 5K content, this may impact your buying decision. The original story remains below.
In other words, the GPUs inside the new iMac are going to be limited to DisplayPort 1.2. That matters, because it takes roughly 17.2Gbps of bandwidth to drive a 4K @ 60 fps signal in a single stream (Single Stream Transport). To summarize the difference between SST and MST, an MST display creates two half-width tiles on the monitor and interleaves two different DisplayPort streams together to create a contiguous image, while an SST display functions like a standard monitor. MST and SST displays typically look identical in common applications, but some games support MST poorly, resulting in menus or functions crammed into half the monitor, or movies playing back in a squashed, half-width format.
Critically, however, MST is the only way to drive a larger-than-4K panel. DisplayPort 1.2 has just enough bandwidth to support a single 4K @ 60 fps SST stream, but 5K is far too large for the standard. When Apple talks about a 40-gigabit TCON, it may have designed a single TCON to output to two DP 1.2 streams — that’s not technically impossible — but it’s not being done with a single stream within the DP 1.2 spec.
Read: No, TV makers, 4K and UHD are not the same thing
Since Tonga doesn’t support HDMI 2.0 or DP 1.3 (which does support 5K SST), DP 1.2 is the only available standard to piggy-back. If Apple had somehow redesigned the TCON to compress a 5K stream into existing DisplayPort 1.2 bandwidth, it wouldn’t need a 40Gbps TCON in the first place. Anandtech notes that there’s another possibility — Apple may indeed have designed its own TCON, overclocked it, customized it for low overhead timing, and be pulling just enough bandwidth out of DP 1.2 to get it done.
Either
way, Apple would have to overclock the DisplayPort signal by 50-100% to
hit the bandwidth it needs for 5K on single stream transport.
Updated, October 24: As per above, it now appears this is a 30Hz panel when operating at 5K.
Scaling is another potential issue — but Apple has always done this one better than Microsoft and 5120×2880 has exactly four times the pixels of the old 2560×1440 monitors, which should make scaling up relatively simple.
Finally, gaming — and here’s where the reality is going to bite. You aren’t going to be doing any gaming on a 5K display at anything like high detail levels. You may not even pull it off at low detail levels, and for a very simple reason: The R9 M290 is a midrange GPU from 2012 boxing way, way out of its weight class on this one. Despite the term, 5K is not 25% more pixels than 4K — it’s almost two times as many pixels.
Not even dual GTX 980s in SLI could drive 60-fps high-detail gaming on that kind of rig. And that means there’s no chance any AMD mobile GPU — even if the R9 M295 is a Tonga implementation — is going to be able to do it either.
Last week, Apple announced a new 27-inch iMac $2,494.00 at Amazon that packs an impressive 5K Retina display. As we’ve already detailed, these new 5K displays are thinner, cheaper, draw less power thanks to a more-efficient LED backlight, and, perhaps most importantly, Apple is selling the whole 27-inch iMac system at a mind-blowing price of $2500. That’s the same price tag on Dell’s 5K monitor.
Updated October 24
One of the major questions regarding Apple’s 5K display is whether or not it would be a 30Hz or 60Hz panel. We’ve had an opportunity to see some demonstrations of gaming on the 27-inch iMac with 5K Retina display and can confirm that it’s a 30Hz panel when operating in 5K mode. Below 5K, the panel shifts to 60Hz, though we couldn’t confirm exactly when the 60/30 shift occurs. It’s a 60Hz panel up to at least 2560×1440.If this panel indeed uses an overclocked DisplayPort 1.2 signal, as we discuss below, it could be 60Hz up to 4K (3840×2160). DisplayPort 1.2 is capable of providing an SST link of 60Hz at that resolution. It’s still a beautiful display regardless, and many may argue (with justification) that the 30Hz limit is inconsequential because the GPU inside the system — a fairly weak mobile AMD GPU — can’t drive gaming above 30 fps at 5K anyway.
Nonetheless, if you edit video above 30 fps, and wanted to create 5K content, this may impact your buying decision. The original story remains below.
Original story: A late-2014 display hooked to an early 2012 GPU
The GPU powering the 27-inch iMac with Retina 5K display is the R9 M290X, with the R9 M295X offered as an optional upgrade. The R9 M295X hasn’t technically been announced, but rumors from months back suggested this would be a Tonga-class GPU. Regardless, the R9 M290X is the minimum spec — and that chip is a rebranded HD 8970M, which was a rebranded HD 7970M, which is functionally equivalent to a desktop Radeon HD 7870.In other words, the GPUs inside the new iMac are going to be limited to DisplayPort 1.2. That matters, because it takes roughly 17.2Gbps of bandwidth to drive a 4K @ 60 fps signal in a single stream (Single Stream Transport). To summarize the difference between SST and MST, an MST display creates two half-width tiles on the monitor and interleaves two different DisplayPort streams together to create a contiguous image, while an SST display functions like a standard monitor. MST and SST displays typically look identical in common applications, but some games support MST poorly, resulting in menus or functions crammed into half the monitor, or movies playing back in a squashed, half-width format.
Critically, however, MST is the only way to drive a larger-than-4K panel. DisplayPort 1.2 has just enough bandwidth to support a single 4K @ 60 fps SST stream, but 5K is far too large for the standard. When Apple talks about a 40-gigabit TCON, it may have designed a single TCON to output to two DP 1.2 streams — that’s not technically impossible — but it’s not being done with a single stream within the DP 1.2 spec.
Read: No, TV makers, 4K and UHD are not the same thing
Since Tonga doesn’t support HDMI 2.0 or DP 1.3 (which does support 5K SST), DP 1.2 is the only available standard to piggy-back. If Apple had somehow redesigned the TCON to compress a 5K stream into existing DisplayPort 1.2 bandwidth, it wouldn’t need a 40Gbps TCON in the first place. Anandtech notes that there’s another possibility — Apple may indeed have designed its own TCON, overclocked it, customized it for low overhead timing, and be pulling just enough bandwidth out of DP 1.2 to get it done.
This kind of issue is common with MST — but did Apple overclock DP to hit higher bandwidths and avoid MST?
Refresh rates, gaming, and scaling
The many clever layers of the iMac with Retina display
Scaling is another potential issue — but Apple has always done this one better than Microsoft and 5120×2880 has exactly four times the pixels of the old 2560×1440 monitors, which should make scaling up relatively simple.
Finally, gaming — and here’s where the reality is going to bite. You aren’t going to be doing any gaming on a 5K display at anything like high detail levels. You may not even pull it off at low detail levels, and for a very simple reason: The R9 M290 is a midrange GPU from 2012 boxing way, way out of its weight class on this one. Despite the term, 5K is not 25% more pixels than 4K — it’s almost two times as many pixels.
Not even dual GTX 980s in SLI could drive 60-fps high-detail gaming on that kind of rig. And that means there’s no chance any AMD mobile GPU — even if the R9 M295 is a Tonga implementation — is going to be able to do it either.
We've had an opportunity to see some demonstrations of gaming on the 27-inch iMac with 5K Retina display and can confirm that it's a 30Hz ... imac5kk.blogspot.com
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