Your new monitor promises 4K at 240 Hz, and the cable spec says that cannot fit. Both are right, because the picture travels lightly compressed.
The short answer
Display Stream Compression (DSC) is a VESA standard that compresses the video signal between the graphics card and the monitor so that higher resolutions and refresh rates fit through an existing connection [1][4]. VESA calls the compression "visually lossless": a typical viewer, under typical viewing conditions, would in most cases not notice a difference from the uncompressed image [2]. DSC 1.2b is part of the DisplayPort and HDMI standards [1]. Our assessment: DSC is not a flaw to avoid but the normal way high-end monitors reach their top modes over DisplayPort 1.4.
How it works
- A light, fast codec. VESA introduced DSC in 2014 [1]. Typical compression ratios range from 1:1 to about 3:1, the decoder is small (typically under 100,000 gates), and it adds no more than one raster line of latency, under 8 microseconds in a 4K system at 60 Hz [2].
- Fixed bits per pixel. In VESA's comparison, DSC 1.2a compresses 30-bit colour 3.75:1 and 24-bit colour 3:1, in both cases to 8 bits per pixel [1].
- The bottleneck it solves. DisplayPort 1.4a carries up to 8.1 Gbit/s per lane, 32.4 Gbit/s over four lanes, which leaves 25.92 Gbit/s after overhead [3]. VESA certifies devices to DisplayPort 2.1b only if they support DSC, among other requirements [2].
- GPU support differs. NVIDIA supports DSC over DisplayPort 1.4 and newer from the Turing generation (GeForce RTX 20 and GTX 16) and over HDMI 2.1 from the Ampere generation (GeForce RTX 30) [4]. On older cards you may not be able to select a monitor's highest resolution or refresh rate [4]. NVIDIA lists "4K at 240Hz or 8K at 60Hz with DSC" as the maximum for GeForce RTX 40 cards [5].
- Side effects. NVIDIA switches DSC on automatically when a display supports it. In DSC mode, the GPU may use two internal display heads for one monitor; with two such monitors on one GeForce card, all four heads are in use and a third monitor cannot be connected [4].
- HDMI moves on. The HDMI 2.2 specification offers up to 96 Gbit/s and supports 4K at 240 Hz uncompressed with full chroma at 10 and 12 bits [6]. Ultra High Speed HDMI cables from HDMI 2.1 are rated for up to 48 Gbit/s [6].
A practical example
Our own calculation for a 4K monitor at 240 Hz with 10 bits per colour, counting only visible pixels: 3,840 × 2,160 × 240 is about 1.99 billion pixels per second. At 30 bits per pixel that is about 59.7 Gbit/s, more than twice the 25.92 Gbit/s that DisplayPort 1.4 can carry [3]. At the 8 bits per pixel from VESA's DSC table it is about 15.9 Gbit/s [1]. Real signals also carry blanking intervals, so the true values are somewhat higher. If such a monitor only offers 120 Hz on an older GPU, missing DSC support is a likely cause [4]. This is a calculation, not a measurement on our own hardware.
Checklist
- Check your GPU generation for DSC over DisplayPort or HDMI before you buy a high-refresh 4K monitor [4].
- Read the maximum resolution note on your graphics card's spec page [5].
- Plan for fewer monitors per card if several of them run in DSC mode [4].
- Look in the monitor menu for the DisplayPort version setting if DSC causes trouble; some displays turn it off when set to DP 1.2 [4].
- Compare the uncompressed modes of HDMI 2.2 with your devices' actual support before you count on them [6].