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Will It Work? · Guide

How much bandwidth a monitor needs, worked out

By Bulan Sarkar, founder of ElectronicsInfoline (since 2000)

Whether a monitor works through a hub, a dock or a cable comes down to one number: the data rate it needs. This works it out from VESA's reduced-blanking timing for the common resolutions and refresh rates, and sets it against what DisplayPort links carry.

Data rate = pixel clock × 3 colours × bits per colour. The pixel clock counts the blank border too: for 4K at 60 Hz it is 522.614 MHz (3920 × 2222 × 60), so 4K60 at 8-bit needs 12.54 Gbit/s. That fits on four lanes of DisplayPort 1.2 (17.28), not on two (8.64).

The formula, and why blanking counts

The link sends each line with a horizontal blank and each frame with a vertical blank. VESA's reduced blanking v2 uses an 80-pixel horizontal blank and a vertical blank of at least 460 µs (VESA). Our timing code reproduces the RB2 pixel clocks VESA published for 5K and 8K exactly: 924.144 MHz for 5120 × 2880 at 60 Hz and 2068.660 MHz for 7680 × 4320 at 60 Hz (VESA).

Active picture and blanking for 3840×2160 at 60 HzReduced blanking v2: each line is 3920 pixel times (3840 visible plus 80 blank); each frame is 2222 lines (2160 visible plus 62 blank). 3920 × 2222 × 60 = 522.614 MHz pixel clock.3840 × 2160 visible8062 blank lines3920 × 2222 × 60 Hz= 522.614 MHz
Figure 1. The link carries the blank border too. 3840×2160 at 60 Hz: 3920 × 2222 × 60 = 522.614 MHz. Times 24 bits per pixel gives the data rate.

Common monitors

ModePixel clock8-bit10-bitSmallest link (8-bit)
1920 × 1080 at 60 Hz133.320 MHz3.2 Gbit/s4 Gbit/sHBR2 × 2
1920 × 1080 at 144 Hz333.216 MHz8 Gbit/s10 Gbit/sHBR2 × 2
2560 × 1440 at 60 Hz234.590 MHz5.63 Gbit/s7.04 Gbit/sHBR2 × 2
2560 × 1440 at 144 Hz586.586 MHz14.08 Gbit/s17.6 Gbit/sHBR2 × 4
3440 × 1440 at 100 Hz531.520 MHz12.76 Gbit/s15.95 Gbit/sHBR2 × 4
3840 × 2160 at 60 Hz522.614 MHz12.54 Gbit/s15.68 Gbit/sHBR2 × 4
3840 × 2160 at 120 Hz1075.804 MHz25.82 Gbit/s32.27 Gbit/sHBR3 × 4
3840 × 2160 at 144 Hz1306.206 MHz31.35 Gbit/s39.19 Gbit/sUHBR20 × 4
5120 × 2880 at 60 Hz924.144 MHz22.18 Gbit/s27.72 Gbit/sHBR3 × 4

Reduced blanking v2, no compression. A monitor running TV timing needs more: VESA lists 594.000 MHz for 4K60 TV timing against 522.614 MHz here (VESA).

What the links carry

HBR2 runs 5.4 Gbit/s a lane, HBR3 8.1 and UHBR20 20 (VESA). HBR2 and HBR3 lose 20 % to 8b/10b coding (VESA), and UHBR20 carries 19.34 Gbit/s a lane (VESA). Modes beyond a link can still run with Display Stream Compression, up to 3:1 (VESA), when both ends support it.

Tips and tricks

  • Refresh rate costs bandwidth almost one for one: 144 Hz needs well over twice what 60 Hz does at the same resolution.
  • Turning 10-bit HDR off saves a fifth of the data rate. That can be the difference on a tight link.
  • A 4K60 monitor stuck at 30 Hz on a hub points to a link that is too narrow, or an HDMI 1.4 port on the hub.
  • DSC needs support at both ends: computer and monitor. A hub in between has to pass it through.

What we would do

Look up your monitor in the table, then find the smallest link that carries it. If your hub or laptop offers less, either lower the refresh rate, turn off 10-bit colour, or move the screen to a port with more lanes. The hub and dock checker does this for a whole setup.

Questions people ask

How much bandwidth does 4K at 60 Hz need? 12.54 Gbit/s at 8-bit colour and 15.68 at 10-bit, using VESA reduced-blanking timing (pixel clock 522.614 MHz). Monitors on older timings need more: VESA lists 533.250 MHz for the first reduced-blanking version and 594.000 MHz for TV timing.

Why not just multiply width × height × refresh × 24? Because the link also carries the blanking interval, a border of pixel times around the picture. Reduced blanking keeps it small (80 pixels a line, at least 460 µs a frame) but not zero.

Does 10-bit HDR need more bandwidth? Yes, a quarter more: 30 bits a pixel instead of 24, at the same pixel clock.

Sources

Each figure on this page is quoted from the source below or worked out from them. Sources checked , each quote read again against the source in a second pass.

  1. VESA, Welcome VESA Workshop Tokyo, Japan 2024 (VESA PDF, page 68, 'Minimal Vblank and Hblank requirements'): VBLANK period way shorter than 300 uSec, let alone 460 uSec listed as the minimum VBLANK period in VESA CVT Standard vesa.org/wp-content/uploads/2024/10/VESA-Tokyo-Workshop_Octo…
  2. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 5120 x 2880 60 Hz IT No DP 1.3 VESA CVT RB 2 924.144 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  3. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 7680 x 4320 60 Hz IT VESA CVT RB 2 2068.660 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  4. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 7680 x 4320 30 Hz IT No DP 1.3 VESA CVT RB 2 1019.896 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  5. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 8k x 4k - Half frame 3840 x 4320 30 Hz IT Yes (2x1) 2 x DP 1.2a VESA CVT RB 2 515.205 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  6. VESA, VESA Display Summit USA 2016 (AMD/VESA presentation), timing table: 8k x 4k - Half frame 3840 x 4320 60 Hz IT Yes (2x1) 2 x DP 1.3 VESA CVT RB 2 1044.993 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  7. VESA, DisplayPort Future Proofing Display Connectivity for VR and 8K HDR (VESA PDF, Display Summit USA 2016, page 11): 3840 x 2160 60 Hz IT, No DP 1.2a VESA CVT RB 1 533.250 MHz 3840 x 2160 60 Hz TV CEA 594.000 MHz vesa.org/wp-content/uploads/2016/06/Display-Summit-USA-2016-…
  8. VESA, Welcome VESA Workshop Tokyo, Japan 2024 (VESA PDF, page 10, 'DP Standard Version Number...'): • DPv1.1 = HBR (2.7 Gbps/lane) • DPv1.2 = HBR2 (5.4 Gbps/lane) • DPv1.4 = HBR3 (8.1 Gbps/lane) • DPv2.x = UHBR rates (10/13.5/20 Gbps/lane) vesa.org/wp-content/uploads/2024/10/VESA-Tokyo-Workshop_Octo…
  9. VESA, DP 1.3 Overview for VESA v1 (VESA/DisplayPort.org PDF, page 4, 'DP 1.3 Link Rate Increase'): 8.1 Gbps link rate, per lane x 0.8 to account for 8b/10b transport coding overhead x 4 maximum number of available lanes 25.92 Gbps total usable data transfer rate displayport.org/wp-content/uploads/2014/09/DP-1.3-Overview-f…
  10. VESA, Why DisplayPort - VESA - Interface Standards for The Display Industry: Up to 8.1 Gbps per lane–32.4 Gbps over 4 lanes Allowing for overhead — 25.92 Gbps over 4 lanes vesa.org/displayport-developer/why-displayport/
  11. VESA, VESA Releases Updated DisplayPort™ Alt Mode Spec to Bring DisplayPort 2.0 Performance to USB4™ and New USB Type-C® Devices - VESA - Interface Standards for The Display Industry: Featuring the highly efficient 128b/132b channel coding shared with USB4, DisplayPort 2.0 delivers a maximum payload of 77.37 Gbps across four lanes (up to 19.34 Gbps per lane) vesa.org/featured-articles/vesa-releases-updated-displayport…
  12. VESA, FAQ - DisplayPort: Developed in collaboration with the MIPI Alliance, VDC-M provides a higher level of compression ratio (up to 5:1) at the same visually lossless quality level as VESA’s Display Stream Compression (DSC) standard (which offers up to 3:1 compression), with the trade-off of higher circuit complexity. displayport.org/faq/

Author: Bulan Sarkar · Report an error

Numbers here are quoted from standards bodies and makers, or worked out from them. Nothing on this page was measured or tested by us.