> However, if you don’t need the full bandwidth that four lanes can give you, a DP link can consist of only one or two main lanes resulting in two
This is how many USB C docks operate. The USB C connector also has four high speed lanes and there's a mode where two are assigned to carry DisplayPort data and two are assigned to be TX and RX respectively for USB. Until DP 1.4 appeared, this meant you were quite limited in display resolution if you didn't have Thunderbolt and wanted faster than 480mbps data speed. With DP 1.3, two HBR3 lanes can carry 12.96 Gbit/s which is almost exacly the requirement for 4k @ 60Hz at 12.54 Gbit/s. DP 1.4 adds compression on top of this. One more beauty of DisplayPort is it's point to point so it's entirely possible your USB C hub will carry the display data from the host to the hub as compressed HBR3 data over two lanes and then hand it out to two monitors over four uncompressed HBR2 lanes to each so a modern USB C hub can, without Thunderbolt, drive two 4k @60Hz monitors and still provide multiple gigabit speed data. It's a very neat trick. This needs full DisplayPort 1.4 support including DSC in the host, for integrated GPUs in laptop CPUs this means AMD Ryzen 4000 and newer or Intel Tiger Lake and newer (older laptops with discrete GPUs might have had it, too).
Handy tip: if your hub is DP 1.4 and drives multiple monitors then it's most likely using a Synaptics MST hub to do this (almost all non-Thunderbolt ones do and many Thunderbolt ones as well) and Synaptics provides a very very little known diagnostics tool called vmmdptool (available in the Windows Store). It doesn't replace a full DP AUX protocol analyzer of course but it's free and for that price it's really handy.
This topic is dear to me because I have fixed the USB C specification related to this and allow me to be damn proud of that: it used to erroneously say the USB data speed in this mixed functionality was limited to 5gbps but it is not, the limit is 10gbps. https://superuser.com/a/1536688/41259
Ps.: When I say Thunderbolt, I am well aware of how Thunderbolt 4 is just USB4 with optional features made mandatory. It's not relevant to the discussion at hand.
Pps.: DisplayPort is the native video standard for USB C, C-DisplayPort adapters and cables are basically passive because they just need to tell the host how to configure the connector lanes. Meanwhile all USB C - HDMI cables and converters are active which constantly work on the DP signal to become HDMI. DisplayPort++ alas is not implemented with the USB C connector. For this reason if any compatibility issues arise it's always better to connect a USB C device to the DisplayPort input on a monitor. A HDMI alternate mode was defined in the past but it remained paper only and it has been declared dead this year at CES.
> Ps.: When I say Thunderbolt, I am well aware of how Thunderbolt 4 is just USB4 with optional features made mandatory. It's not relevant to the discuss at hand.
Dear God, I hope this situation settles down in the near future. As it is I have years of USB-C-looking cables that all do different things but are visually indistinguishable.
I wish the manufacturers would just adopt the USB IF marketing names and logos. https://i.imgur.com/H3unbD5.png would be a lot simpler.
I also wish the USB IF defined colors for high speed lanes absent vs 5/10/20 gbps capable high speed lanes and then 60W/100W/240W power. All it needed were two color bands on the plastic part of the plug. If colors are too gaudy then go Braille-style, have a 2x2 grid on top and bottom of the plug where bit 0 is a little hole and bit 1 is a little bump. That's 16 possible data speeds and 16 possible power levels and so far we have only needed 4 for data and 3 for power.
Intel could've added a separate row for Thunderbolt.
What is the benefit of adding compression here? Does that mean that the bandwidth might or might not be sufficient depending on which pictures are being shown on the screen?
If so, that doesn't seem very reliable and if not, what's the point of compression?
> DSC is most often used in a Constant Bit Rate (CBR) mode, where the number of coded bits generated for a slice is always a constant that is generally equal to the bit rate multiplied by the number of pixels within the slice
Ah, so basically this is typically used as lossy compression with a constant bit rate, which makes the bandwidth usage predictable/constant but depending on the pictures being shown (and the noise level?), it can lead to some visual degradation (which they say is usually imperceptible).
If you consider how ~1:18 is considered visually lossless for x.264, a compression of 1:3 is very very likely can be achieved visually lossless indeed.
This is how many USB C docks operate. The USB C connector also has four high speed lanes and there's a mode where two are assigned to carry DisplayPort data and two are assigned to be TX and RX respectively for USB. Until DP 1.4 appeared, this meant you were quite limited in display resolution if you didn't have Thunderbolt and wanted faster than 480mbps data speed. With DP 1.3, two HBR3 lanes can carry 12.96 Gbit/s which is almost exacly the requirement for 4k @ 60Hz at 12.54 Gbit/s. DP 1.4 adds compression on top of this. One more beauty of DisplayPort is it's point to point so it's entirely possible your USB C hub will carry the display data from the host to the hub as compressed HBR3 data over two lanes and then hand it out to two monitors over four uncompressed HBR2 lanes to each so a modern USB C hub can, without Thunderbolt, drive two 4k @60Hz monitors and still provide multiple gigabit speed data. It's a very neat trick. This needs full DisplayPort 1.4 support including DSC in the host, for integrated GPUs in laptop CPUs this means AMD Ryzen 4000 and newer or Intel Tiger Lake and newer (older laptops with discrete GPUs might have had it, too).
Handy tip: if your hub is DP 1.4 and drives multiple monitors then it's most likely using a Synaptics MST hub to do this (almost all non-Thunderbolt ones do and many Thunderbolt ones as well) and Synaptics provides a very very little known diagnostics tool called vmmdptool (available in the Windows Store). It doesn't replace a full DP AUX protocol analyzer of course but it's free and for that price it's really handy.
This topic is dear to me because I have fixed the USB C specification related to this and allow me to be damn proud of that: it used to erroneously say the USB data speed in this mixed functionality was limited to 5gbps but it is not, the limit is 10gbps. https://superuser.com/a/1536688/41259
Ps.: When I say Thunderbolt, I am well aware of how Thunderbolt 4 is just USB4 with optional features made mandatory. It's not relevant to the discussion at hand.
Pps.: DisplayPort is the native video standard for USB C, C-DisplayPort adapters and cables are basically passive because they just need to tell the host how to configure the connector lanes. Meanwhile all USB C - HDMI cables and converters are active which constantly work on the DP signal to become HDMI. DisplayPort++ alas is not implemented with the USB C connector. For this reason if any compatibility issues arise it's always better to connect a USB C device to the DisplayPort input on a monitor. A HDMI alternate mode was defined in the past but it remained paper only and it has been declared dead this year at CES.