Ask the Editors: What is 4K Passthrough? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 15 Old 09-30-2015, 10:48 AM - Thread Starter
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Ask the Editors: What is 4K Passthrough?

Q: What is 4K passthrough? I see that newer AV receivers support it, but then I also see only one HDMI input that supports HDCP 2.2. Does this mean that 4K passthrough does not need HDCP 2.2? Doesn't 4K require HDMI 2.0 and HDCP 2.2? Does that mean not all of the inputs support "true" 4K?

- TheRatPatrol

A: You're not alone; this is a topic of much confusion. 4K passthrough is an AV receiver's ability to accept a 4K video signal from a source device and pass it through to a 4K display. To pass a copy-protected 4K signal—which will include virtually all commercial 4K/UHD content for the home—the HDMI output of the source device must be connected to an HDMI input on the AVR that supports HDCP 2.2 copy protection. The AVR's HDMI output is then connected to an HDMI input on the display, which must also support HDCP 2.2. In the example you cite, 4K passthrough would only work with the AVR's one HDMI input that supports HDCP 2.2.

These days, it's common for AVRs to have many HDMI inputs, only one or some of which support HDCP 2.2—if there are any at all. When shopping for an AVR, be sure at least one input supports HDCP 2.2; if it has no such inputs, the signal from any upcoming Ultra HD Blu-ray player or 4K/UHD streaming device will not pass through the AVR and on to the display. This does not apply to the streaming apps within the display itself, which do not send any video signals via HDMI to the AVR. (They might send audio via the display's optical output or HDMI Audio Return Channel, but that shouldn't be a problem for HDCP 2.2.)

There are some related issues to consider when shopping for an AVR. For example, HDMI 2.0 can operate at a bitrate of 10.2 or 18 Gbps, and it's usually not clear which bitrate is implemented in any HDMI 2.0-equipped device. At 10.2 Gbps, the connection can carry 2160p at 60 frames per second with no more than 4:2:0 color subsampling and 8-bit resolution, which is insufficient for high dynamic range (HDR). The lower bitrate can convey up to 12-bit resolution and 4:2:2 color subsampling at 30 and 24 frames per second, which is sufficient for HDR at these lower frame rates. Still, I strongly recommend getting an AVR that operates at the higher HDMI bitrate of 18 Gbps if possible, because that is more future-proof than the lower bitrate.

Another issue is HDR itself, which I believe is critically important for the future of video entertainment. To convey an HDR signal from a source device—say, an Ultra HD Blu-ray player or 4K/UHD streaming device—through an AVR and on to the display, all devices must have HDMI 2.0a connections. The "a" signifies that the connection can convey HDR metadata, which is required for an HDR display to properly reproduce HDR images.

However, keep in mind that just because HDMI 2.0a specifies this capability, that doesn't mean a hardware manufacturer must implement it, even if the connection is identified as HDMI 2.0a. So be very careful about determining exactly what capabilities are actually implemented in a given product, not just what HDMI version number it uses. The version number represents the set of capabilities that can be implemented, not which ones are actually implemented.

If you've got an AV question, please send it to Scott Wilkinson (scott@avsforum.com) or Mark Henninger (imagic, mark@avsforum.com) via PM or email.
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Last edited by Scott Wilkinson; 09-30-2015 at 09:26 PM.
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post #2 of 15 Old 09-30-2015, 12:23 PM
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Quote:
Originally Posted by Scott Wilkinson View Post
However, keep in mind that just because HDMI 2.0a specifies this capability, that doesn't mean a hardware manufacturer must implement it, even if the connection is identified as HDMI 2.0a. So be very careful about determining exactly what capabilities are actually implemented in a given product, not just what HDMI version number it uses. The version number represents the set of capabilities that can be implemented, not which ones are actually implemented.
What? The manufacture can advertise a feature (HDMI 2.0a) but it doesn't have to work. Could you expand on this statement a little?
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post #3 of 15 Old 09-30-2015, 12:34 PM
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This is like the early days of HDMI all over again. How many receivers were made with HDMI inputs that couldn't handle certain types of audio or video signals?

My receiver advertises as having 4K passthrough but it pre-dates HDCP 2.2. So for a lot of forthcoming content thats basically worthless.

I won't be going 4K for a good while in the room that unit is in, but I'm sure others will be disappointed to learn their UHD bluray players won't work correctly.

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post #4 of 15 Old 09-30-2015, 01:02 PM
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Quote:
Originally Posted by CountryBumkin View Post
What? The manufacture can advertise a feature (HDMI 2.0a) but it doesn't have to work. Could you expand on this statement a little?
HDMI 2.0a is a HDMI version not a feature (though I think HDMI are asking them to not advertise a particular version). A HDMI version can have various features of which a manufacturer may or may not support particular features.

Wikipedia says, on the HDMI page, "Note that a given product may choose to implement a subset of the given HDMI version. Certain features such as deep color and xvYCC are optional".
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post #5 of 15 Old 09-30-2015, 01:03 PM
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^^^^^exactly. However this year set of AVR receivers from denon have everything you need.
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post #6 of 15 Old 09-30-2015, 01:09 PM
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Quote:
Originally Posted by CountryBumkin View Post
What? The manufacture can advertise a feature (HDMI 2.0a) but it doesn't have to work. Could you expand on this statement a little?
A TV can have an HDMI 2.0a input, but not be physically capable of displaying HDR. A version number of HDMI does not imply support for every feature the HDMI supports.

Case&point: HDMI 1.4 added support for 4k resolution, 3D, audio return channel, and Ethernet-over-HDMI. I have a TV that lacks all 4 other those features except 3D, and yet the TV is advertised as HDMI 1.4 compliant.

Another case&point: 10.2 vs 18 Gbps HDMI 2.0. HDMI 2.0 added support for higher bandwidth. But devices aren't required to support it.
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post #7 of 15 Old 09-30-2015, 04:31 PM
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I like this 'ask the editors' idea. I didnt know how much i was wondering that same question till i just saw it answered!

I look forward to more of these
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post #8 of 15 Old 09-30-2015, 05:59 PM
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Geez talk about confusion! What happened to "plug and play"? Maybe they should have used DisplayPort instead?
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post #9 of 15 Old 10-02-2015, 12:12 AM
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I'm still a little confused. I thought that one of the main purposes of an AVR was to switch the inputs and "pass thru" the input video signal to the output display. So, why is "pass thru" being described as some kind of new feature? How can a UHD-capable AVR not have "pass thru"? Or does "pass thru" infer some amount of video processing (such as upscaling) which allows for a non-UHD signal to be output as a UHD signal? I've seen "UHD pass thru" threads elsewhere where people have complained about earlier AVRs being advertised as "UHD pass thru" but not supporting HDCP 2.2 .. how is that even possible without lying, unless they're restricting themselves to video from UHD GoPros or video cameras which almost no one owns yet?

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post #10 of 15 Old 10-02-2015, 10:47 AM
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Quote:
Originally Posted by IanR View Post
one of the main purposes of an AVR was to switch the inputs and "pass thru" the input video signal to the output display.
Accurate.
Quote:
Originally Posted by IanR View Post
So, why is "pass thru" being described as some kind of new feature?
It's the "4k" part that is new.
Quote:
Originally Posted by IanR View Post
How can a UHD-capable AVR not have "pass thru"?
I think those terms are nearly synonymous, though I would generally consider UHD to be the newer term that implies support for 4k, HCDP 2.2, HDR, 12bit, etc.
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Or does "pass thru" infer some amount of video processing (such as upscaling) which allows for a non-UHD signal to be output as a UHD signal?
Pass thru does not imply any amount video processing/upscaling.
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people have complained about earlier AVRs being advertised as "UHD pass thru" but not supporting HDCP 2.2 .. how is that even possible without lying, unless they're restricting themselves to video from UHD GoPros or video cameras which almost no one owns yet?
We've had "4K passthru" receivers for years now. They all lacked HDCP 2.2. So they weren't lying; they could pass unencrypted video from PCs or GoPros. Only the 2015 model receivers (for the most part) have HDCP 2.2. This is a limitation imposed by Hollywood, and really has nothing to do with lying or planned obsolescence. Had Hollywood decided to publish 4k videos only requiring standard HDCP, then it would be no problem.

Last edited by kitti; 10-02-2015 at 02:27 PM.
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post #11 of 15 Old 10-02-2015, 02:14 PM
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Originally Posted by TheRatPatrol View Post
Geez talk about confusion! What happened to "plug and play"? Maybe they should have used DisplayPort instead?
Right. I'm fairly geeky with this kind of stuff, but it's starting to get out of hand.
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post #12 of 15 Old 10-02-2015, 08:42 PM
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Scott,

A while back you had a guest on your show talking about DisplayPort and how much better it is than HDMI. What are your thoughts on DisplayPort, can it handle 4K, HDCP and HDR better?

Thanks
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post #13 of 15 Old 10-03-2015, 11:00 AM
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Pass thru does not imply any amount video processing/upscaling.
Pass through, if it doesn't pass through the signal as if the receiver wasn't part of the signal chain, is false advertising and I hope receiver manufacturers get sued who do not offer firmware updates so that they do not strip this metadata from the signal.

There is no reason why an HDMI extender or switch should work better at "passing through" the signal, the whole signal, and nothing but the whole unadulterated signal.

Any receiver that does not pass-through every single bit that is coming into its input towards its output and claiming "HDMI pass-thru" should be sued for false advertising.

The reason manufacturers (and standards bodies like HDMI) get away with these sort of abusive, greedy shenanigans is because they are allowed to by apathetic consumers. This is actionable, IMO. But IANAL.
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post #14 of 15 Old 10-05-2015, 09:57 AM
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Pass through, if it doesn't pass through the signal as if the receiver wasn't part of the signal chain, is false advertising and I hope receiver manufacturers get sued who do not offer firmware updates so that they do not strip this metadata from the signal.

There is no reason why an HDMI extender or switch should work better at "passing through" the signal, the whole signal, and nothing but the whole unadulterated signal.

Any receiver that does not pass-through every single bit that is coming into its input towards its output and claiming "HDMI pass-thru" should be sued for false advertising.

The reason manufacturers (and standards bodies like HDMI) get away with these sort of abusive, greedy shenanigans is because they are allowed to by apathetic consumers. This is actionable, IMO. But IANAL.
I haven't heard of any receivers that that have falsely stated they offer HDMI pass-thru. Name one?
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post #15 of 15 Old 10-13-2015, 07:42 PM
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Ugh. Whoever thought that decoupling the HDCP version from the HDMI version (and worse: having them so very close in numbers) was a good idea should be institutionalized.

File this @#$%ing debacle under the larger umbrella term "bag of hurt". See also: "incremental stupidness".
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