AirPlay Quality is Bad

I've never liked Airplay mirroring, though confess it's better for me than it used to be,

It does not send pixel perfect screen information to the AppleTV.

The Mac/iOS device compresses the display contents in real time to send to AppleTV to decompress and display.

The compression reduces bandwidth needed to send the data, but quality is dependent on at least - 3 factors:

1 - Processing resources on the sending device to compress the screen content - relatively static screens, say desktop mirroring, will take less processing power than rapidly changing content, and with the latter the sending device may already be using a lot of processing power to render the screen display say it's playing video or displaying a game running.

2 - Local network bandwidth - the slower the local connection the lower quality the stream has to be to avoid issues with frame loss etc. Don't forget that if you were using Airplay mirroring to send video streaming from the internet (assuming site allows this), then the wifi had to share bandwidth for incoming video and outgoing Airplay stream.

3 - Sending device screen resolution vs target output device resolution - conversion may be necessary. It would make more sense to me if the display size has to be upscaled at the receiving end for this to be done by AppleTV (or the TV). If however Airplay upscaled on sending as well this would incur more processing steps and bandwidth to send. I therefore suspect AppleTV does any scaling to meet the set output resolution.

AC

Airplay (depending which type you are using) either streams the actual video file (basic airplay) or a copy of the screen of the device (mirroring) you are airplaying from. Clearly, the actual file should be the same whether it's streamed directly (i.e. without airplay) or via airplay. The device screen, however, may well be of considerably lower resolution than the screen of your 4K TV, however, the TV should be more than good enough to provide a good image on your TV providing your TV is able to make a reasonable job of upscaling, unfortunately I have seen numerous 4K TV's that fail to upscale adequately in order to maintain affordability.

More likely though if your airplay image is degraded, it is because you are mirroring and your local network has problems.

Mirroring requires a lot from the network as it is sending the full display in realtime. Degradation or lag will be caused when there is any issue detected within the network and troubleshooting would need to occur to resolve. For video and photos it is best to just use basic airplay.

It is best to not have any other activity occurring (other streaming, web use, network backups etc)

If on wifi try connecting the Apple TV via ethernet. Get a diagnostic report, as outlined above, to see the current status of the network (signal as shown on the device won't account for issues and won't be accurate for troubleshooting purposes)

Test on another network

It is not network related. It is related to the iphone (7, maybe also others). The problem doesn’t exist with ipad (2017) as it works perfect there. So foto screen mirroring with ipad is sharp, with iphone 7 not. Not even with the new apple tv 4K.

Exactly, I think it is related to your first point, the processing power of the sending device. I can understand that they would have programmed this in the iPhone to safe battery life. The processor should be perfectly able to send the content.

My iPad doesn’t have this problem and sends the photos directly in highest resolution to the apple tv/tv. Perhaps because there is more battery power available. The iphone should send at least directly in high resolution when connected to the charger.

Send some feedback to Apple:

Feedback - Apple TV - Apple

I don't think any of us as users can begin to guess exactly how Airplay is implemented on individual devices, and I don't doubt your experience, but I'd not be surprised at all if compromises are made on lower powered/older (even quite recent) devices. Personally like you I'd prefer best quality whenever possible.

Don't forget that if Airplay mirroring then you are mirroring the 7's screen.

iPhone 7 resolution (1334x750 or 1920x1080 for the Plus) is a lot lower than an iPad with Retina display (2048x1536) or iPad Pro. So mirroring iPhone 7 will give a lower resolution Airplay stream than an iPad with retina display (as a minimum). Another thing to mention is that I can't remember if Airplay resolution is limited from iOS in particular. Until AppleTV 4k there would have been no point sending a stream of higher quality than 1920 x 1080 over Airplay (assuming it can even reach HD res) as display would have been impossible at the higher res. In fact my beef with the original AppleTV was that it should have had 4k on release (as did Amazon's Fire TV update at the time), for no other reason than the iPhone 6s released could capture 4k video and AppleTV would have been the ideal output device for this, bandwidth allowing of course.

Say you were displaying individual photos and not worried about screen mirroring I'd far prefer AppleTV to be able to directly navigate an attached iOS device's photo library and pull and render the files itself for best quality irrespective of sending device. Trouble is it doesn't do that, and even when sharing Photos from a Mac in the past it always 'processed' the image to some extent lowering quality/resolution.

Not Airplay then, but at least my iphone screen is mirrored on the HDMI tv. The result is exactly the same as with Airplay using apple tv 4k. It is therefore not related to the apple tv/ airplay/ network but perhaps a deliberate iphone developer decision.

I ruled out the whole network by verifying Airplay with the AV HDMI cable. Exactly the same behavior as described earlier. So same error with iphone, no error with ipad.

That is not the same thing and does not count towards troubleshooting, airplay is done over the local network and that is where the issue lies.

Actually, you said it was from your phones too, however moving on...

This is most likely a network issue, If you are still having problems, try turning off bluetooth if it's enabled, also try using 802.11n for wifi if you are currently using either 802.11b or 802.11g. You might also try removing any other software related to screen sharing or streaming to remote displays that you might have installed at some time.

Additionally, the following article(s) may help you.

Troubleshooting AirPlay

Troubleshooting Wi-Fi networks and connections

Recommended Wi-Fi settings

Wifi Diagnostic Software (for Mac users)

You may also find some help on this page, where I’ve collected some of the more unusual solutions to network issues.

It is exactly the same behaivior on the av cable without any network involved.

I have a perfect wifi by the way. It is ultra fast and no problems there. If some else can test it and has other results I am open for advice.

Another device working doesn’t rule out the network. You need to do proper troubleshooting

If you are tethering a device via hdmi then that is not using Apple TV nor airplay. What have you done to actually troubleshoot the network