How to control volume using apple tv app on iphone?

That is using airplay to stream content from your phone, and that is controlling your phone’s volume, which is then what is being passed to the Apple TV. You are not adjusting your TVs volume directly. That is why it is not able to occur in other circumstances, it has nothing to do with a bug. The phone would require an IR receiver to be able to control your TVs volume.

From that article:

“Want to change the volume using your iOS device? Receivers with HDMI-CEC volume support let you adjust volume with the volume buttons on the side of your iOS device. You just need to plug your Apple TV directly into your receiver and not into your television.”

TV alone won’t cut it for this functionality.

That is correct

CEC is used to control other devices over HDMI. It is allowing your Apple TV to also turn off your TV

If you are using your TVs internal speakers then there is no CEC involved as it is internal and not being transmitted over HDMI. That is being controlled by IR and the phone does not have that ability.

I mean that is incorrect.

As was already mentioned the TVs speakers would be controlled by IR, and that it is not possible to control this with the app because the phone’s hardware does not have the ability to transmit a signal using IR.

You would need a receiver connected with external speakers and the receiver would need to support CEC.

Your TV is operating over IR, which is not possible to do when using the app.

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That is incorrect.