TrueHD 7.1 vs. Dolby Digital AC-3 5.1

Curious about the quality differences between these two formats as I notice the difference in volume when I switch back and forth between them and possibly the mixing?? For example, right now I’m watching a BluRay 4K HDR Remux of Ready Player One with Apple TV 4K (Audio Output set to ‘Auto’) and Infuse 6.4 using Plex as a media server on a decent 5.1 Surround Sound System. By default it plays TrueHD 7.1 and I tried switching to Dolby Digital AC-3 5.1, which from reading around seems to be a compressed audio format as opposed to TrueHD 7.1, but 5.1 sounds better to me so far. Shouldn’t 7.1 uncompressed format sound better? The 5.1 audio is definitely louder but I can just turn 7.1 volume up, but the 5.1 also sounds more ‘full’ if that makes any sense. I feel like 7.1 is hollow or maybe it’s not downcoding to 5.1?? Can someone help explain what’s going on here? Is it maybe playing 7.1 and I’m missing 2 speakers so I’m not getting some of the audio signal? Possible my receiver is not downcoding 7.1 to 5.1? My receiver is a 7.1 receiver but I’m using it as a 5.1 system and using a high quality optical cable. I’ve read that HDMI can pass uncompressed audio but optical is limited, but without noise. Any input or feedback is much appreciated, excited to get the most out of Infuse and watching higher quality 4K movies and uncompressed audio!

First of all optical only supports up to 5.1 and then only via bitstream/DD. PCM over optical is only 2.0 which is what infuse decodes to by default. Thus either the channels are just getting dropped or they are downconverted to DD5.1. Recommendation to plug ATV directly into receiver with HDMI for to get full 7.1 signal.

Digital audio signals, such as 5.1/7.1 multi-channel PCM, Dolby Digital Plus, Dolby TrueHD, Dolby Atmos, DTS-HD Master Audio, DTS:X, and Auro 3D Audiocannot be transferred via Digital Optical connections. These types of audio signal formats require the use of HDMI connections

Hi, thanks for the response. The reason I don’t plug Apple TV into my receiver via HDMI is my receiver does not have HDMI inputs. Also, I do not have a 7.1 speaker system. So I have my Apple TV plugged directly into my tv via HDMI and sound output from my tv via Optical/TosLink cable directly to my receiver.

Unfortunately if you want to hear the best audio you would still need to plug in to HDMI to get full lossless 5.1/7.1 audio, otherwise I believe it just defaults to the lossy 5.1. Not sure if you are using just an audio-only receiver or an old one (I still have one too) but the new receivers do a better job of up/down mixing audio as well as much better room correction.

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I see. I have an older higher quality receiver, but I’m confused. It’s a 7.1 channel receiver. Why would they make a 7.1 receiver with only Optical/Toslink outputs??

Does yours have a multi-channel input (8 individual rca analogue)? I think that is the only way. Other than that the extra channels are used to upconvert the signal to 7.1 or use the other two channels for another zone.

If it doesn’t have HDMI then it was most likely made before blu-ray players came out so it only serviced DVD which only went up to 5.1. I have a 7.1 Sony from 2005 I still use but just for audio

I think it’s from 2003, it does have individual inputs for each channel

Do you think it’s better to choose Dolby Digital 5.1 from the drop down audio menu vs. going into Apple TV Settings and changing Audio to 5.1? Or should it be the same?

Drop down is maybe better quality but that only works if the file you have actually included it.