AppleTV 4 surround sound issue

Jon Walker wrote:

If your AVR displays dialog normalization you can see that AC-3 is not pass-through.

Am confused by this statement. If the AVR display of a dial norm message such as this: (I.e., I always get a Dial Norm +4 notification when playing an AC3 file/DVD/BD audio track when using a "Dolby" compatible TV mode or DVD/BD device.)

I'm not what this means because dialog normalization isn't a constant however my point was if you compare the the dialnorm display for the "same" AC-3 source (e.g. play the same movie from your iTunes server) you'll see one value on the ATV3gen and another value (or in my case no value) on the ATV4gen.

In that case the ATV 3 could have tampered with dialnorm and the ATV 4 is pass-through. Do you have the means to check the original AC-3 file for metadata like dialnorm?

HAL9000.2 wrote:

Yes, that statement by bodosom isn't correct as dialnorm is part of the AC-3 specs. It's not an indicator whether an AC-3 stream has been tampered with or not.

I was unclear. You have to compare the same AC-3 stream out the ATV4g and something else. In my case I compare and ATV3,1(8.3) to an ATV5,3(9.0.1). The data below is from the web interface in my AVR.

MI:Rogue Nation on ATV3,1(Dolby,Auto):

MI:Rogue Nation on ATV5,3 (Surround,Dolby):

Note that the latter calls the Input Signal analog (thats the hint I mentioned) the OSD for the same info calls both input signals DD. As you noted dialnorm is in the bitstream and is set by the sound editor (or equivalent) for each movie in the range 1

bodosom wrote:

Jon Walker wrote:

If your AVR displays dialog normalization you can see that AC-3 is not pass-through.

Am confused by this statement. If the AVR display of a dial norm message such as this: (I.e., I always get a Dial Norm +4 notification when playing an AC3 file/DVD/BD audio track when using a "Dolby" compatible TV mode or DVD/BD device.)

I'm not what this means because dialog normalization isn't a constant however my point was if you compare the the dialnorm display for the "same" AC-3 source (e.g. play the same movie from your iTunes server) you'll see one value on the ATV3gen and another value (or in my case no value) on the ATV4gen.

Feh, I meant to say "I'm not sure what this means .... (and mine is -4dB)".

HAL9000.2 wrote:

Do you have the means to check the original AC-3 file for metadata like dialnorm?

No but it's unlikely dialnorm would be zero and certainly not across multiple inputs. The default is -4dB but it's not fixed and I suppose -4dB could be displayed as 4 since it's always negative. The simpler explanation is the ATV4g is doing "adjustments" and then setting dialnorm to zero so the downstream device won't undo those adjustments. I think most people are of the opinion that those adjustments should only be made if compression has been enabled. Apple is apparently of the opinion that it's "better" to always process even the processing is a no-op with respect to volume.

bodosom wrote:

... As you noted dialnorm is in the bitstream and is set by the sound editor (or equivalent) for each movie in the range 1

I clearly can't type today.

"in the range 1 [to 31]"

However rereading does make me wonder if that range is how it's displayed on the encoder with an implicit negation and if the typical value of -27dBfs is set with 27 (absolute value) or 4 (offset from 31).

Dialnorm is a number between 1 and 31. The values translate to -1dB to -31dB below full scale. -31 = no gain reduction, -1 = 30dB of gain reduction.

Please see https://en.wikipedia.org/wiki/Dialnorm

Thanks for the link to the referenced article. It basically confirms what I had originally thought. Am, however, still wondering if the display value is based solely on the AC3 encode, the AVR calibration, or a net result of the two in combination. (Just curious as to whether you happen to know for sure.)

The dialnorm number is part of the AC-3 stream metadata. It is set when the bitstream is encoded. Basically it signals the average dialog level in the program. For example, if the average level is -27dBFS then dialnorm has to be set to 27. The AVR then reduces program level by 4dB (31-27=4) during playback.

HAL9000.2 wrote:

Dialnorm is a number between 1 and 31. The values translate to -1dB to -31dB below full scale. -31 = no gain reduction, -1 = 30dB of gain reduction.

I've read the Wikipedia article too. That doesn't explain the "notation" used by the pictured Pioneer which shows DIAL NORM +4. Obviously +4 doesn't mean 27dB of gain reduction (4 on a scale of 1 to 31) it means add 4 to -31dB to get the default -27dB which you would say is a dialnorm of 27.

In any case the ATV4g is (currently) modifying the bitstream in Dolby mode presumably to support volume management and presumably by doing a decode/encode. Another example would be a particular Netflix Orginals episode at -5dB or a movie at -4dB. Both show 0 (or missing) on the ATV4gen.

I agree this behavior suggests that Apple TV 4 is doing a live and probably lossy AC-3 re-encode. Bitstream pass-through should be default behavior instead or at least an optional settings for people that don't want the device to tamper with audio quality.

HAL9000.2 wrote:

In that case the ATV 3 could have tampered with dialnorm and the ATV 4 is pass-through. Do you have the means to check the original AC-3 file for metadata like dialnorm?

My earlier answer of no was based non-local sources. That's unduly limiting. As far as I know you can't determine much about FairPlay audio streams and of course streamed data is, well, streamed. However mediainfo (at least under Linux) will display dialnorm if it's not under DRM. I checked an on-disk unencrypted movie that reported zero on the ATV4gen and -4dB on the ATV3gen and the dialnorm is reported as -27dB.

So yes for on-disk unencrypted AC-3 and no for anything else. As far as I know.

I agree this behavior suggests that Apple TV 4 is doing a live and probably lossy AC-3 re-encode. Bitstream pass-through should be default behavior instead or at least an optional settings for people that don't want the device to tamper with audio quality.

I tend to disagree. The current evidence only suggests that the metadata is being manipulated. Until such time as the output is compared with and without TV4 processing between the source and AVR, it seems more likely that the encoded "audio" data itself is being passed to the receiver unchanged—e.g., like copying data from an M4V file container to an MOV file container without actually transcoding any of the stored data. Apparently my definition of "pass through" strategy is more liberal than that of others. If I encode an AC3 track as 1.0 mono and the AVR receives an AC3 bitstream with 1.0 audio data untranscoded, to my mind the data was "passed through" by the bitstream whether or not any metadata was changed or lost. The same holds true for 2.0, 2.1, ..., 5.1 encoded audio. This is exactly what appears to be happening to my files in the "Dolby Surround" mode when sourcing the AC3 encoded audio rack.

I'm not what this means because dialog normalization isn't a constant however my point was if you compare the the dialnorm display for the "same" AC-3 source (e.g. play the same movie from your iTunes server) you'll see one value on the ATV3gen and another value (or in my case no value) on the ATV4gen.

While the "+4" may be a constant, it may not actually be a dialnorm value. Could it actually be the "stored adjustment" the receiver is supposed to apply to any recognized AC3 bitstream based on the calibration performed during initial setup of the receiver and speakers?

As to playing the "same" AC3 source, I currently get the +4 value streaming "the same" M4V library files encoded by me from the iTunes server, Air Video server (via indirect Air Play only), Air Video HD server, Plex Server, or Air Play (as compatible) directly to an TV2, TV3, TV4, LG BD Player, Oppo BD Player or LG HDTV (audio redirected back to the AVR), as well as, indirect Air Play playback from both iPad and iPhone mobile devices. DVD and BD optical discs also have the same "+4" display value (when applicable) and I most recently noted "on the fly" DTS 5.1 to AC3 transcode streams from the Plex Server for RAID stored backups of DVD/BD content also have this display value. All of which brings me back to the question of whether or not the "+4" display might merely refer to a programmed adjustment. (I.e.,, I even see this for 1.0/2.0 AAC audio tracks which the TV4 outputs via an AC3 5.1 bitstream connection in the TV4 "Dolby Surround" mode which are definitely the product of TV4 non-pass through "machinations" as programmed by Apple.)

Jon Walker wrote:

I'm not what this means because dialog normalization isn't a constant however my point was if you compare the the dialnorm display for the "same" AC-3 source (e.g. play the same movie from your iTunes server) you'll see one value on the ATV3gen and another value (or in my case no value) on the ATV4gen.

While the "+4" may be a constant, it may not actually be a dialnorm value. Could it actually be the "stored adjustment" the receiver is supposed to apply to any recognized AC3 bitstream based on the calibration performed during initial setup of the receiver and speakers?

As I said, what seems a long time ago, only if your AVR is misleading you since it says DIAL NORM. Dialog Normalization isn't constant and it's independent of AVR settings unless your decoder is broken and presumably fails to meet Dolby certification. As previously noted mediainfo (available on most common platforms) will display dialnorm in an unencrypted disk file. I have values from -27dB to -31dB. By the way my AVR displays dialnorm -31dB differently from no dialnorm.

If you have Netflix check Jessica Jones episodes 1 and 2. Episode one has dialnorm -5dB and episode two has dialnorm -4dB on both my ATV3 and my TiVo. Both are reported as 0/missing via the ATV4gen (with Surround Dolby).