Encoding Dolby Surround with CTR Chan @ Full Volume?

[BassPig]

I'm working on a challenging soundtrack that was mastered in 24-bits and has more than 85dB range of levels between the loudest sounds and background ambient level (pyrotechnics).

I'm cracking through hurdles one by one with maximizing the audio level on the Dolby AC3 surround track, and I've managed to achieve full 0dB levels on the front left and right channels after turning the dialog normalization to -31dB and shutting off RF modulation protection and any compression.

My main leftover challenge is getting the Center channel up to full levels. It is always being attenuated by 3dB by the encoder. This, no matter how I set the Center Mix Level, whose highest level is -3dB. The is no 0dB level available for the center mix. It seems absurd not to allow the full dynamic range for the center channel. Center is going to be half the dynamic range as LF and RF and, despite it receiving the hottest signal in much of the event, it ends up with the weakest output in proportion to the other front channels.

I'm analyzing the results of each encode by taking the AC3 file into HeadAC3, decoding it to a 5 channel WAV PCM and inspecting that in SoundForge 9, so see the levels actually encoded.

What am I missing here? Or is this simply not possible with Dolby tracks?

[Midzuki]

What am I missing here?

Aften.exe

[BassPig]

Thanks, but I don't want to re-encode lossy audio to yet more lossy audio. As it is, I'm trying to stuff something that should remain in 24-bit depth into Dolby AC3 (because it's the only multichannel format that Adobe Encore CS3 understands) and I'm trying to author a Blu-ray disc. (I really wish I could encode Dolby TrueHD, because the AC3 has already lost so much of the transient attack of the pyro explosions that it's quite sad already. Can't really afford to lose much more.)

The problem is that, without resorting to dynamic compression, the ambient sound levels are down in the power supply hum of the Blu-ray player and there is only 5 samples conveying the amplitude of the signals down there in 16-bit Dolby AC3. The master 24-bit track sounds like you're there on the airfield, but the AC3 has lost much of the edge, like MP3 loses the attack on snare drum hits and sounds mushy. The rise times on the explosions, viewed in a waveform editor, are about 40uS. I think it would be a worse situation to put the file through additional normalizing, since there's not enough bit depth to begin with and normalizing the AC3 file would just result in a gritty sounding center channel. I was hoping there was a setting I'd missed in the Dolby Encoder that would alleviate the center 3dB attenuation. I guess I need a player with a -135dB noise floor to properly play back this disc. I can get it loud enough with enough preamp gain, but the hum and hiss when the player is paused is almost conversation level. On the 24-bit master, there's no hiss and one can clearly hear the national anthem playing on a car radio 650' away on the other side of the airport, before the fireworks start launching. The recording mics were only 170' from the large mortar launchers, so the SPL was easily in the 120dB range, whilst the ambient was around 25-30dB. Given the extraordinary dynamic range that I am trying to pack into the Blu-ray, I need to get every dB of loudness out of the encoder and no compression of any kind.

BTW, if anyone knows how to get Adobe Encore to import a 640kpbs AC3 file, I would like to know what you did to work around the bug in Encore. The best I was able to do was 512kbps. Anything higher crashed Encore on import. Blu-ray is supposed to accept 640kbps and I'm authoring in Blu-ray mode in Encore.

[Ghitulescu]

Doesn't CS3 accept LPCM? I'm pretty sure it does. This way you'll have your audio fully "unprocessed" on the blue disk ....

[tebasuna51]

...
My main leftover challenge is getting the Center channel up to full levels. It is always being attenuated by 3dB by the encoder. This, no matter how I set the Center Mix Level, whose highest level is -3dB. The is no 0dB level available for the center mix. It seems absurd not to allow the full dynamic range for the center channel. Center is going to be half the dynamic range as LF and RF and, despite it receiving the hottest signal in much of the event, it ends up with the weakest output in proportion to the other front channels...
The Center Mix Level -3dB is only used when downmix to stereo, not used when play in 5.1 environment. The max value is -3 dB because the mix is make in Left and Right channels then you have the full Center channel power.

Other cuestion is the dialog level inside the Center channel. Play only the lossless Center channel and check if you have here the problem.

And don't mistake dynamic range (difference between low and high volume) with sound volume (proportional to RMS value). Maybe you have high peaks (from sound ambience) but low RMS in dialogs (less than -31 dB) in the Center channel.

[BassPig]

CS3 won't accept LPCM tracks with more than two channels, unfortunately. :( (Specifically, it will transcode them to 16-bit stereo audio, throwing away the center and rear channels completely!)


I discovered the ENCODED center channel is being reduced by 3dB when I decompressed it to a 5-channel LPCM file in HeadAC3 and viewed that file in SoundForge 9. LF and RF channels were exactly the same levels as the discreet mono WAV files used to encode it, but the Center channel was -3dB compared to the Center .wav file that it was used to encode it.

I set the Dialog Norm to -31, as that results in the highest playback volume on the player. I tried the opposite end of the scale at some point, but that resulted in audio that was about -30dB on the loudest (0dBfs in the original Wav files) peaks.

The trouble with 16-bit is harmonic distortion is pretty high on the -85dB ambient sounds. With the playback levels up there so that the original SPL is achieved, the 16-bit version has a gritty, slightly distorted sound to it, compared to the 24-bit master. With all the optimizations I did, it's improved, but I sure could use another 3dB in the center.

As it is, I had to order an Oppo BDP-83 player, as my Sony BDP-S301's power supply isn't quiet enough, and I spent an evening tearing into my Carver C4000 preamp to chase down a ground loop hum and make the necessary modifications to null that out by routing supplemental ground busses to the input PCB. I'm hoping the Oppo, with it's ability to trim every channel to +10dB, will give me 10dB more s/n ratio from the player, in addition to any improvement through quieter analog electronics over my Sony player. I gained over 6dB just by modifying the ground system in my C4000 preamp. I also installed a noise filter in the line cord to the preamp rack, which killed some of the buzz from the big QSC power amps in the other rack.

When you're digging out sounds in the -100dB range, every little hiss and hum and buzz becomes an issue!