Lots of people have asked the question recently about how to play back the HD audio on Blu-Ray and HD-DVD discs on their HTPC's and the response/dicussion this created has answered some, but not all of the holes in my own knowledge of this subject.
Subsequently, I have done some investigation to try to understand this fully and thought that I would post what I have discovered. Please note that I include some quotes from other forums, reviews and articles.
First things first, Dolby TrueHD is Dolby's lossless audio and DTS-HD MA is the DTS version of lossless audio. Basically DTS-HD MA, Dolby TrueHD are both exact copies of the studio masters (albeit in a lossless compression) and arguably the same in terms of quality (albeit with some sample rate differences that I won't go into). Also there is PCM which is a totally uncompressed and exact replication of the studio master.
The main thing to understand is that you cannot pass either of these HD audio formats to your amp via optical or digital coax as there isn't enough bandwidth in either of these connections. The only way to pass it digitally is via HDMIv1.3, although all versions of HDMI will carry 8-channel PCM. The only product available at the moment that can do this from a PC is the Asus HDAV1.3 sound card (with which you'll need a HDMI outputting gpu as well). Some ATi and Nvidia graphics cards and integrated gpu's can pass 8 channel PCM via HDMI but Power DVD always downsamples the stream to 16-bit/48khz anyway so it isn't a true bit for bit version of the HD codecs (although you do get 8 discrete channels if you have a 7.1 set up).
So what about outputting it over analogue signals then? Well as I mentioned, Cyberlink has confirmed that;
'Please note that according to AACS compliance rule, player has to protect AACS decrypted content from digital copy. Since there's no secure protection between AP and audio driver yet, PowerDVD has to downsample High-def Audio to 48 kHz before sending to driver'.
So while some people have been claiming to be able to do this it isn't possible without the HDAV1.3 that can handle Protected Audio Playback Systems (PAPS) and full Advanced Access Content System (AACS) certification.
The PS3 is, unfortunately, not fully compatible with the HD audio codecs either. While it does have the ability to decode TrueHD and DTS HD (from update 2.30) internally it is only able to output it as PCM (a method for sending an analogue signal, digitally). The PS3 has no support for bitstream output of the HD codecs although this shouldn't impact on sound quality as PCM output just means that the PS3 is doing the decoding of the HD formats rather than your amp. You would need an AV amp that can accept 8-channel PCM over HDMI for this.
The final point that I was unsure of was about ripped, or backed up, copies of Blu-Ray and HD-DVD discs which seemed to me a viable way of avoidng that pesky AACS system. It seems to be possible but there is no way of proving that it works, just peoples view on whether it sounds better or not. Here is a post from AVSforum that explains how it's done;
'Currently, it is easy to rip data, but there are few directshow decoders that can handle audio playback. One such decoder is contained in PowerDVD, but can only be accessed via the PowerDVD player itself and it downsamples the audio during decode. The Sonic decoders handle DTS well and can be used externally (albeit with a few caveats). Libav can handle TrueHD quite well and is incorporated into MPC-HC amongst others. Nero decoder is apparently best at handling E-AC3 and can be used externally. Unfortunately, most of these decoders are difficult and/or costly to obtain.
However, rather than loading various filters to handle different audio codecs at play time, some forum members are electing to transcode the source audio into LPCM/FLAC (using the above decoders) and mux it with the video into an alternate container. This only needs to be done once and then most players will handle the much easier to decode LPCM/FLAC audio during playback. As long as the transcode is performed with the highest quality, it is finally possible to (generally) listen to the source audio in as close to its full resolution as practicable, without the downsampling of PowerDVD.
If there was a single external directshow decoder that could handle all the high resolution audio codecs, without downsampling, then we wouldn't have to go through this complicated process and could playback directly from disc. But until libav is improved to handle everything well, then multiple decoders still have to be used at some point.'
Adding to all of this complication is the fact that a lot of the discs around are being reported as only carrying 16-bit/48khz HD audio it makes me wonder if it's really worth the hassle at the moment. I for one will wait for a while longer before buying a Blu-Ray drive and any hardware needed for the audio.
I await with baited breath for any corrections and/or additions to this!
A link to a good explanation of the audio formats on Blu-ray and HD-DVD;
http://www.highdefdigest.com/news/s..._FAQ:_Blu-ray_and_HD_DVD_Audio_Explained/1064
If you want to know what's on a disc that you have then search for a program called 'BDinfo'.
A link for info on 8 channel LPCM;
http://www.anandtech.com/video/showdoc.aspx?i=3411&p=1
Subsequently, I have done some investigation to try to understand this fully and thought that I would post what I have discovered. Please note that I include some quotes from other forums, reviews and articles.
First things first, Dolby TrueHD is Dolby's lossless audio and DTS-HD MA is the DTS version of lossless audio. Basically DTS-HD MA, Dolby TrueHD are both exact copies of the studio masters (albeit in a lossless compression) and arguably the same in terms of quality (albeit with some sample rate differences that I won't go into). Also there is PCM which is a totally uncompressed and exact replication of the studio master.
The main thing to understand is that you cannot pass either of these HD audio formats to your amp via optical or digital coax as there isn't enough bandwidth in either of these connections. The only way to pass it digitally is via HDMIv1.3, although all versions of HDMI will carry 8-channel PCM. The only product available at the moment that can do this from a PC is the Asus HDAV1.3 sound card (with which you'll need a HDMI outputting gpu as well). Some ATi and Nvidia graphics cards and integrated gpu's can pass 8 channel PCM via HDMI but Power DVD always downsamples the stream to 16-bit/48khz anyway so it isn't a true bit for bit version of the HD codecs (although you do get 8 discrete channels if you have a 7.1 set up).
So what about outputting it over analogue signals then? Well as I mentioned, Cyberlink has confirmed that;
'Please note that according to AACS compliance rule, player has to protect AACS decrypted content from digital copy. Since there's no secure protection between AP and audio driver yet, PowerDVD has to downsample High-def Audio to 48 kHz before sending to driver'.
So while some people have been claiming to be able to do this it isn't possible without the HDAV1.3 that can handle Protected Audio Playback Systems (PAPS) and full Advanced Access Content System (AACS) certification.
The PS3 is, unfortunately, not fully compatible with the HD audio codecs either. While it does have the ability to decode TrueHD and DTS HD (from update 2.30) internally it is only able to output it as PCM (a method for sending an analogue signal, digitally). The PS3 has no support for bitstream output of the HD codecs although this shouldn't impact on sound quality as PCM output just means that the PS3 is doing the decoding of the HD formats rather than your amp. You would need an AV amp that can accept 8-channel PCM over HDMI for this.
The final point that I was unsure of was about ripped, or backed up, copies of Blu-Ray and HD-DVD discs which seemed to me a viable way of avoidng that pesky AACS system. It seems to be possible but there is no way of proving that it works, just peoples view on whether it sounds better or not. Here is a post from AVSforum that explains how it's done;
'Currently, it is easy to rip data, but there are few directshow decoders that can handle audio playback. One such decoder is contained in PowerDVD, but can only be accessed via the PowerDVD player itself and it downsamples the audio during decode. The Sonic decoders handle DTS well and can be used externally (albeit with a few caveats). Libav can handle TrueHD quite well and is incorporated into MPC-HC amongst others. Nero decoder is apparently best at handling E-AC3 and can be used externally. Unfortunately, most of these decoders are difficult and/or costly to obtain.
However, rather than loading various filters to handle different audio codecs at play time, some forum members are electing to transcode the source audio into LPCM/FLAC (using the above decoders) and mux it with the video into an alternate container. This only needs to be done once and then most players will handle the much easier to decode LPCM/FLAC audio during playback. As long as the transcode is performed with the highest quality, it is finally possible to (generally) listen to the source audio in as close to its full resolution as practicable, without the downsampling of PowerDVD.
If there was a single external directshow decoder that could handle all the high resolution audio codecs, without downsampling, then we wouldn't have to go through this complicated process and could playback directly from disc. But until libav is improved to handle everything well, then multiple decoders still have to be used at some point.'
Adding to all of this complication is the fact that a lot of the discs around are being reported as only carrying 16-bit/48khz HD audio it makes me wonder if it's really worth the hassle at the moment. I for one will wait for a while longer before buying a Blu-Ray drive and any hardware needed for the audio.
I await with baited breath for any corrections and/or additions to this!
A link to a good explanation of the audio formats on Blu-ray and HD-DVD;
http://www.highdefdigest.com/news/s..._FAQ:_Blu-ray_and_HD_DVD_Audio_Explained/1064
If you want to know what's on a disc that you have then search for a program called 'BDinfo'.
A link for info on 8 channel LPCM;
http://www.anandtech.com/video/showdoc.aspx?i=3411&p=1
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