Sunday, October 14, 2007

How Much Storage Space Will I Need For HD Audio

I've been following several threads (including this AVS Forum thread) that talk about how the audio tracks of an HD DVD or Blu-ray movie when played on your HTPC using PowerDVD Ultra are downrez'd from 24bit/48kHz to 16bit/48kHz because of copy protection issues. I think they say you cannot pass the full resolution digital audio of protected (flagged) sources unless you have a protected path. The protected path is all part of the solution that makes it impossible to copy high resolution audio and video using DRM schemes like AACS for audio and HDCP for video. It seems PowerDVD is trying to comply by just converting every audio track down to 16bit/48kHz even if the tracks are not flagged to be protected. I also thought this rule only applied to digital audio and not analog audio. Nobody seems to have a complete handle on what's really going on here, but I did read that Microsoft and Cyberlink (the developer of PowerDVD) are trying to work together to get this resolved. Hopefully they'll come up with some solution so we can listen to the audio tracks from these discs in all their high resolution glory from our PCs.

commented that maybe people aren't even going to get 24bit/96kHz or 24bit/192kHz soundtracks anyhow because the movie studios will probably only provide at best 24bit/48kHz soundtracks. (Actually, I would probably be satisfied with that, but I'd still like to hear 24bit/96kHz surround and/or 24bit/192kHz stereo tracks for concert performances.) His post implies that since the studios aren't going to include anything with a sampling rate higher than 48kHz, then maybe the problem with PowerDVD downrez'g isn't such a big deal. They're still converting the bitdepth from 24bit to 16bit and I'd like to hear them in 24bit. He referenced an article that appeared in EngadgetHD that said the real reason is because of storage space. There isn't enough space to include the uncompressed high resolution audio. But that's why they use lossless compression like Dolby TrueHD and DTS-HD MA to give you the same high quality using less space. These losslessly compressed HD tracks should still maintain the same resolution of 24bit/48kHz or higher (if available).

Just to give you an idea of how much storage space you need for uncompressed HD audio files, take a look at the following chart:
Bit DepthSample Rate# of channelsBit Rate (Mbps)File Size for 1 minute
File Size for 90 minutes
16bit44.1KHz2 (stereo)1.411
10 MB
900 MB
16bit48KHz2 (stereo)1.46
11 MB
990 MB
16bit48KHz6 (5.1 surround)4.6
33 MB
2.9 GB
24bit48KHz6 (5.1 surround)6.9
49.5 MB
4.35 GB
24bit48KHz8 (7.1 surround)9.2
66 MB
5.8 GB
24bit96KHz6 (5.1 surround)13.8
99 MB
8.7 GB
24bit96KHz8 (7.1 surround)18.4
132 MB
11.6 GB
24bit192KHz2 (stereo)9.2
66 MB
5.8 GB

As you can see, these HD tracks take up a lot of space. However, those numbers are for uncompressed audio. Don't get confused by the way they measure bit rate and storage size. When calculating the bit rate they refer to million bits per second (divide by 1,000,000) and for storage requirements, they refer to megabytes (divide by 1024 a couple times and also multiply by 8 bits to get a byte).

I don't know exactly how much space we'll save by using the losslessly compressed codecs. The only information I could find regarding the efficiency of these codecs was from this FAQ for Dolby's TrueHD. If I'm interpreting this correctly, with a 24bit recording, they achieve compression ratios of about 2 to 1, for a file size savings of about 50%. So we might be able to store 90 minutes of a 24bit/96Khz 5.1 surround sound track in 4.3 GB of space.

Since HD DVD has a storage capacity of 15 GB for single-layer and 30 GB for dual-layer discs (and 51 GB for single sided triple-layer discs); and, Blu-ray's capacity is 25 GB for single-layer and 50 GB for dual-layer, you'd think they'd have enough space at least for one of the losslessly encoded HD tracks.

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