Dolby Digital HD
Dolby Presents Lossless High Resolution Multi-Channel Audio
Dolby Digital is a long standing lossy audio compression method. It allows surround sound to be squeezed into the Mpeg2 format. Mpeg2 is the audio video digital compression format that gives us Digital Cable TV, DVD movies, and Satellite TV. To keep the audio portion of any DVD movie from taking up too much space it’s compressed, like a zip file. This has been Dolby Digital’s forte. The trouble with lossy compression is that there are literally sounds being jettisoned for compression. For most people audio compression makes no difference to the audio experience, but for the true audiophile this is unacceptable. DTS, the alternative surround soundtrack maker, provides technology that diminishes the lossy effect.
Generally, sound is converted to digital using a process called Pulse Code Modulation (PCM). Using PCM, the more digital samples that are taken and higher bit rate creates better sound quality, especially through a very detailed audio system. CDs, for instance, have used the 16bit / 44 kHz method. That means 16 bits per cycle at 44 thousand cycles per second. Newer digital audio sources, like Dolby Digital and DTS (including the high res DVD Audio), use a 24bit/96kHz sampling rate, that’s a lot more bits. For Dolby Digital and DTS, this raw digitized audio signal is compressed using a lossy compression algorithm, this is where valuable sound quality is lost. DVD Audio is another format that doesn’t compress at all. A DVD audio music disc will play back the 24/96 completely uncompressed, providing the DVD with more audio detail. This uncompressed format is called a high resolution audio format.
The high resolution audio formats (DVD Audio and Sony’s competing format called SACD) have not become commercial successes. However, modern next-gen disc storage methods (HD DVD Blu-Ray) have much higher storage than DVD. This should allow studios to produce discs with uncompressed multi-channel audio tracks if they so choose.
This is where Dolby True HD comes in. Dolby Labs call it a 100% lossless coding technology. This means that it's not compressed after it is sampled. True HD will support up to eight channels of 24-bit/96kHz audio. However, the True HD moniker doesn't necessarily mean that the audio you're listening to is actually sampled at that high a rate and not compressed. What it does mean is that the format the audio is encoded in can support it. We’ve seen it before with DTS 24/96: not all DTS soundtracks are encoded at this high a sampling rate. It’s really up to the studio engineers designing the discs to make the best use of the format.
In the near future you might see a copy of X-Men2 on Blu-Ray Disc that is encoded in True HD. It’s unlikely Fox would use TrueHD if it wasn’t going to take full advantage of its technology or they could just release it in what will be the standard lossy encoding method, Dolby Digital Plus.
Hopefully we will see our favorite films released on Blu-Ray using Dolby TrueHD. So far neither optical format (HD DVD or Blu-Ray) have announced Dolby TrueHD as the standard audio format. For those of us who really appreciate high quality sound, we hope Dolby TrueHD does better than DVD-Audio and SACD.
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