The Alphabet Soup of Audio Codecs – Mass confusion confounds some consumers when they are faced with the plethora of different audio logos on HDTVs and DVD, universal or Blu-ray players. These are mostly digital codecs – proprietary logirhythms used to process audio signals at the source end to better fit the “pipe” or media on which they are recorded, and then reconstituting them for your listening pleasure at the other end – using a mirror image of the code. Back in strictly analog days it was just Dolby B, C or S, though there were other approaches such as dbx II and CX. Now the two main players are Dolby and DTS.
What these processes do is to compress the vast amount of sampled audio data in various ways so it can fit on the broadcast signal or the signal on your optical DVDs or Bllu-rays. Just as with the varieties of MP3, most of them throw out a good part of the audio information that has been found to be the least-heard psychoacoustically. This is separate from such elements as the sampling rate and word length (number of bits used). The higher those numbers, the better the fidelity will be, in general.
Dolby has three codecs: The most-compromised and oldest is Dolby Digital 5.1 or 2.0, used on DVDs for years and in broadcast – although it’s further compromised there by a long chain of processing and the fact that local stations often don’t broadcast the 5.1 soundtrack, converting it to plain stereo. The mid-rung is Dolby Digital Plus. This can ramp up to 7.1 surround channels, data-reduces less, and is the minimum specified for Blu-ray. The top Dolby codec is Dolby TrueHD, which is lossless – that means you supposedly get exactly the same data coming out your end that went in at the other end, even though it was data-reduced in between. It handles up to 18 megabits per second and can support as many as 14 channels.
DTS also offers a confusing threesome: DTS Digital Surround runs at 1.5Mbps and supports 5.1 channels. Depending of course on how it was applied, the basic DTS often sounded better than the basic Dolby on DVDs. DTS Hi-Res Audio goes up to 6Mbps and can support up to eight channels. If there are all sorts of extras on a DVD this codec might be used to save space instead of the top DTS codec. Which is DTS-HD Master Audio, a lossless format just like the top Dolby offering. It is said to be bit-for-bit the same as the studio master, can handle up to eight channels, and is supported on Sony’s PS3. Over and above all six of these – and widely used early in the Blu-ray game due to decoders for the advanced codecs not being included in first generation players – is Uncompressed 5.1 channel PCM. It’s not data-reduced at all since Blu-rays have 50GB of storage space, but mostly it used 48K/16-bit sampling rather than the 96K/14-bit option. At the latter it should be the best surround sonics of all, and doesn’t require a special logo, but often it is output only via the six-channel analog outs, and some players don’t even have those anymore, having settled on glitch-prone HDMI connections instead.
There are also logos for various analog matrix surround sound processes such as Dolby ProLogic II & IIx, DTS Neo:6 and Circle Surround. These processes take a standard stereo audio signal and process its ambient/difference information, feeding it to 5 or 6 channels for surround effect. They can be quite convincing with appropriate source signals. There are also several virtual surround technologies which try to do just the opposite: they take a multichannel source and process it to create a pseudo-surround effect from just two front speakers or stereo headphones. It’s a subjective thing which seldom equals the soundfield of multiple speakers.
