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Understanding High Definition Audio

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One of the big selling points of Blu-ray is the promise of lossless audio to go with those gloriously sharp high-definition pictures, but to be quite frank the audio formats found on Blu-ray discs have been a complete pain in the backside ever since the first players and discs were unveiled, causing more confusion than a Dutch-language movie about Quantum Physics directed by David Lynch. More than two years down the line the situation isn't much clearer, which is why we felt it would be useful to discuss some of those issues.

The main problem is that not all of the Blu-ray players and AV receivers doing the rounds are equipped to handle these formats, which in turn means that every review of a Blu-ray player we run is packed with ifs, buts and maybes as we attempt to explain the ramifications for home cinema systems with differing capabilities.

Receivers equipped with the right features and connections are growing in number and dropping in price, which has alleviated the problem to some extent, but the whole business of hearing HD audio still seems horribly convoluted.

Before we highlight some of these complications it's worth quickly recapping what each of these formats are and what they offer. Dolby TrueHD and DTS HD Master Audio are lossless audio formats, which means the sound is bit-for-bit identical to the studio master and therefore allow you to hear movie soundtracks exactly as the director intended.

Dolby TrueHD supports bit rates of up to 18Mbps, and up to eight full-range channels of 96kHz/24-bit audio (or up to 5.1 channels of 192kHz/24-bit audio). The format can actually handle more than eight channels, but it's restricted to eight by the Blu-ray standard. DTS HD Master Audio also supports 7.1 discrete channels at 96kHz/24-bit but supports bitrates up to 24.5Mbps.

The sound quality is therefore superior to the regular Dolby Digital and DTS formats that came before them, and both formats are backwards compatible with legacy amplifiers.

In order to hear these formats, you need to connect your Blu-ray player to an AV receiver, and there are several ways to do so. The simplest method is using an HDMI connection, which allows you to transfer the raw audio bit stream to the receiver where the decoding (and other audio processing) can take place. The problem is that Dolby TrueHD and DTS HD Master Audio bit streams are only supported by HDMI v1.3 connections, which means both the player and receiver need to feature this type of HDMI socket.

However, if the receiver lacks HDMI 1.3 inputs, all is not lost. Some Blu-ray players can convert Dolby TrueHD and DTS HD Master Audio soundtracks to PCM - which is supported by all types of HDMI connection - which allows you to enjoy the higher-resolution benefits of these formats, as well as any secondary audio streams that are featured on the disc.

And should the receiver not sport any HDMI inputs at all, then some Blu-ray players feature multichannel analogue outputs that let you transfer the decoded TrueHD and Master Audio signals to the relevant phono inputs on the receiver.

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