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Dolby creates an elevated listening experience

In this week's feature, the Prof reveals why Dolby Home Theatre v4's new suite of tools is music to the ears

Let’s face it, the sound quality coming out of most PC speakers is a bit crap, and yet people regularly use laptops for listening to movies and TV programmes. The sound quality of online content varies widely too, having significant differences in level and dynamic range. It would be nice if such machines could offer a better listening experience, with some sort of surround. Dolby’s Home Theater v4 tackles these problems with a new suite of tools. It’s surprising how effective ‘faux’ surround can be, created by upmixing ordinary two-channel stereo to extract some content that can be used to create an immersive spatial effect. The new Dolby system, built into a number of Acer and Lenovo PCs, achieves this using a form of frequency domain matrix decoding. Essentially this is Pro Logic-like decoding that extracts centre and surround channel content out of left and right front channels, but the frequency domain processing enables more precise identification of the dominant elements of the scene in different frequency bands. The result is a decoder that can create reasonably convincing 5.1 or 7.1 surround out of any two-channel input. If the stereo source contains properly matrixed surround content, all the better – the result will be effectively decoded. A surround virtualiser enables the surround effect to be heard even through two front loudspeakers, by using a form of binaural processing and crosstalk cancelling to make you think there are speakers behind you. Computer sound replay is one area where this makes some sense, because it’s much more likely that the listener will be sitting in a stationary spot between a pair of loudspeakers. Among other tools, this new system provides dialogue enhancement by detecting speech content automatically and processing it for greater clarity. Level anomalies are ironed out, and the audio is conditioned in a way that is customised for the computer model in question, so as to minimise distortion, flatten the response and maximise loudness. There’s also a form of intelligent equaliser that attempts to ensure consistency of timbre across different programme content. The company has also been branching out – this time in the vertical direction – with a new flavour of matrix surround decoding known as Pro Logic IIz. This extracts two additional front channels that are aimed at loudspeakers mounted above the conventional left and right ones. The content that’s extracted from ordinary surround material is intended to be diffuse rather than directional, so it mainly adds an increased sense of spaciousness in the vertical dimension. For content creators that want to take advantage of the new height channels as a discrete mixing destination there is an encoding option that allows left and right height information to be matrixed. According to a recent AES paper by Tsingos et al, presented at the AES Game Audio conference, the height channels are each phase shifted by 90° and attenuated before being mixed with the left and right surround signals, so as to be 180° out of phase. (See the accompanying diagram.) This has some things in common with conventional Dolby Stereo encoding of the centre channel, but in this case two channels are matrixed rather than one. Although this encoding matrix information is offered royalty-free by Dolby, preliminary agreement is required from them before using it in commercial products.