Dolby Mobile explained - PSNEurope

Dolby Mobile explained

The first of our in-house professor’s ten-minute technology reviews, designed to keep you up to date with the latest in audio engineering, writes Francis Rumsey.
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People now listen to reproduced sound more on headphones than loudspeakers. Young and old with wires coming out of their ears are everywhere. The reason for this is not hard to discern – audio has gone mobile. Either in the car or out and about, people listen to music and watch movies while moving around and they no longer sit in their living rooms to listen to albums from beginning to end – at least not very often. Whatever you may do in the studio to ensure a high quality listening experience, be sure that your beloved master will be squashed, squeezed and rendered in the gap between most listeners’ ears through a pair of ear buds. Among the companies that have risen to the challenge of providing higher quality mobile audio is trusty Dolby Labs. Dolby Mobile is actually a suite of processes, combined in various ways to make the mobile audio experience more rewarding. These include a method for enhancing bass response, various spatial enhancement and processing algorithms, a high frequency restorer, dynamics and EQ control. Depending on the mode and the source material, some or all of these may be employed. You will increasingly find the technology installed in mobile devices such as those by LG and HTC, and it seems to have popped up along with HD video in Nokia’s new flagship N8.
Get out of my face One problem with loudspeaker stereo when reproduced over headphones is that it sounds inside the head. To get it out of the head, so that it sounds more like it would when replayed in a room, Dolby Mobile incorporates a form of binaural processing along with simulation of the acoustics of a good listening room. Essentially this attempts to put virtual loudspeakers around your head (see Figure 1), whose outputs are convolved with the reflection characteristics of a listening room. The result is audio that seems to be ‘out there’ rather than confined to the narrow space between your ears. ‘Sound Space for Headphones’ and ‘Mobile Surround’ do this for two-channel and five-channel stereo respectively. The most recent version of Dolby Mobile also incorporates Dolby Digital Plus, its latest surround sound decoding system, which enables 5.1 audio content to be played back either over headphones or loudspeakers, using an HDMI output from the mobile device. Dolby’s Sound Space Expander is designed to widen the rather narrow stereo image that you tend to get from little loudspeakers in handsets or docking stations. Again using binaural processing, more widely spaced phantom left and right signals are created, along with a phantom centre to secure the middle of the image. The result is a sound stage that extends considerably beyond the loudspeakers, said to wrap around the listener. The ‘Mono-to-Stereo Creator’ module aims to make mono files, such as some podcasts, less spatially boring to listen to. Using a modern twist on an old-fashioned technique, a second channel is created by delaying it slightly to add spaciousness. However, in order to avoid the nasty comb-filtering effects usually resulting from the addition of delayed signals, the second channel delay is varied with frequency. The lows and the highs Ear buds can have a limited low frequency response and data reduction processes such as MP3 or HE-AAC, which feature in many mobile sound files, can squash the high frequency end of the spectrum if used aggressively. Excessive dynamic range can also be a problem in mobile contexts – either you destroy your ears or your headphones or you can’t hear the quiet stuff. Here, Dolby Mobile attempts to restore the missing ends of the spectrum with ‘Natural Bass’ and ‘High Frequency Enhancer’. Unlike some other bass enhancement algorithms, such as that by Philips, Dolby Mobile doesn’t attempt to give the impression of lower bass by adding artificial harmonics of missing bass frequencies. Instead it works by boosting the bass of low level signals and compressing the peaks of loud bass sounds, so as not to destroy your headphones by pushing them beyond their limits. The amount of boost is matched to the headphone or loudspeaker and the bass content of the signal. High frequencies are restored by synthesising the missing components sometimes reduced or removed by data reduction. For example, High-Efficiency AAC doesn’t transmit the high frequency parts of the spectrum but attempts to replicate them by transposing lower-frequency elements during reproduction. Although this can be quite successful, a little extra help may be needed from technologies such as this one. High frequency enhancement can also help to restore the apparent width of the stereo image, says Dolby. Dynamic range is managed by the ‘Sound Level Controller’, which acts in a clever way to compress the signal, taking account of differences between centre-panned signals and those on the periphery of the image. Assuming that the most critical dialogue or singing voice is usually somewhere near the centre, the most unpleasant effects of pumping and sudden squashing on transients can be avoided by processing this content differently from the left and right information. How about my MIPS, RAM and ROM? MIPS (millions of instructions per second) are important to mobile platform developers. Us audio chaps generally don’t get offered many of them because they are rare commodities that eat battery power and get stolen by video or comms people. It’s also important that audio stuff doesn’t use too much memory. Dolby Mobile comes in at somewhere between 14 and 51 MIPS, depending on the mode and platform, and it would like to have between 9 and 28 kB of data RAM. With a typical mobile processor such as the ARM9E motoring along at around 220 MIPS, we’re talking about 10% or so of its capacity. Hear Dolby Mobile at http://www.dolby.com/professional/technology/mobile/mobile-demo.html

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