Design Article
Pro audio techniques applied in mobile devices to improve sound quality
Earl Schreyer, Fairchild Semiconductor
7/26/2011 10:11 PM EDT
While mobile phones continue to emerge as a key communications device and become more feature rich with multimedia applications, there still remains the challenge of achieving good sound quality from the small low profile speakers. Pro Audio techniques such as Dynamic Range Compression (DRC) and limiting can be designed in and applied to the speaker amplifying device to improve the user experience.
DRC and limiting are traditional recording and broadcasting techniques used to raise the volume of subtle audio (such as human voice), in order to bring out those nuances, but at the same time automatically attenuate the dynamic peaks as not to overload the level. This technique is also used on the final mix in recording to increase the average level of the entire track without overloading during louder passages.
In addition, the use of compressors to boost perceived volume is used by broadcasters who want their station to 'jump out' at the same volume as comparable stations on the dial. This can also be useful for "in-car" listening to compensate for natural vehicle and road background noise.
In common to all the above applications, if the lower level amplitudes can be raised while the higher level peaks are attenuated; the overall volume can be increased without distorting and over loading the electrical level or the speakers on the louder dynamics of the audio. This article describes how these techniques can be applied and how they can aid in improving communication as well as entertainment in modern mobile devices.
Compression and limiting have been around for a long time. In 1959 Sherman Fairchild licensed a compressor/limiter designed by Rein Narma giving life to the legendary Fairchild 670 compressor/limiter.
The Fairchild 670 has found its way onto many classic recordings over the years including famous Beatles recordings at Abbey Road Studios in the 60's. The unit is still revered to this day, can go for upwards of $30,000 in good condition, and many software plug-ins have been designed to emulate its tone and function. This unit was all hand wired, had over 20 vacuum tubes, 14 transformers and weighed in at a hefty 65lbs.
Mr. Fairchild couldn't have known that over 50 years later, Fairchild Semiconductor would be integrating dynamic range compression/limiting, as well as 1-W Class-D amplifiers, and stereo Class-G headphone amplifiers on to a single silicon die roughly 2mm on a side.
Dynamic range compression and limiting can be useful in mobile devices with speaker amplifiers to raise the average level of the audio output (turn it up) while compressing or limiting the louder peaks of the signal to reduce distortion and speaker overload. A compressor is essentially an automatic gain control. Loud sounds above a certain threshold are automatically turned down while quiet signals are unaffected.
The dynamic range of a system is defined as the largest signal the system can handle linearly divided by the smallest signal it can process (or the noise floor lower limit). Therefore, the effect is technically compressing the dynamic range of the system because it is attenuating the higher peaks as compared to the lower level signals. However this is desirable in certain situations to raise the lower level signals.
Compression therefore has the potential to improve audibility in a mobile phone conversation. In addition, for entertainment such as audio tracks for video, it allows the smaller speakers to bring out the more subtle parts, but not overload on the louder dynamics for a better user experience.
A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. The amount of gain reduction is determined by a ratio; a ratio of 4:1 for example means that if the input level is 4 dB over the threshold, the output signal level will be reduced to 1 dB over the threshold. The gain (level) has been reduced by 3dB.
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kdboyce
7/26/2011 10:52 PM EDT
The audio quality problem with small speakers in portable devices is being attacked on several fronts, with DRC, AGC, and limiting being among the 'classical' ways to help the situation. Active multi-band EQ is showing up more often, and OEM's are tending to place more emphasis on acoustics design to help counter some of the speaker deficiencies. Even psycho-acoustics methods (tricking your ears and brain) are being tried with some measure of success.
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gdavis9999
7/27/2011 1:31 PM EDT
Boyce, good comment. Here at Fairchild, we believe that one of the key attributes of DRC (dynamic range compression) is to achieve higher SPL (sound pressure level) for louder sound. The advantage is you do not have to use the “brute force” method of driving more current into the speaker element to gain some added perceived volume. Although more power is ultimately needed to make a large change to achieve higher output volumes, we see DRC as a supplement to that, without the downsides of added current such as higher power dissipation and possible speaker damage.
We will issue a major audio announcement soon that includes our first audio ICs optimized for mobile and tablet applications. We also have multiple devices currently in design that will continue to provide value in “making small speakers sound louder and better™”. This includes psycho-acoustic approaches. Thanks again for your comments, they are always welcome.
Greg Davis
Senior Marketing Manager
Fairchild Mobile Audio Products
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kdboyce
7/28/2011 4:04 PM EDT
Greg, your point is correct. Thank you for further information on Fairchild's activities. I am sure the market will respond well to solutions that work, as they are sorely needed.
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Dr. HD
7/28/2011 10:23 AM EDT
I think it's important to be clear that "louder" absolutely does *not* always equal "better", but this mistaken belief in some quarters has lead to the "loudness war" - sound recordings have copious amounts of DRC & limiting applied to them, robbing them of all dynamics and making them sound worse (see this excellent youtube video detailing the problem: http://www.youtube.com/watch?v=3Gmex_4hreQ, further info can be found here: http://en.wikipedia.org/wiki/Loudness_war and here: http://turnmeup.org/). The problem of applying DRC & limiting during mastering is that there's no lossless way to get the dynamics back; Dolby and others have some systems that can analyse sound data in real-time and try to re-create the dynamics. Hopefully the irony of this process is not lost on the reader and they can appreciate that it would be better if the dynamics had not been removed from the music in the first place!
Having said all of this, there are indeed several scenarios where DRC & limiting can enhance the listening experience:
1.) If the background acoustic noise levels are high (in a moving vehicle for example) DRC & limiting ensures you can hear all aspects of a music track without deafening yourself and your passengers during the loud parts.
2.) For late-night movie watching it can be used to ensure you can hear all the dialogue without waking your neighbours when the next bout of gunfire, explosions etc. hit the screen.
3.) It can be helpful when playing back music on smaller, less efficient loudspeakers such as those found inside smartphones, tablets and laptops.
Clearly the best place for DRC & limiting is on music playback devices themselves and *not* at the time the music/soundtrack is mastered. In this way, DRC & limiting can be applied when it is useful, and deactivated at other times for maximum audio quality.
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kdboyce
7/28/2011 4:12 PM EDT
Dr. HD,
The paragraphs make your post, and points, easier to read/understand. No apologies needed for them :)
The points you make in your first post are on the mark. Obviously if you can control the playback system, then you may be able to optimize the sound - from the mastering process down to the ears. This is why you will often find things like THX, and Dolby certified, etc. labels on consumer audio.
But in general, especially in portable devices, you don't have much control over the final hardware. Even to the extent that you might, the poor quality of the speakers will screw you up, as well as the adverse effects that can come from poor or limited acoustic design possible in such devices.
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Dr. HD
7/28/2011 5:23 PM EDT
Sure, when you're mastering music you don't have control over the quality of the playback system. But surely you're not suggesting that music or movie soundtracks should be mastered to the lowest common denominator? Mastering audio by DRCing and limiting it to death so that it sounds passable on poor quality playback hardware or in noisy listening environments inevitably makes said audio sound worse on even semi-decent equipment.
As a designer of high-quality audio power amplifiers the loudness war really depresses me. What is the point of trying to build power amps with 120 dB signal:noise ratio and 0.0001% THD if the music's dynamic range is only 6 dB or even worse? I hope that other manufacturers will follow Fairchild's lead. Having DRC and limiting, that the user can turn on and off when necessary, in the playback hardware itself would make it easier for the recording industry to de-escalate from the loudness war and hopefully begin again to master music with decent dynamics.
P.S. Really not sure what happened with the paragraphs thing: when I first posted my original comment, it appeared as one contiguous block of text and that's what I was apologising for. But when I later reloaded the page (after making my second post apologising for the poor formatting of the first), the paragraphs had magically re-appeared. It would be nice if we could edit and/or delete our comments because my second one's making me look like a bit of dolt!
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kdboyce
7/29/2011 2:40 AM EDT
No, I am not suggesting music be mastered to the lowest common denominator. I am very much for improving the playback equipment.
What I meant was that many of the products on the market now, and currently being designed, are being limited (no pun intended) more by the quality of the speakers and the inability to do really good acoustic design. Aside from size issues, many OEM's are not willing to put in really good speaker based audio since to do so may 'screw up' their nice industrial thin and light design. So they rely on headphone connections and very good SNR in that path.
National Semi, Wolfson, and TI have audio subsystems targeted at the portable device market, many of which incorporate one or more of the methods in the article
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Dwain
10/27/2011 10:21 AM EDT
Louder usually comes in a distorted form, when audio has less distortion, for a given power, it is often perceived as quieter.
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