Software: the future of audio

Last night, on a whim, I decided that I would like my active crossover software to display some sort of indication of the output levels being sent to the DACs. This is quite important, and something that I should have tackled quite a while ago. Basically, we should be worried about clipping, and also ‘overs’ i.e. those interpolated samples that are generated by DAC reconstruction filters in between the recorded samples and which have the potential to clip even though the recording does not, directly. By messing around with various types of driver correction and so on, am I running the risk of clipping? Or, am I wasting DAC resolution by needlessly attenuating my DAC outputs too much?

Here is how easy it was to display the information in a useful and aesthetically pleasing way:

  • I created six vertical rectangular areas on the active crossover app’s screen – one bargraph for each DAC output.
  • I decided upon a linear percentage display (not dB) and an update rate of 10 Hz
  • A timer was set to trigger at 10 Hz (the timer is provided by the GTK GUI library) and call the function to draw the six bargraphs
  • In the output function for the DACs, I take the absolute value of each sample as I write it to the DAC and compare it to the maximum recorded so far for that channel (out of six channels). I overwrite the maximum if it is exceeded. There is a ‘mutex’ interlock around the maximum value to prevent the bargraph drawing function from accessing it at the same moment.
  • The bargraph drawing function for each channel accesses that maximum recorded value and saves it. The maximum value for that channel is then reset to zero. The saved value is compared against that bargraph’s previous displayed value. If it is greater, a coloured rectangle is drawn directly proportional in length to the value. If it is less, the previous value is multiplied by 0.9, and the rectangle drawn to that height, instead. With this simple system, we have a PPM-style display that shows signal peaks that slowly decay.
  • The bargraph display function also records an absolute maximum for that channel, which doesn’t get reset. This value is displayed as a red horizontal line, thus showing the maximum output level for that particular listening session.

The result is one of those attractive arrays of VU meters that dances in response to the incoming signal levels. The results were interesting, and will alert me to any future mis-steps with regard to clipping – it still doesn’t tackle the issue of ‘overs’ directly, however.

But the reason for mentioning it, is to show the power and simplicity of engineering with software. To build a PPM meter in hardware and wire it all up, would not be trivial, and would take days, weeks or months for a commercial product. In software, it takes less than an hour and a half to construct it from scratch. Audio processing functions are equally simple to create and integrate within the system. It seems clear that once the basic DSP ‘engine’ is in place, complex audio systems can be put together like Lego. A perfectly capable three-way speaker can be built in days. It is not too hard to see how a three-way, six channel DSP system could simply be scaled up to create something like the Beolab 90.

Is this an exciting trend, or the end of everything that makes audio interesting? I think it is the former, but I can see that many traditionalists might disagree.

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6 thoughts on “Software: the future of audio

  1. A general thought on your site (not related to this post in particular).

    Just reading through your posts and I agree with a lot of what you say. I completely agree that digital is, in almost all cases, a superior way of reproducing music; and, in fact, about 75% of my time is spent listening to digital sources.

    However, I also enjoy analog, and damn I love glowing tubes. I haven’t fooled myself into thinking that my turntable and tube amp somehow sound “better” than digital, but the tactile experience of turning on a tube amp, waiting for it to warm up, cuing up a record, and watching it spin on my Michell Gyrodec is almost as enjoyable as the music itself. I also–believe it or not–like getting up every 20 minutes to flip a record.

    Completely irrational, I know! Particularly shameful since I am a scientist. However, we aren’t robots, and the ritual and aesthetics of analog, for some, influences was the music is experienced in a very positive way. I listen to a digital source on a high-quality, low-cost system much like yours all day, every day, but I look forward to coming home and listening to my records.

    I haven’t been through your entire site yet, but you might benefit from exploring the psychiatric literature on how experience modifies perception. Much of audiopilia, good and bad, rests on this phenomenon.

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  2. Actually just got through everything and I have a couple questions for you, if you don’t mind taking a moment:

    Given your feelings about coherence in multi-driver speakers, how do you feel about Fostex/similar systems with single drivers?

    Since I can’t build my own, do you have any recommendations for reasonably priced modern passive speakers that would mimic (more or less) your design?

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    1. Hi John. Many thanks for your comments.

      Re. single drivers, the problem is that they cannot reproduce all frequencies equally well. The only practical way to produce low frequencies is with a large diaphragm, but it is too slow and heavy to reproduce high frequencies. Even if you manage to make a very light speaker it will still ‘beam’ at high frequencies due to the finite size of the driver, and the different path lengths from the driver to points in space which causes partial cancellation. Even if you make a small speaker with the capability of extreme cone travel, it will produce large amounts of Doppler distortion. By assigning different frequencies to different sizes of drivers, all of these problems hopefully go away. The driver with concentric tweeter (such as KEF use) is a logical way of trying to achieve this and hopefully get close to a single driver’s coherence.

      I don’t know enough about passive speakers to know which other designs are going to be the most coherent. However, I think it is the case that sealed bass will be better then bass reflex. A sloping or stepped baffle indicates that the design may be time coherent. I never heard any, but would KEF 105 fit the bill? (I love the look of them). Of course it is possible to apply DSP to a passive speaker to improve its characteristics. Have you seen the Devialet SAM system? It is also possible to do this using a DEQX or MiniDSP, or one of several PC-based software packages. The main problem would be in designing the filters, which is where the Devialet SAM system comes in because of its database of correction filters for commercial speakers.

      I agree about the tactile nature and ‘ceremony’ of vinyl. However, I haven’t yet fully automated the starting up of my system, and there is some delay as it boots up, so even my system is not completely free of ceremony. Also, I think that any system with huge speakers has something impressive about it. ‘Digital’ often seems to go hand in hand with clever all-in-one compact boxes or satellite speakers with separate concealed subwoofers. I like the look of old fashioned systems.

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  3. Thanks for your reply. I certainly agree about the look of old-fashioned systems–I certainly do not want alien robots in my living room! Along with “simpler is better” I think audiophiles are prone to think that if something is actively ugly it must sound better.

    I’m putting together a system for another room in my house, and the specific speaker I was considering was Omega’s Outlaw Super 7XRS, which consists of a single 6″ Fostex-like ported full-range driver paired with a sealed 12″ active sub in the same cabinet; they provide a seal for the port if you choose to block it. Of course part of the appeal of this speaker is that it is quite attractive!.

    The Devialet system looks very interesting; however, the entry point appears to be US$6500! Although I’ve spent more than you on my current system, the total cost of speakers, turntable, and integrated amp is quite a bit less that.There’s also quite a bit of risk involved in buying something technology-based from a small company.

    Which brings up one last thought: While technology has the potential to improve the musical experience, all such products, by nature, are prone to obsolescence. How many people are still running the early, and highly expensive, room correction systems? How long until the $6500 to $30,000 Devialet systems essentially become expensive garbage? You (and to a much lesser extent, me) have the skills and knowledge to extract value from these systems for many years, even after Devialet goes out of business or stops supporting these products. What about the average consumer of high-end audio?

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  4. Software is indeed the future of audio. Even without too much know-how it is possible to implement a multi-way active system using off the shelf components.

    Personally, I use Foobar2000 with the crossover component, a $10 5.1 usb soundcard, two 3 way loudspeakers and two ‘roadkill’ amplifiers to implement a two way active system, to be upgraded to 3 way in the very near future. Foobar2000 plays just about every format from DSD to mp3.

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