"What Happened to the Negative Frequencies?"
Nothing happened to them, of course, as I will show, but you mustn't forget that they are always there.
Everyone in this room will be familiar with the acronym "FFT." The Fast Fourier Transform is both elegant and ubiquitous. It allows us to move with ease between time-based and frequency-based views of audio events. You will all be familiar with the following example. Here is the waveform of a short section of a piece of music:
And here is the spectrum of that music:
This usefulness of the FFT algorithm—…
Is There Something There?
Over the almost 35 years during which I have taken part in or organized listening tests, I have become convinced that what is fundamentally important is to respect the listeners—to listen to what they tell me. Yes, there may be a trivial explanation for what they hear. But there may be something there. When I first heard of so-called LP "demagnetization"—where an LP sounds better after being subjected to the action of, for example, a bulk tape eraser—I was skeptical. But I didn't dismiss the reports; I just filed them away for further investigation, if and when…
Nothing Is Real
It is a common put-down of audiophiles: "You're imagining things." But is this a meaningful criticism? Is there a real difference between "reality" and "illusion"? Or was Professor Dumbledore on to something?
I have been interested in human perception almost as long as I have been working in magazines. This sound is something with which everyone in this room will be familiar: a 1kHz tone at –20dBFS.
[Play 1kHz, –20dBFS sinewave tone]
What I'd like you to do now is to imagine the same tone for 10 seconds.
I believe a scan of your brain…
Measuring Sound Quality
Table 1: What is heard vs what is measurable
This is a table I prepared for my 1997 AES paper on measuring loudspeakers. On the right are the typical measurements I perform in my reviews; on the left are the areas of subjective judgment. It is immediately obvious that there is no direct mapping between any specific measurement and what we perceive. Not one of the parameters in the second column appears to bear any direct correlation with one of the subjective attributes in the first column. If, for example, an engineer needs to measure a loudspeaker's…
Case Study 1: Recording
Back in 1987, the AES published the anthology pictured above of historic papers on "Stereo." It includes a document (celebrating its 80th anniversary this year) that pretty much defined the whole field of stereo reproduction, including the 45°/45° stereo groove and the moving-magnet stereo cartridge. That document, a 1931 British Patent Application written by the English engineer Alan Dower Blumlein, is worth quoting at length:
"The fundamental object of the invention is to provide a sound recording, reproducing and/or transmission system whereby there…
Case Study 2: Loudspeakers
During a visit to Canada's National Research Council many years ago, I noticed, stuck to the wall of the prototype IEC listening room, a page of results from one of Floyd Toole's seminal papers on the blind testing of loudspeakers. The scoring system was the one that Floyd developed and that I subsequently used for blind tests at Stereophile: "0" represents the worst sound that could possibly exist, "10" the perfection of live sound. On this scale, a telephone, for example, rates a "2." The speakers in Floyd's test pretty much covered the range of possible…
Case Study 3: Digital Recording & Playback
The title of this lecture asks "Where did the negative frequencies go?" Once we enter the world of digital audio, they are very much present. Here is the spectrum of the music waveform I showed earlier:
And this is the spectrum of the same signal after it has been sampled in the time domain:
The positive (red) and negative (blue) spectra are mirrored around the sampling frequency and all of its harmonics, the latter extending to, if not infinity, then to something practically close to it. If you wish to play back…
Case Study 4: Amplifiers
To many audio engineers, the amplifier is a solved problem. Static distortion and noise levels can be restricted to well below the threshold of human hearing at all audible frequencies and at all power levels short of clipping. Yet the darned things continue to surprise by sounding different—perhaps only slightly different, and sometimes for trivial reasons, such as too high an output impedance. But over the years I have been measuring amplifiers, some things have fallen out of the cloud of measured data: factors that are shared by amplifiers that sell well to…
Summing Up
First, I would like to thank the Audio Engineering Society, not only for inviting me to give this lecture but also for making a wealth of invaluable information on audio available on its website. Second my thanks to DRA Labs, Audio Precision, and Miller Audio Research, for allowing me to make use of their measurement tools—absolute accuracy and repeatability are the twin goals of those who measure components, and these companies' tools have been a major help over the years; Larry Archibald, for more than a quarter century ago making me an offer to move to the US that I couldn…
John Atkinson reviewed the Musical Fidelity M1DAC in January 2013 (Vol.36 No.1):
Sam Tellig enthusiastically reviewed this inexpensive solid-state D/A processor in March 2011. "What I mainly heard from the M1DAC was nothing: an absence of artifacts, if you want to get fancy," he wrote. "There was no fudging of detail, no smearing of transients. Purity of tone was exceptional."
Some months after that review was published, the M1DAC was updated. Whereas the original version's USB input was limited to 48kHz sample rates and below and 16-bit data, the new version uses the popular…
