Until recently, all problems in digital audio systems were blamed on either the analog/digital converters (ADCs) used in mastering or the digital/analog converters (DACs) needed for playback (footnote 1). As the performance of both ADCs and DACs improved, however, a previously unrecognized mechanism for distortion was unmasked: jitter. As we shall see, jitter—or, more correctly, word-clock jitter—can be a significant limitation in the technical and sonic performance of digital audio systems (footnote 2).
If anyone ever thinks to compile a list of the 100 seminal audio papers that should be found in every tech-aware audiophile's filing cabinet, Harry Olson's "Direct Radiator Loudspeaker Enclosures" deserves to feature in it. Originally presented at the second Audio Engineering Society Convention, in October 1950, it was published in Audio Engineering in 1951. In 1969—in a rare and certain acknowledgement of its classic status—the AES republished it in its Journal (footnote 1).
At the February 1991 Audio Engineering Society Convention in Paris, Audio Precision's Dr. Richard Cabot (see my interview in January 1991, Vol.14 No.1) proposed a new technique for measuring noise modulation in D/A converters (footnote 1). The method, based on psychoacoustic principles, attempts to predict the audible performance of D/A converters. Now that Stereophile has added digital-domain signal generation and analysis to our Audio Precision System One, we can employ Dr. Cabot's technique and see if there are any correlations with subjective performance.
The promise of "perfect sound forever," successfully foisted on an unwitting public by the Compact Disc's promoters, at first seemed to put an end to the audiophile's inexorable need to tweak a playback system's front end at the point of information retrieval. Several factors contributed to the demise of tweaking during the period when CD players began replacing turntables as the primary front-end signal source. First, the binary nature (ones and zeros) of digital audio would apparently preclude variations in playback sound quality due to imperfections in the recording medium. Second, if CD's sound was indeed "perfect," how could digital tweaking improve on perfection? Finally, CD players and discs presented an enigma to audiophiles accustomed to the more easily understood concept of a stylus wiggling in a phonograph groove. These conditions created a climate in which it was assumed that nothing in the optical and mechanical systems of a CD player could affect digital playback's musicality.
Looked at from one viewpoint, DVD-Audio and SACD appear to be exercises in sheer profligacy. In the case of DVD-A, why provide a maximum bandwidth almost five times what is conventionally taken to be the audible frequency range, and couple it to a dynamic-range capability far in excess of that achievable by the microphones used to record the sound? In the case of SACD, why provide a potential bandwidth in excess of 1.4MHz, only to fill more than 95% of it with quantization noise?
Jitter is not what digital sound quality induces in the listener; rather it is the instability in the clock signal that controls exactly when the analog waveform is sampled in the original A/D conversion, or when the digital word input into a DAC results in an analog voltage being produced at the chip's output. "So what?" is the response of digital advocates, "As long as a digital one is recognized as a one and a digital zero as a zero, then how can there be any difference in sound?" goes their argument, normally culminating in a fervently expressed "Bits is bits!"
The Compact Disc clearly hasn't read the script. At a time when, in the autumn of its commercial life, the format is supposed to be stepping aside to allow younger blood to succeed it, CD has instead in recent years enjoyed something of a revival in audiophile opinion. While SACD and DVD-Audio, rather strutting and fretting their hour upon the stage, are doing more plain fretting than anything, the best in CD sound quality has improved sufficiently for some to question whether we need the new media at all.
The fundamental object of the invention is to provide...the listener a realistic impression that the intelligence is being communicated to him over two acoustic paths in the same manner as he experiences in listening to everyday acoustic intercourse....—Blumlein, et al, British Patent #394,325, issued June 14, 1933
Charles Hansen said it best, in a recent e-mail: "People have been holding back from criticizing this technology because they weren't certain that some new discovery hadn't been made." Ayre Acoustics' main man was talking about "upsampling," whereby conventional "Red Book" CD data, sampled at 44.1kHz, are converted to a datastream with a higher sample rate. (Because of its association with DVD-Audio, 96kHz is often chosen as the new rate.)
In his very English way, Sony's then managing director for the UK, Tim Steele, was getting a touch, er, desperate. His oh-so-cultured voice rose a smidgen as he resorted to a direct selling of the benefits of what he was talking about. "Look, you're all sitting on riches," was his fundamental pitch. "You can sell music-lovers your entire back catalog all over again—at a higher price!"
Although Philips invented the Compact Disc, it was only when Sony got involved in the early 1980s that it was decided—at the prompting of conductor Herbert von Karajan, a close friend of Sony's then-president Akio Morita—that the CD should have a long enough playing time to fit Beethoven's Ninth Symphony on a single disc (footnote 1). Even if the conductor was using very slow tempos, and even given the minimum pit size and track pitch printable at the time, the 16-bit data and 44.1kHz sampling rate they settled on gave them a little margin.
Most people who now listen to tube amplifiers began with a transistor amp, and know from experience that a tube amp of a given measured power output sounds louder than its nominally identical transistorized equivalent. The unofficial consensus is that you need two to four times the transistor power to achieve the same loudness as you would using tubes. In other words, given the (subjectively) undistorted sound level a 25W (footnote 1) tube amplifier can provide, if you want the same loudness from solid-state technology you would have to replace it with at least a 50W transistor amp (footnote 2).
One of the challenges I faced in optimizing the performance of the Thiel CS7.2 loudspeakers that I reviewed in February 2000 was controlling and tuning their interaction with my listening room. Intuition, experience, trial and error—all came into play, as did several of the procedures and calculations covered by Jonathan Scull in his "Fine Tunes" column.
This series of articles is based on a paper presented at the 103rd Audio Engineering Society Convention, New York, September 1997. The preprint, "Loudspeakers: What Measurements Can Tell Us—And What They Can't Tell Us!," AES Preprint 4608, is available from the AES, 60 East 42nd Street, Room 2520, New York, NY 10165-0075. The AES internet site, www.aes.org, offers a secure transaction page for credit-card orders.