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发帖数: 4090 | 1 http://www.stereophile.com/features/368/index.html
By Robert Harley • Posted: Nov 1, 1993
Not that long ago, digital audio was considered perfect if all the bits
could be stored and retrieved without data errors. If the data coming off
the disc were the same as what went on the disc, how could there be a sound-
quality difference with the same digital/analog converter? This "bits is
bits" mentality scoffs at sonic differences between CD transports, digital
interfaces, and CD tweaks. Because none of these products or devices affects
the pattern of ones and zeros recovered from the disc, any differences must
be purely in the listener's imagination. After all, they argued, a copy of
a computer program runs just as well as the original.
As our knowledge of digital audio has become more sophisticated, however, we
've learned that the timing of those ones and zeros is of utmost importance.
It isn't enough to get the bits right; those bits have to be converted back
into music with the same timing reference as when the music was first
digitized. It turns out that timing errors in the picosecond (ps) range—the
time it takes light to travel inches—can audibly degrade digitally
reproduced music. These timing errors—called jitter—are only now beginning
to be understood (footnote 1).
Although I have a pretty good feel for how jitter in a digital processor can
degrade sound quality, what I don't begin to understand is why CD
transports sound so different. Some have a smooth treble, soft bass, and a
deep soundstage, while others are bright, have tight and extended bass, and
poor soundstaging. My auditioning of the C.E.C. TL 1 belt-drive transport (
reviewed in Vol.16 No.7) deepened the mystery: The TL 1 had the most
distinctive sonic signature of any transport I've heard, with an extremely
smooth treble, lushly liquid midrange, and a soft, somewhat sluggish bass.
The TL 1's presentation was in sharp contrast to the Mark Levinson No.31
transport's tight, punchy, highly detailed rendering. If jitter is the cause
of these sonic differences, why don't poor (high-jitter) transports all
have the same sonic signature? What mechanisms create such a broad palate of
sonic flavors?
There are two possible answers. The first is that, besides the bits and the
timing of those bits, sound quality is influenced by a third, unknown factor
. The second—and much more likely—answer is that the jitter's spectral
content affects certain sonic aspects differently. Jitter can be randomly
distributed in frequency (like white noise), or have most of its energy
concentrated at specific frequencies. The jitter's characteristics probably
determine each transport's sound. Is this the mechanism behind the different
sonic signatures of CD transports?
We may have taken the first step toward answering that question. Stereophile
has acquired a unique test instrument that measures jitter in a CD
transport's digital output. The analyzer takes in an S/PDIF or AES/EBU
signal from a transport and outputs the transport's jitter content. The
jitter can be looked at on an oscilloscope, measured with an RMS-reading
voltmeter, listened to through an amplifier and loudspeakers, analyzed with
FFT techniques, or plotted as a function of frequency with 1/3-octave
spectral analysis. The jitter test instrument, designed by UltraAnalog's Dr.
Rémy Fourré and described in his Stereophile article last month ("Jitter
and the Digital Interface," Vol.16 No.10, p.80), is a powerful tool for
revealing the different jitter performances of various CD transports (
footnote 2).
I used the analyzer to measure the jitter in a wide range of CD transports,
most of them previously reviewed in these pages. The Stereophile test bench
and surrounding area looked like "transport city," with more than a dozen
high-end models awaiting testing. Also on hand for measurement was a "jitter
-reduction" device, Audio Alchemy's Digital Transmission Interface (DTI).
Because Stereophile has already reported on the sound of many of these
products, we can look at the measurements and see if there's a correlation
between a transport's sound quality and its measured jitter.
I'll report on the test methods and results later in this article. First,
let's look at how a transport's jitter affects the sound quality of a
digital processor connected to it.
How transport jitter affects DAC sound quality
In "The Jitter Game" (Stereophile, January 1993, p.114), I explained how
jitter in a digital processor's word clock affects the processor's sound
quality. The word clock is the timing signal that controls when the digital-
to-analog converter (DAC) converts the digital audio samples into an analog
output. Timing errors in the clock produce voltage errors in the DAC's
analog output signal, degrading the processor's sonic and technical
performance.
That article focused on jitter in digital processors; at the time, we had no
way of measuring transport jitter. Since then, we've learned much more
about the relationship between word-clock jitter, the digital processor, and
the CD transport. It turns out that word-clock jitter in a digital
processor—the point where jitter becomes audible—is a result of many
variables, including the transport, the digital interface, and the digital
processor itself.
The transport's S/PDIF digital output drives the digital processor's input
receiver. The input receiver generates a new clock by locking to the
incoming clock in the S/PDIF datastream with a Phase-Locked Loop (PLL). This
so-called "recovered" clock then becomes the timing reference for the
digital processor. When your digital processor's "lock" or "44.1kHz" LED
illuminates, the processor has locked to the incoming clock signal. If this
recovered clock is jittered, the word clock at the DAC will also be jittered.
It is commonly believed that transport jitter is rejected by the input
receiver and not passed to the recovered clock. Unfortunately, that's true
only above a certain frequency, called the "jitter attenuation cutoff
frequency." Below this cutoff frequency, the input receiver and PLL simply
pass the incoming jitter to the recovered clock. The popular Crystal CS8412
chip has a jitter attenuation cutoff frequency of 25kHz, meaning that the
device is transparent to transport jitter below 25kHz. (This specification
is clearly stated in the CS8412's data sheet [downloadable as a PDF file---
Ed.].) The input receiver essentially acts as a low-pass filter to jitter.
Note that jitter energy with a frequency between DC and 40kHz produces
audible degradation.
A second source of word-clock jitter is the input receiver's intrinsic
jitter. Input receivers vary greatly in their intrinsic jitter, from 40
picoseconds in the UltraAnalog AES 20 input receiver, 200ps for the Crystal
CS8412, up to 5000ps (5ns) in the Yamaha YM3623 chip. (The Yamaha receiver's
jitter can be reduced with a few circuit tricks.)
We can quickly see that the sonically degrading word-clock jitter in a
digital processor is influenced by several variables:
1) the transport's jitter;
2) S/PDIF or AES/EBU interface-induced jitter (the digital interconnect);
3) how well the digital processor's input receiver rejects transport and
interface jitter;
4) the input receiver's intrinsic jitter; and
5) how well the clock is recovered and handled inside the digital processor. | x***4
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【在 Z*****o 的大作中提到】
: http://www.stereophile.com/features/368/index.html
: By Robert Harley • Posted: Nov 1, 1993
: Not that long ago, digital audio was considered perfect if all the bits
: could be stored and retrieved without data errors. If the data coming off
: the disc were the same as what went on the disc, how could there be a sound-
: quality difference with the same digital/analog converter? This "bits is
: bits" mentality scoffs at sonic differences between CD transports, digital
: interfaces, and CD tweaks. Because none of these products or devices affects
: the pattern of ones and zeros recovered from the disc, any differences must
: be purely in the listener's imagination. After all, they argued, a copy of
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