The debate of CD vs. vinyl is a popular religious issue amongst
music fans, audiophiles, DJs and techies. The arguments are usually
peppered with scientific factoids ranging from the pedantically
correct to the hilariously mistaken. Here I intend to explore the
technical issues in a balanced manner. Reasoned scientific feedback, debate and corrections are most welcome.
I shall try to avoid the quagmire of
subjective aesthetic judgement; having never owned any vinyl records
I
couldn't say which format sounds "better".
The Frequency Domain
Frequency is pitch - the difference between a low, deep sound and a
high piercing sound. As we hear sounds as a sum of frequencies, most of
the important information is in this domain, and accurate reproduction
is correspondingly important.
The sample rate of a CD is 44,100 samples per
second. According to the Nyquist theorem this is sufficient to
capture
all the information in a signal that contains no frequencies above
22,050 Hz.1 This bandwidth is more than enough. The human
ear is
most sensitive
at around 3,000 - 4,000 Hz. (The fundamental tones of a piano
range roughly 25 Hz to 4,200 Hz) The upper frequency limit for
human hearing is roughly 20,000 Hz, while for a middle aged male this
figure may be nearer 15,000 Hz. At both extremes of the audible
frequency range the perceived loudness relative to the signal power is
less than that for moderate frequencies.
The reason for this extra sampling overhead is actually to make the
CD player's job a little easier. When recreating an analogue signal
from digital data it is necessary to filter artefacts generated above
the Nyquist frequency (in this case 22,050 Hz). Analogue filters
with
sharp cut-off slopes are expensive, so a 2,050 Hz gap to ramp down is
quite helpful. Still, a poorly designed filter might unduly attenuate
the audible high end or have a "ringing" frequency response across the
full range.
Sound behaves linearly, which means that
frequencies above 20,000 Hz
cannot somehow affect, alter or "shape" the lower,
audible
sounds. Nevertheless, audiophiles often believe that these
high frequencies actually
do make a difference. The best scientific support I have seen for this
view is a
paper2 which suggests that brain activity may be somehow
affected by
the presence of hypersonic sounds.
Frequency domain issues with vinyl have an entirely different
nature. First
off, vinyl records are not recorded with a constant frequency
response.
To properly
reproduce low frequencies would require larger grooves, so records are
pressed with the low frequencies reduced. In addition, as a strategy to
drown out noise, higher frequencies are boosted on the
recording. A
RIAA equalisation amplifier mostly corrects the frequency response on
playback. Problems introduced by cheap turntables include "wow and
flutter", which refers to
frequency shifts caused by variation of playback speed and "rumble",
low frequency noise from the motor.
Dynamic Range
The dynamic range is the resolution of the recording - the ratio
of the loudest possible sound to the softest, expressed in the
logarithmic decibel (dB) scale.
For CDs, each sample has a 16-bit range (65536 values). This might
not seem to be a great deal but it actually translates to roughly 96 dB
dynamic range. I would argue that this should be sufficient for any
music, when we consider the range of human hearing.
By definition, decibel levels are always comparative - the
difference between the loudness of two sounds. When you see an absolute
figure quoted, ("A lawn mower measures 90 dB") this is in relation to a
defined zero point - 0 dB is defined as the softest sound audible to a
person with perfect hearing. This means that if you set the volume of
your CD player such that you can hear all the sound it contains, the
loudest point on the track will measure at least 96 dB on the absolute
scale. Listening at a higher volume than this for too long will
reduce
the need for a large dynamic range.3
The biggest general misconception about analogue technologies is that
they
have
infinite resolution, which is never true. (In this example, a
PVC
molecule is small, but not
infinitesimal). In practice, the dynamic
range of vinyl records is affected by the age and quality of the
recording, and the density of the tracks. The best-case seems to be
around 60 dB. The dynamic range will be adversely affected at low
frequencies because of the equalisation issue detailed above.
Errors and Noise
CDs are recorded with interleaved redundant
information built
in.
This allows the decoding machinery to correct or interpolate bad data
caused by damage to the CD surface. However, should the damage be too
great, the CD will simply skip.
Any wear to the surface of a vinyl record will cause a gradual
deterioration in the quality of the recorded signal. This manifests as
hiss and reduces the effective dynamic range. Scratches
and
other damage can result in pops and clicks that momentarily drown out
the recorded sound.
Conclusions
CDs technology is mostly an improvement on vinyl
technology.
Stop worrying and just enjoy the music.4
- Frequencies above the Nyquist point will cause interference if
not removed before sampling. This is one motivation for sampling at
higher rates in the studio.
- Oohashi, Tsutomu,
Emi Nishina, Manabu Honda, Yoshiharu Yonekura, Yoshitaka Fuwamoto,
Norie Kawai, Tadao Maekawa, Satoshi Nakamura, Hidenao Fukuyama, and
Hiroshi Shibasaki. Inaudible high-frequency sounds affect brain
activity: hypersonic effect. J Neurophysiol 83: 3548–3558, 2000.Also
available online at: http://jn.physiology.org/cgi/reprint/83/6/3548.pdf
- In UK workplaces hearing protection is mandatory for sound levels
above 90dB.
- I've never listened to so much music as I did when I was 17, using
a cheap cassette player. There are factors more important than
bidirectional copper cable.