Since the Compact Disk (CD) was introduced 15 years ago, digital audio has transformed
the way we listen to music. Compared with traditional analog sources such as LPs and
tapes, radical advances were made in convenience and durability. In addition, the fact
that a digitized audio signal can be copied perfectly at low cost has been a great boon to
the recording industry.
Unfortunately, these benefits have come at cost (sometimes subtle and sometimes
dramatic) in audible performance, due to limitations of the standardized format as well as
its implementation. For this reason, audiophiles, music lovers, and recording engineers
have been eagerly awaiting a high resolution digital audio format virtually from the time
that the CD was announced.
Analog and Digital — How They Work
When we listen to music, we are experiencing pressure changes (sound waves) in the air
around us. Edison’s magnificent achievement was to capture and store a representation
of these pressure changes. Combined with a method of duplication, musical performances
could now be widely distributed for unlimited playback.
Edison captured the continuously varying sound waves as a continuous variation in the
groove on a wax cylinder. This is referred to as “analog recording” because the
variations in the groove are analogous to the variations in the air pressure. Since its
invention in 1877, analog recording was the only practical recording method for a full
century.
In the late 1970s, advances in semiconductor technology made it practical for the first
time to record audio digitally. With digital recording, the continuously varying sound
waves are approximated as a series of numbers and then stored as a string of
“ones” and “zeroes”. It is this storage of only two states (one and
zero) that gives digital audio its advantages of robustness and ease of duplication. Since
we cannot listen to a string of ones and zeroes, the digital signal must be converted back
to analog during playback.
Digital — Perfect Sound Forever?
The performance limitations of digital recording arise when the original analog signal
is converted to digital and again when the digital signal is converted back to analog. The
primary issue is the question of resolution when approximating the analog signal as a
series of numbers. That is, how close must the approximation be to avoid audible
degradation?
When the Compact Disk format was developed in the early 1980s, there were practical
limitations as to how high the resolution could be. The standard chosen was a 44.1 kHz
sampling rate with 16 bits of resolution. That is, the value of the analog signal is
measured 44,100 times per second and then rounded to the closest of 65,536 (2^16)
different possible values.
The sampling rate determines a maximum upper frequency response of roughly 20 kHz,
while the resolution level sets the maximum dynamic range at 96 dB. This was adjudged
adequate for CD’s intended purpose, which was a convenient consumer-grade playback
format. Although never intended for extremely high levels of sound reproduction, the
advantages of digital recording made it the de facto standard of both quality-conscious
consumers and professional recording engineers alike within 10 years of its introduction.
DVD - A Breakthrough in Digital Audio
Although people have been aware of the potential for improved sound available from
higher sampling rates and greater resolution, technical limitation have made it
impractical to implement a new standard until now. The arrival of DVD — the Digital
Versatile Disk — has dramatically transformed the possibility for improved digital
sound.
A DVD offers over seven times the storage capacity of a CD. This additional capacity
allows for a radical improvement in the resolution of the audio signal. The DVD
specification allows for many different resolution levels, but the dominant standard for
high quality audio samples at 96 kHz to a resolution of 24 bits. This translates to
16,777,216 (2^24) different possible levels. Combined with a more than doubled sampling
frequency, DVD audio offers over 500 times the resolution available from CD!
The improvement must be heard to be appreciated. The extended upper frequency response
bypasses the previous difficulties found on CD. Massed violins sound correct for the first
time on digital playback, with the individual instruments easily differentiated and a
complete lack of harshness. Cymbals and other percussion instruments are now reproduced
with a finesse that allows the craftsmanship of the instrument makers to be fully
comprehended. The increased resolution at low signal levels fully captures the fine
details of the recording, such as the ambiance of the recording venue. The dynamic range
is truly spectacular, allowing a complete freedom from compression for the first time.
Conclusion
The new high-resolution digital audio formats that are made possible by the DVD
represent a breakthrough in music reproduction. Not only does this new medium achieve new
heights in sonic fidelity from program sources, but will also provide a firm foundation
for improving the performance of all the other components in the playback chain.
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