Analog sound vs. digital sound
|
|
Since the first publication of digital sound recordings listeners have disagreed over the respective sound quality of analog and digital sound.
Briefly, an analog recording is a mechanical representation of the original sound encoded on a substrate such as the groove of a gramophone disc or the magnetic field of a magnetic tape. The reproduction of the sound will reflect the nature of the substrate and any imperfections on its surface.
A digital recording, on the other hand is produced by encoding the original sound as digital information which can then be decoded for reproduction. It does not include the surface sound of the disc or tape, although it is subject to noise and imperfections in capturing the original sound. A damaged digital medium, such as a scratched compact disc may also yield degraded reproduction of the original sound, due to the loss of some digital information in the damaged area (but not due directly to the physical damage of the disc).
Specifically, proponents of analogue recordings argue that it is superior to digital for the simple reason that digital recordings are an approximation of a waveform. That is, a sampling rate and resolution must be taken into account. For example, in a CD, digital sound is encoded as 44.1 kHz, 16 bit audio. This means that the original wave is 'sliced' 44,100 times a second - and an average amplitude level is applied to each sample. The variety of different amplitude values available is dependent on the resolution. 16 bit means that a total of 65,536 different values can be assigned, or quantized to each sample. Therefore, the higher the sample rate and resolution, the higher the quality of the audio, because a wave closer to that of the original audio can be stored. For comparison, DAT can store audio at up to 48 kHz, whilst DVD Audio can be 96 or 192 kHz and up to 24 bits resolution. This affords a significant increase in sound quality. The Nyquist-Shannon sampling theorem showed that a sampled signal can be reproduced exactly, as long as it is sampled at a frequency greater than twice the bandwidth of the signal. Quantization, however, is not included in this theorem, and adds quantization noise, decreasing in level as the bit resolution increases. Also, in some cheaper systems, aliasing can become a problem, though this can be remedied by using steeper filters and oversampling.
Many people claim that the analog sound is "truer" because it is not reconstructed. They claim that digital sound simply does not sound as natural to them. Others claim that digital is more natural because it is not subject to the same imperfections and non-linear distortion as an analog medium. And some suggest that analog is technically of lower quality than digital but sounds subjectively better. For the general listener, however, there appears at present to be no simple way of demonstrating or proving the difference in fidelity.
Similar claims have been made about the sound of analog synthesizers compared with the sound of digital synthesizers.
Quantization and very low signal levels
A real difference between analog and digital sound occurs at very low sound levels. For analog sound, there is no hard "floor" (lowest sound level) beneath which recording is not possible. Instead, the desired signal simply slips farther and farther into the noise floor as its amplitude is reduced. This is not true for digital sound. As the amplitude of a digitally-recorded sound is reduced, it occupies fewer and fewer bits and the quantization errors (quantization noise) become a larger and larger portion of the reproduced sound. At some point, the digital sound is represented simply by the flipping of the least significant bit and, if the amplitude is reduced further, by no bits at all; the reproduced sound simply does not exist at all.
More bits (for example, 24 versus 16) allow the arbitrary lowering of this floor but they cannot eliminate it. The deliberate introduction of noise (called noise shaping) can mitigate this effect, however, and restore behavior that is much more like analog recording.
Was it ever digital?
For modern recordings, the controversy between analog recording and digital recording is becoming moot. No matter what format the user uses, the recording probably was digital at several stages in its life.