Shape of the waveforms
Proponents of analog recordings argue that it is superior to digital for the 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 sampled 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. Higher quality systems will probably sound better than cheap ones, regardless of type. One simple listening test is to play a vocal track; the 's' sounds will hiss on a CD player with a bad DAC and they will be barely audible on a record player with a bad pickup.
Similar claims have been made about the sound of
analog synthesizers compared with the sound of
digital synthesizers and about
analog video and
digital video.
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Quantization and very low signal levels
Some sustainers of analog sound claim that 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 statement is suspicious and could only be true for analog signals that are strong enough to be above the (unavoidable) mechanical, electrical and thermal noise level in the recording and playback cycle (mechanical transducers (microphones,
loudspeakers), amplifiers, recording equipement, mastering process, reproduction equipment, etc) .
It makes in fact little sense claiming that an analog signal can "use" all the available physical resolution of a medium and be accurately recorded, when that same
signal can be weaker, at low levels, than the sum of all external
noise,
interference, and unwanted signals that are recorded at the same time. This applies of course to both analog and digital systems.
Mathematically, this can be expressed by means of the
signal to noise ratio. On an 8-bit digital system, there are only 2^8= 256 possible signal amplitudes, of which there are 256 discrete amplitudes relative to the minimum signal level, which results in a dynamic range of just
48.165 dB, which is inferior to most
cassette tape systems, so in fact 8-bit recordings tend to sound noisy and scratchy, and miss low-level signals.
- Note that a decibel is one-tenth of a Bel. It is a somewhat strange concept that characterizes the logarithmic nature of human senses. Now to make it more complex, the amplitudes discussed in this article are voltage levels. To convert a voltage level ratio to a Bel, simply divide them and calculate the logarithm to base 10. Then multiply by 10 to get decibels. Unfortunately, Ohm's Law comes into play; the power of the sound is approximately the square of the voltage level. Summary: The human hearing range is around 120 dB. A digital recording has, at best, a range of 20 * log10 (2 ^ number of bits).
On the other hand, a system with a 16-bit quantization has a dynamic range of
96.33 dB, which is generally considered
Hi-Fi and way beyond the signal to noise ratio of most consumer audio systems, and it's difficult, in practice, to find an analog sound recording system that can offer a better sensitivity at a reasonable price and implementation complexity.
In practice, each additional quantization bit adds a notable
6 dB in signal to noise ratio, e.g.
144 dB for
24 bit quantization (24 x 6 = 144), which is however very rarely (if ever) achieved in practice, with
21-bit (
126 dB) and
20-bit (
120.4 dB) being more practical, see
DAC and
ADC for more details.
To make a comparison,
cassette tapes are generally below
70 or even
60 dB; FM broadcasts are more or less the same; an average
vinyl record, if in good condition, can sometimes surpass
85 or
90 dB and a properly mastered CD can approach or even exceed
90 dB.
For example, a 0.5 V
peak to peak input
line signal, quantized at 16-bits, would require an equivalent minimum input sensitivity of just 7.629 microvolts, or an equivalent 15.3
ppm sensitivity by part of the whole recording system and medium, which is only achievable with studio-grade equipment, perfectly crafted and preserved medium, and cannot be achieved during reproduction by the majority of consumer audio systems, at a physical-electrical level.
