Analog sound vs. digital sound
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Audiophiles and sound engineers have been debating the respective merits of analog and digital sound reproduction ever since the appearance of digital sound recordings. This is one of the enduring controversies in the field, both in professional and consumer audio, much like that pitting partisans of vacuum tube amplifiers against those of solid state (transistor) electronics. In consumer audio, the opposition is usually between vinyl LP recordings and compact discs.
An analog recording is one where the original sound signal is modulated onto another physical signal carried on some media or substrate such as the groove of a gramophone disc or the magnetic field of a magnetic tape. A physical quantity in the medium (e.g., the intensity of the magnetic field) is directly related to the physical properties of the sound (e.g, the amplitude, phase and possibly direction of the sound wave.) The reproduction of the sound will in part reflect the nature of the substrate and any imperfections on its surface.
A digital recording, on the other hand is produced by first encoding the physical properties of the original sound as digital information which can then be decoded for reproduction. While it is subject to noise and imperfections in capturing the original sound, as long as the individual bits can be recovered, the nature of the physical medium is immaterial in recovery of the encoded information. 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).
Sometimes, analog proponents are not so much against digital technology per se, but rather one implementation in the Red Book audio CD standard, as opposed to newer, higher resolution digital formats like SACD or DVD-Audio. Conversely, some digital criticisms of analog techniques refer to specific implementations like gramophone records or cassette tape that may have poor signal to noise ratios compared to digital formats, but analog modulation techniques like frequency modulation used in professional analog audio will typically offer much better SNR than consumer grade equipment.
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Arguments made in favor of analog sound
- Shape of the waveforms: sound reconstructed from digital signals is claimed to be harsher and unnatural compared to analog signals
- Lower distortion for low signal levels
- Absence of quantization noise
- Absence of aliasing
- Not subject to jitter
- Euphonic characteristics
Arguments made in favor of digital sound
- Lower noise floor
- Dynamic range
- Signal to noise ratio
- Absence of generation loss
- Resistance to media deterioration
- Immunity to wow and flutter
- Ability to apply redundancy like error-correcting codes, to prevent data loss
Shape of the waveforms
Proponents of analog 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. 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 film and digital video.
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 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.
Problems with early digital recording technology
Many of the criticisms levied against digital sound reproduction stem from the early days of the technology, where limitations in real-world implementations meant actual digital recording and playback sometimes fell short of the theoretical performance possible, but the situation has improved since. Analog sound reproduction was already a mature technology when digital recording and compact discs first appeared. First-generation digital recording and reproduction equipment suffered from the inevitable teething troubles of a relatively immature technology. Audio professionals also needed some time to build a body of knowledge, as analog techniques could not always be directly transposed to the new digital medium.
Much progress has been made since. Progress in electronics and economies of scale from mass production of CD platers led to improvements in Digital to analog converter technology. Professional digital recording equipment correspondingly improved.
Was it ever entirely analog or digital?
Complicating the discussion is that recording professionals often mix and match analog and digital techniques in the process of producing a recording. Analog signals can be subjected to digital signal processing or effects, and inversely digital signals are converted back to analog in equipment that can include analog steps such as vacuum tube amplification.
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. In case of video recordings it is moot for one other reason; whether the format is analog or digital, digital signal processing is likely to have been used in some stages of its life, such as digital timebase correction on playback.