Most professional engineers got their start with exclusively analog hardware in the studio. Today modern digital recording is considered to be significant step-up from what it was years ago. A lot of the early digital gear was riddled with problems, but new developments and technology spawned digital recording systems that can now deliver pretty much all that was once promised—clear transparent sound. However clear transparent sound can often deliver almost sterile sounding results that are not always what we are after. The technical confines and deficiencies of analog have become an essential part of the recordings that we all know and love. Some of the sounds resulting from overdriven analog equipment have become recognizable effects in their own right.
In short, enjoyment of sound isn't necessarily about precision and accuracy so much as it's about mood, character and subtle nuance that many modern digital recordings lack. With regards to audio, some aspects of analog technology introduce artifacts that are perceived by many as pleasant and more musical—and this is what lies at the heart of the concept of 'analog sound’. Anybody who has worked with analog will readily admit that it can be expensive a hassle to maintain. It’s no surprise then, that so many people turn to software and hardware tools that aim to reintroduce some ‘character’ into digital production chains.
ANALOG SOUNDThe term ‘analog’ encompasses a whole world of microphones, tape, valves, transformers and other electronics. There are several factors that make up 'analog warmth' and our ears almost certainly respond favorably to a combination of all of them. In order to familiarize ourselves with these factors, it is helpful to compare the effects of harmonic and non‑harmonic distortions (such as those caused by transformers and inductors), magnetic recording tape, (and the mechanical artifacts of the tape machine itself) and active circuitry, including valves or solid‑state devices.
We also have to consider frequency response and dynamics. Tape devices and many valve‑stages dating from the earlier days often had restricted high‑frequency performance and a more robust bottom end, for example, and they also tended to reduce the dynamics of signal transients, through thermal or magnetic saturation. Due to the direct influence early analog equipment had on audio, it also influenced many recording and mixing decisions, such as high‑frequency EQ boosts being applied with the knowledge that the top‑end emphasis and transients would be smoothed by the recording chain.
TAPE MACHINESMost audio enthusiasts are probably aware to some extent of the sonic influence of magnetic tape, but fewer will have considered the influence of the tape machine itself. The biggest problem for any mechanical tape transport system is speed control, and the imperfect nature of this control produces artifacts that are referred to as 'wow and flutter'. There are actually four distinct variants of this, namely 'drift' (which shows up below 0.1Hz), 'wow' (0.1‑10Hz), 'flutter' (10‑100Hz), and 'scrape flutter' (in the 1‑5kHz region). To explain the last term, which you may not have heard, as tape is dragged across the tape heads under tension, its movement sets up a vibrational resonance along the length of unsupported tape between the heads and/or the preceding rollers or guides — just like a violin string being excited with a bow. This resonance will tend to cause the tape to vibrate against the head, effectively causing it to move in a series of short, rapid jerks, which we call scrape flutter.
Fifty years of tape‑machine design evolution reduced wow and flutter to extremely low levels by the 1980s, but it couldn't be removed completely, and even the mighty Studer A820 two‑track machine's specifications quoted a wow and flutter figure of 0.04 percent when the tape was running at 15ips (inches per second). This is a tiny amount, certainly, but word‑clock stability — which is the equivalent of wow and flutter in modern digital systems — can't even be measured, as digital systems are orders of magnitude more stable in the time domain. The minute cyclical speed fluctuations of flutter, and particularly scrape‑flutter, create subtle 'side‑bands' (see 'Technical Terms' box) and noise modulation around the recorded audio. These add a perceptible low‑level 'grunge' to the sound, and while better-designed and maintained tape machines suffered lower levels of this grunge, it was always there to some extent. Although technically flutter is a fault, many argue that its side‑bands and noise‑modulation effects are an intrinsic part of the sound character of all analogue tape recordings, and that we've come to accept (and expect) them as part of recorded sound — and as part of what we call analogue warmth. These once‑common recording attributes are absent from all digital recording chains, and I don't know of any software plug‑ins that claim to recreate the specific grunge effects of wow and flutter.
Analog tape is inherently 'non‑linear', with its effect being determined by a combination of tape formulation, record and replay head construction, tape speed, tape width, record and playback equalizations and the level and waveform of high‑frequency bias. These parameters introduce distortions of harmonic content, particularly at low frequencies, and frequency and phase‑response irregularities, and they reduce dynamic range, mainly affecting high‑frequency transients through magnetic saturation and 'self‑erasure' effects.
TRANSFORMERSTransformers have been an integral part of analog audio signal paths right from the earliest days, frequently being employed at every input and output, as well as between amplifying stages, in many cases. Finding 10 or more transformers in the signal path wasn’t uncommon fifty years ago. Harmonic distortion in transformers is caused by hysteresis, for low‑level signals and saturation for high‑level signals. The effect is always greatest for low frequencies and results mainly in third‑harmonic distortion. Different factors in the design of a transformer affect the level of audio distortion, but key amongst these is the material used for the magnetic core. Core metals with a high nickel content typically result in the least hysteresis distortion—but tend to be relatively expensive—while simpler, soft‑steel cores tend to give far higher distortion figures but are much more affordable. The factors associated with transformer distortion and character are many and varied.
ACTIVE GAIN STAGESActive gain stages usually introduce a degree of distortion as part of the business of raising signal levels, but the nature of that distortion varies considerably, depending on the circuit topology, the kind of active devices employed, and even the nature of their power supplies. One such device is a valve, or tube, and while most people associate valve amplifiers with the concept of a ‘warmth’, it's entirely possible to design solid‑state circuitry using discrete transistors or integrated circuits that can sound just as 'warm', if setup properly. Neve mixing consoles, for example, don’t have any valves so don't assume that if it doesn't have valves it won't sound good.
Similarly, solid‑state devices are used in different circuit topologies that tend to dominate the kind of harmonic distortion they produce. Circuit topology is far more influential over the amount and character of distortion than the type of active device actually being used. In class‑A topologies, for example, the level of distortion components falls with reducing signal level, whether valves or transistors are being used. Quiet signals have little distortion, while louder signals suffer more distortion. However, in class‑AB circuits, the amount of distortion remains more or less constant, regardless of signal level, so the distortion components become far more audible as signal level falls. On top of this, in class‑B topologies that exhibit 'crossover distortion', the distortion productions which may be not musically related to the source signals at all and can lead to a very unpleasant sound.
Power‑supply design can also be a big factor in overall sound quality. It's not unusual, for example, for vintage valve power amps to suffer from saggy power supplies that dip when handling loud signals, resulting in a kind of dynamic compression or even a dynamic modulation effect. But the problem isn't limited to valve amps, and is potentially worse for solid‑state designs, because they tend to operate with lower supply voltages and higher supply currents, while valve amps require the reverse.