Analog vs Digital Synthesizers
- Andrew Southworth
- May 24, 2018
- 5 min read
Synthesizers can generally be broken down into two categories: analog and digital. There are many purists that will say digital synths can never compete with their analog counterparts, and analog synths have a certain 'warmth' or 'organic' sound to them. Why do they say this? In this blog i'll try to explain the differences between the two so you can decide for yourself. First we need to talk about how each of these synths typically work on a high level. **Note: this applies for both standard synths and modular synths (i.e. Eurorack).
The Common Synth Structure
Most 'east coast style' synthesizers, both analog and digital, will follow a structure shown in the block diagram below. One or more oscillators will create a waveform triggered by a keyboard, this waveform will be fed through a filter, through an amplifier, and then to the output.
In the case of the analog synth the oscillator is referred to as a VCO (voltage controlled oscillator), the filter is referred to as a VCF (voltage controlled filter) and the amplifier as a VCA (voltage controlled amplifier). In the case of a digital synth everything is processed using DSP (digital signal processing). These differences are further described below.
Analog Synth Details
In an analog synthesizer, the signal is created and processed completely using voltages traveling through physical electronic devices. The voltage from a keyboard will cause the VCO to oscillate with a particular waveform determined by the device used, and the frequency of the waveform is proportional to the voltage applied to the device (1V, 1.083V, 3V, 4.166V etc). The output of the VCO is an actual voltage waveform traveling through the circuit to the next device in the signal path. The image below shows some common waveforms that may be output from a VCO.
Without getting into the details of how each component works (VCO, VCF, VCA, etc.), it is important to note that for each device in the signal path the actual voltage in the circuit is being controlled by the electrical components. The VCF circuit is designed such that higher or lower frequency components of the voltage signal can be filtered out using either a low pass filter or a high pass filter.
Digital Synth Details
Digital Synthesizers typically aim to accomplish the exact same goals as an analog synth, except the processing is all done digitally and then outputted with a DAC (digital to analog converter), which converts the signal in the digital domain to the analog domain. For example if we consider a synthesizer that is only comprised of an oscillator (no filtering, amplification, or effects), it is easy to visualize what a DAC is doing. Instead of the keyboard sending a discrete voltage value to the oscillator, a particular switch or series of switches are 'turned on' by the keyboard (simplifying this greatly for clarity) and that switch tells the oscillator to output a certain waveform at a certain frequency. However since digital components can only create constant voltages for discrete amounts of time, the digital oscillator will create what resembles a set of stairs to approximate the waveform desired. This is opposed to a VCO which is capable of generating a completely smooth waveform with no stair stepping. The image below shows the differences between a digital oscillator and a VCO (the digital one being the one that resembles a staircase).
The image above is exaggerated for visualization purposes, but it gets the point across. Typically a good synth DAC will be able to generate a signal that has 2^24 (16,777,216) unique voltage levels at a speed of at least 44,100 samples per second (for a 24 bit depth and 44.1kHz sampling rate device). The bit depth determines how small the height of the 'stairs' can be, and the sampling rate determines how fast over time each 'stair' will occur. As these numbers increase the approximation of the waveform will become more and more accurate. The two images below will help you visualize bit depth and sample rate.
So far we've just talked about the oscillator for the digital synthesizer, but these details about bit depth and sample rate apply to the entire signal path. Typically a digital synthesizer will actually do all of its processing before it gets outputted to an 'analog' signal using the DAC. When the oscillator is told to create a certain waveform it will not generate that waveform in reality, instead it will likely be generating it in software. A particular numerical value will represent the voltage of the waveform at a particular time step, and each time step will be processed through various algorithms to filter the sound, amplify the sound, or modify the sound in any way. The entire signal path from hitting the keyboard will be created in software until it is outputted in its final form using the DAC. This process is known as DSP (again, digital signal processing), and it is grossly simplified here but at a very high level that is how it works.
The Case for Analog Synths
One of the major arguments for why analog synths are better, is that they are imperfect. The voltages traveling through each device will be effected by the tolerance of the components and the traces on the circuit board, the calibration of the equipment and the environment the synthesizer is in will effect the waveform created, the waveform will be much smoother than a digital approximation of that waveform, and the waveform may actually have other imperfections such as 'ringing' or even artifacts that are frequency dependent. Every analog synth will have a unique fingerprint associated with it, even if its not directly noticeable by ear.
These artifacts make the synth sound more 'real', more 'organic', or 'warmer'. Digital synthesizers are just recreating what analog synthesizers do, and the recreation can never truly be the same due to how DAC's work. Digital synthesizers will sound the same every time you turn them on, and my digital synth will sound just like your digital synth.
The Case for Digital Synths
One major argument for digital synths is that you can recreate all of these desired imperfections in DSP (the Roland Boutique analog series aims to do exactly this, using their Analog Circuit Behavior technology). Most of the artifacts such as waveform ringing, frequency dependent factors, and a slight amount of random-ness can be created in software using DSP. The digital synthesizer may be approximating the output of an analog waveform, but the approximation is happening 44,100 times per second with 2^24 (16,777,216) unique voltage values. Theoretically your ear isn't even capable of perceiving changes this small (due to the Nyquist sampling theorem).
Why should you pay drastically more for an analog synth when your ear theoretically can't perceive the differences if the DSP is done precisely enough? Analog synthesizers won't last as long, and will require servicing to keep them working properly. Digital synths can take advantage of DSP to calculate infinitely diverse sounds with features such as FM (frequency modulation) synthesis, reverb algorithms, delay algorithms, mixing waveforms with digital samples, and much more only using software. The only limit is processing speed and that gets better and cheaper every year.
Conclusion
Overall I hope you have learned that there are great arguments for both analog and digital synthesizers. You shouldn't let purist attitudes get in the way of what instrument you buy, instead you should use your ear. I personally have both analog and digital synthesizers, and I use each one where it makes the most sense. A quality digital synth can easily sound miles better than a crappy analog synth, and most people can't tell the difference between a good analog synth and a good digital synth recreating that analog synth.
Resources:
http://web.mit.edu/klund/www/weblatex/node2.html
https://blog.reigndesign.com/blog/from-bytes-to-beats-digital-to-analog-conversion-using-as3-arduino/
https://documentation.apple.com/en/soundtrackpro/usermanual/index.html#chapter=B%26section=2%26tasks=true
https://ledgernote.com/reviews/best-audio-interface/
Komentar