The history of digital is more complex than valve and solid state.
Digital refers to the software rather than hardware, and as a result most digital amps are actually solid state amps with digital software.
Digital technology has been used in other musical instruments for a long time, such as keyboards and synthesizers. In the realm of guitars, digital was used in the effects pedal market around the late 80s; here is where it gets confusing as digital is not really an amplifier at all but an effect. When digital was used in effects pedals, it was to create sonic effects such as a delayed sound, reverb, clipping of the signal, etc.
It was only until the mid 90s where digital technology became part of the amplifier market. Here digital signal processing (DSP) was the method used. DSP is the study of audio signals, and how they can be emulated by digital processors. As a result, digital ceased creating sonic effects, but began emulating sound (this has since been known as digital modeling).
One of the first digital modeling equipment was the Roland VG8, introduced around 1995. The VG8 was a unit (approx. 30cm x 12cm x 15 cm) which would capture the output of an electric guitar, analyze the signal’s wavelength, which is processed and reassembled into a new signal. The user had a number of options as to how the VG8 would process the guitar signal, all of which were emulations of particular pieces of guitar equipment, such as different amp sounds. The VG8 retailed for around £2000 and was definitely aimed at the professional market. It should be noted that the VG8 was not an amplifier, it is a processor which created a new signal which would then need to be amplified.
Digital modeling was not an instant success due to its rarity amongst the average guitar player. Those who could afford it felt that digital was astonishing, but its sound quality was not the same as the amps it was emulating. As the years went by, digital modeling would improve in all aspects.
Around 1999 digital modeling began to renew interests with the introduction of Line 6’s POD. the POD was a unit the size of two hands, and not only was it a DSP unit, it was designed to take the place of the preamp.
<Quick lesson: An amplifier consists of two circuit stages, the preamp and the poweramp. The preamp essentially takes a low level signal (the type produced by a electric guitar) and amplifies it to a level that can be handled by the poweramp. It is also the preamp stage where the majority of tonal shaping of the signal takes place. The poweramp stage takes the signal from the preamp, and literally powers it so that it can accepted by speakers thus producing a sound.>
The POD could be used in conjunction with a poweramp/speakers, or the player could bypass that stage by using headphones (to play alone), or plug straight into a mixing desk.
The POD provided more emulations that any other digital gear, and it also allowed more flexibility as to how the signals were produced (the user could match certain amps with certain speakers). The best characteristic about the POD was the quality of sound, if you asked the POD to emulate a vintage Marshall stack amp, it actually sounded like it!
The new incarnation of digital modeling posessed fantastic sound quality, portability and versatility. Best of all, it retailed for around £150 – affordable.
The fact is, digital modeling provided everyone a chance to play with the same equipment that professional used, and it also provided many practical advantages. However the POD was not perfect, and many felt it the sound it produced was cold and sterile in live situations, a problem which is being solved as we speak.
Today, digital modeling can be bought in individual forms like the POD (or the latest incarnation, the Pocket POD which is essentially the same except it will fit in your pocket), and it can also be bought in amplifier forms (where the digital processor is built into where the preamp would usually sit).
It is a piece of equipment used by the beginning guitarist, and the professional musician. All the major amplifier brands have realised it’s advantages and have released their own versions. Some have even developed hybrid amplifiers which combine digital with either solid state or valve or both.
Another notable form of digital modeling is in the form of software only. There are products where the user installs the software onto their computer, and the computer becomes the central medium.
Below: Revalver, a digital modelling software.
As someone who owned the original POD, I agreed with the sonic criticisms it received. However recently I bought a new version of the POD and all I can say is it has improved vastly – the good just got better!
It is my belief that digital would fulfil most players practical needs, but the popularity of valve is that it fulfils an emotional desire.