Behringer T1952 - Manual (Page 26)

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TUBE COMPOSER T1952
level of noise, hum or other ambient background hiss, which can disturb the quality of the program material.
Normally these noises are inaudible if the level of the desired signal lies significantly above the level of the
noise. This perception by the ear is based on the “masking” effect: noise will be masked and thus becomes
inaudible as soon as considerably louder sound signals in the same frequency band are added. Nevertheless,
the further the level that the desired signal decreases, the more the noise floor becomes a disturbing factor.
Expanders or noise gates offer a solution for this problem: these devices attenuate signals when their
amplitudes drop, thereby fading out the background noise. Relying on this method, gain controlling amplifiers,
like expanders, can extend the dynamic range of a signal and are therefore the opposite of a compressor.
In practice, it is shown that an expansion over the entire dynamic range is not desired. With an expansion ratio
of 5:1 and a processed dynamic range of 30 dB, an output dynamic range of 150 dB will be the result,
exceeding all subsequent signal processors, as well as human hearing. Therefore, the amplitude control is
restricted to signals whose levels are below a certain threshold. Signals above this threshold pass through the
unit unchanged. Due to the continuous attenuation of the signals below this threshold, this kind of expansion
is termed “downward” expansion.
The noise gate is the simplest form of an expander: in contrast to the expander, which continuously attenuates
a signal below the threshold, the noise gate cuts off the signal abruptly. In most applications this method is not
very useful, since the on/off transition is too drastic. The onset of a simple gate function appears very obvious
and unnatural.
4.2 The tubes used in the TUBE COMPOSER
A closer look at developments and trends in audio technology shows that tubes are currently enjoying a
renaissance, in a time when even amateur musicians are free to use digital effects processors and recording
media, and ever more affordable digital mixing consoles are becoming a natural part of the equipment of many
semiprofessional studios. The manufacturers try with ever new algorithms to get the most out of DSP’s (Digital
Signal Processors), the heart of any digital system.
Still, many audio engineers, particularly old hands often prefer using both old and new tube-equipped devices.
As they want to use their warm sound character for their productions, they are ready to accept that these
“goodies” produce a higher noise floor than modern, transistor-based devices. As a consequence, you can find
a variety of tube-based microphones, equalizers, pre-amps and compressors in today’s recording and
mastering environments. The combination of semiconductor and tube technologies gives you the additional
possibility of using the best of both worlds, while being able to make up for their specific drawbacks.
4.3 Tube history
Due to many patent litigations, it is difficult to determine exactly when the tube was “born”. First developments
in tube technology were reported between 1904 and 1906. It was a research task of that time to find a suitable
method for receiving and rectifying high frequencies. On April 12, 1905, a certain Mr. Fleming was granted a
patent for his “hot-cathode valve” which was based on Edison’s incandescent lamp. This valve was used as a
rectifier for high-frequency signals. Robert van Lieben was the first to discover (probably by chance) that the
anode current can be controlled by means of a perforated metal plate (grid), one of the milestones in the
development of amplification tubes. In 1912, Robert van Lieben finally developed the first tube for the
amplification of low-frequency signals. Initially, the biggest problem was to produce sufficient volume levels,
which is why resonance step-ups (though impairing the frequency response) were used to maximize the
attainable volume. Later, the objective was to optimize the electroacoustic transducers of amplifiers in such a
way that a broad frequency band could be transmitted with the least distortion possible. However, a tube-
specific problem is its non-linear amplification curve, i.e. it modifies the sound character of the source material.
Despite all efforts to ensure a largely linear frequency response, it had to be accepted that tube devices
produce a “bad” sound. Additionally, the noise floor generated by the tubes limited the usable dynamics of
connected storage media (magnetic tape machines). Thus, a one-to-one reproduction of the audio signal’s
dynamics (expressed as the difference between the highest and lowest loudness levels of the program
material) proved impossible. To top it all, tube devices required the use of high-quality and often costly
transducers and sophisticated voltage supplies.
With the introduction of semiconductor technologies in the field of audio amplification it soon became clear that
the tube would have to give way to the transistor, as this device featured an enormously enhanced signal-to-
noise ratio, less complex power supply and improved frequency response. Plus, semiconductor-based circuits
can be realized much more easily—for less money. Two decades later, the introduction of binary signal
4. TECHNICAL BACKGROUND