V700 Stereo Mastering

The Colorizer U784 allows the colorization of a stereo signal by adding distortion products of low quality audio equipment and the side effects of dynamics units. Four independent sections can produce the distortion of FET compressors, diode based compressors, and low quality, IC based amplifiers. The distortion can be added to the original signal with crossfade controls independently. Like the harmonics generator U794 that produces and adds clean 2nd and 3rd harmonics, the Colorizer is a useful tool to simulate vintage gear and improve a mix in uncommon ways.

The Colorizer U784 is focussed on unobstrusive colorisation and not on pushing effects. The control ranges are well designed to accomplish modifications that are barely audible; however, strong effects are possible as well.

While the harmonics generator U794 allows precise modifications of the different harmonics, the Colorizer simulates common side effects of gear. Both devices belong to the same group but the results are entirely different.

The Sections
The device has four independent sections that produce different colors. Each section has a crossfade control that allows to mix the original with the generated signal. These controls are designed to allow precise addition of a very small percentage of the color. When the controls are all to the left, the original signal remains unaffected. Settings all to the right switch off the original and use the output signal of the particular section instead. These positions are helpful for adjustment of the parameters.

High and Low
At a glance these sections look like equalizers, which actually is the case. However, these equalizer are of very bad quality and add the typical color of low slew rate operational amplifiers even when used in the 0 dB position. Such effects appear when audio passes amplifiers that have such a low slew rate that the output of the amplifier cannot follow the input signal precisely. At a certain combination of level and frequency, the slew rate of the audio signal exceeds the maximum slew rate of the amplifiers, which results in distortion. Any signal form above this limit is converted to a triangle wave. In the early days of integrated circuits this effects was very common, since all integrated amplifiers up to the early 80th were not able to process high level high frequency signals without that kind of distortion. Even today this effects is still important if inappropriate operational amplifiers are used for audio.

Both equalizer bands of the U784 colorisator produce this kind of distortion, which can be modified and adapted by the EQ settings. While the slew rate of the low band is very low and useful for colorizing the bass range. Colorization already takes place with the boost/cut control at 0 dB. Adjusting that boost to the frequency range of the kick drum or bass guitar increases the effect a lot and restricts the colorisation to this frequency band when mixed to original signal appropriately.

The high equalizer is based on the same principle but uses faster amplifiers that are more appropriate for colorization of mid and high frequencies. Boosting very high frequencies results in 'cloudy' highs, while the mid range sounds aggressive and harsh.

FET
The FET section is basically the audio path of a FET compressor. FET's (field effect transistors) are a special kind of transistor that can be used to control the level of an audio signal; however there are many restrictions and limitation that need to be considered when using these components. The level across the FET is limited and higher levels cause heavy distortion, while the necessary attenuation before and amplifcation after the FET that is necessary to reduce the level across the FET, results in a bad noise performance. For this reason the dynamic range of such compressors is very poor and even with the poor noise performance there is still a basic distortion, even if no regulation takes place. The distortion increases with the gain reduction and is directly connected to the use of the FET.

The FET section produces exactly that typically kind of distortion by using of one of the FET types that are commonly used in such compressors. The BIAS control allows adjustment of a certain voltage that alters the distortion. The scaling of the bias control ist just from 0 to 10; however, 0 does not mean that there is no distortion. The control just shifts the input level range where distortion occurs and should be used to adapt the effect to the signal.

Diode
The diode section works like the FET section; however the control elemet that causes distortion is a diode bridge. Diode bridges have been widely used with tube compressors and transistor compressors as well. Vintage compressor like the U73 used this principle to control the audio level instead of a tube, to avoid the problems with using tubes for regulation. Diodes also result in the same compromise between distortion and noise that was disscussed before. The distortion of the diode bridge is different from the FET and allows another option for colorisation. The BIAS control adapts the operation level and causes more or less distortion if the input level is constant.

The Module
The U784 Colorizer is a single slot V700 standard module that can be used in all shop wired V700 frames. The device is has electronically balanced inputs and outputs and can handle levels of + 30 dBu. The usual hard bypass is included.