Review: Audio Media April '97
by Jayson Chase

Like its predecessors, the SPL Stereo Vitalizer MK2 is designed to sharpen and enrich the sound of the final production, whether in the recording studio, making good use of the dynamic equalisers and the stereo expander control (which enables signals that are placed off-centre of the stereo image to be detected and fed into the opposite channel, and phase reversed - a great way to increase the size of the overall image); or in broadcast applications, when signals sent via ISDN or music lines can suffer from a reduction in quality and a diminishing stereo image.

The Knobs and Buttons
The Stereo Vitalizer MK2 has an active process button which acts as a hard-bypass function that enables the circuitry to be switched in and out of operation. This allows quick and easy A/B analysis. One set of controls operates both channels, which ensures that both paths are processed equally. The input control, labelled 'Drive', sets the level at which the filter network will operate. The amount of processing is determined by the level at which the 'Drive' is set, and there is a clip LED which will illuminate 3 dB before the onset of clipping. The drive level can be adjusted between -20 dB and +6 dB.

There are three adjustable parameters dedicated to frequencies ranging from 15 Hz to 22 kHz. The bass section is fitted with two separate controls. The first is labelled 'Sound', and is responsible for the overall feel of the low frequencies. When this control is rotated in an anti-clockwise direction (Soft) the low frequencies will become warmer, and seemingly more robust with a deeper feel. Rotate the control in a clockwise direction (Tight) and the sound becomes drier, with increased definition. The second control for low frequencies is the compressor, which is dedicated to operate within the low frequency range; the threshold, attack and release are adjusted internally. The ratio can be selected by the user, from the off-position to near limiting at position 10.

The Mid-Hi Tune is based on a broadband shelving filter with a frequency range from 1 kHz to 20 kHz, the adjustments here being made by amplitude-dependent phase shifting - all frequencies above the selection made at the Mid-Hi Tune control will be modified by the 'Process' control, whilst those that fall below the selection are affected by damping. This is where the Stereo Vitalizer Mk2 differs from the conventional graphic equaliser. The subjective loudness of the program material is maintained whilst the broadband spectrum is altered using a linear frequency response similar to the human hearing. This action may be executed without producing the effects of comb filtering with the low or high frequency filters.

Process Control
The Process control determines the intensity of the mid-range frequencies; it prevents adjustments that are made to the low frequencies from affecting those in the higher ranges, by maintaining the ratio between Bass and Mid-Hi Tune to the original signal. As the value at the 'Mid-High Tune' is lowered, the sound images become brighter due to more mid-range frequencies being included in the processing. When the value is raised, sharp or excessively bright frequencies will be reduced, due to a smaller band of mid-range frequencies being treated. Much of the psychoacoustic manipulation operation takes place at the Process section; the unique circuitry of the Stereo Vitalizer Mk2 allows bands of frequencies to be altered whilst being able to preserve a balance throughout the spectrum.

The high frequency filter, which can operate within a frequency range between 2 kHz and 20 kHz, does not depend on the 'Exciter' principle of generating high frequencies and their associated harmonics. The Stereo Vitalizer Mk2 makes use of a steep filter network that controls the changes in phase relationships to emphasise the existing high frequencies and harmonics of the original sound source. The Intensity control determines the amount of boost that is applied to the frequencies that are selected by the High EQ.

In Use
For the purpose of this review, I decided to put the Stereo Vitalizer Mk2 to the test using a finished production recorded in 1963 by Graeme Goodall at Federal Record Studios. The introduction of the Stereo Vitalizer Mk2 made an immediate impression. By selecting 'Soft' and minimal compression at the 'Bass' controls, and liberal amounts of 'Process' around 2 kHz, the rhythm section breathed new life into the mix. The double bass became more vibrant, the movement through the scales achieved new definition, and the vocals were brought forward. The High EQ was beneficial for adding brightness to the upper spectrum, but it also raised the level of the residual noise.

I found the Stereo Vitalizer Mk2 to be an all round performer, the unique equaliser section combined with the 'Stereo Expander' will allow the user to tackle most applications and, as demonstrated with even very old recordings, the unit can prove a very useful studio tool. For the ultimate enhancer, however, my heart still lies with the SPL Tube Vitalizer.

Jayson Chase puts the latest member of the SPL Vitalizer series to the test, and finds the same processing quality as it's predecessors, but with some new and exciting features.

Jayson Chase is a producer at Unique Broadcasting. The views expressed in this article are the author's own and are not necessarily those of his employer.

Find out more about the SPL Stereo Vitalizer 2