Thursday, April 25, 2013

FM Acoustics 268-C Linearizer - How it works part 3 of 4

Symmetrical or non-symmetrical correction

On the left side of the Linearizer section there are two switches. The left one allows activating or bypassing the entire Linearizer section (hardwire bypass). The switch next to it is a special switch that allows selection of either symmetrical or non-symmetrical  addition/subtraction of the audio signal.

Why symmetrical and non-symmetrical?

Linearization must not be the same for boost and cut curves as the ear reacts differently when adding a certain band of frequencies than when  subtracting  it. In reality, the most objectionable sounds are resonances. Therefore, often a different response is required when  attenuating  frequencies than when increasing  frequencies (one does not really want to create a resonance!) The function of the controls are best illustrated with a few actual frequency response curves.

Fig. 1 shows the response when the symmetrical/non-symmetrical switch is in the symmetrical position (out) and set to a 6dB subtraction at  800Hz. The result is a mild, musically pleasing correction curve.

Fig. 2 shows the same setting of the linearizer section but this time with the non-symmetrical switch pushed in. The curve is quite distinctly different from that in Fig. 1 resulting in more concentrated correction in the non-symmetrical position.

Fig. 3 shows signal addition at 800 Hz. This curve is identical for both the non-symmetrical as well as the symmetrical switch position. The reason for this is that the ear reacts differently to a boost in frequency response than to an attenuation. Attenuated frequencies in  music are often not sharp but rather soft "valleys" and can be linearized by equally mild addition as Fig. 3 shows.

This is another characteristic that all current frequency range correction circuits do wrong. It is a great feature of the FM 268-C to have the possibility of linearizing the signal in a  musically correct way.

Fig. 4 shows - with the non-symmetrical switch pushed in - the response when the 800 Hz and the 3.2 kHz band are subtracted (each is set at  -3.6). As these are unique additive/subtractive Linearizers the linearizer banks can be combined to provide an unlimited number of curves! Because of this combining possibility the numbers printed on the front panel do not correspond to an absolute dB level but are changing with the settings of adjacent bands.

The smooth subtraction curve is centered at 1.6kHz, midway between the 800Hz and 3.2kHz (in music terms better explained as being precisely centered between the lower and higher two adjacent octaves).

It is easy to see the absolutely smooth response with no ripple (valleys and hills) as is usually the case. Adjacent bands add perfectly without ripple or discontinuity and transient reproduction is superb.

Compare this with Fig. 5, which shows the frequency response of a typical equalizer. Apart from the non-desirable response curve and the ripple in the pass band which proves the existence of non-linearity, the phase response is poor ruining most of what was on the original  audio signal.

Fig. 5 Attenuation of a typical equalizer with frequency controls set as in Fig. 4.