THAT4305 Просмотр технического описания (PDF) - Unspecified

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THAT4305

Pre-trimmed Analog Engine® IC

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Document 600067 Rev 00

Page 11

slightly with temperature unless the applied voltage

is appropriately compensated.

If we let TdB=20, AdB=0 dB, and k=0.75, this be-

havior yields the transfer function shown in Fig-

ure 13. As predicted by the above equations, this

results in a 4:1 compression ratio above the thresh-

old of -20 dB (relative to the RMS detector's 0 dB

reference level. The output level increases by 10 dB

over a 40 dB change in input level.

20

10

0

-10

-20

-30

-40

Out

-50

In

-60

-70

-80

-90

-100

-100 -80 -60 -40 -20

0

20

dB In

Figure 13. Transfer function of a feedforward compressor

In Figure 14, we have again extended the basic

feedback compressor with a threshold adjustment

(TdB), a threshold (the ideal diode), a means to vary

the sidechain gain (k), and a make-up gain adjust-

ment (AdB) just as in Figure 8. However, in this

case, the detector level is based on the compressor’s

output.

Once again we start with

OutdB = IndB + GdB

For output signals above the level determined by

the threshold setting,

OutdB > TdB .

So for signals above the threshold,

GdB = −k(OutdB − TdB) + AdB .

Substituting yields

OutdB = IndB − k(OutdB − TdB) + AdB ,

which can be reduced to

OutdB

=

IndB + TdB + AdB

(1+ k)

.

For output signals below the level set by the

threshold setting, the signal at the output of the

ideal diode threshold is 0 (dB), so

GdB = AdB , and

OutdB = IndB + AdB

If we let TdB=10, AdB=20, and k=10, this behav-

ior yields the transfer function shown in Figure 15.

The compression ratio of 11:1 allows a rise of only

about 4.5 dB over a 50 dB range.

IndB

GdB

S

A dB

Out dB

RMS

Ideal

Diode

Out dB

-k

S+

-

TdB

Figure 14. Feedback compressor with threshold, gain, and

ratio adjustments

0

-50

Out

In

-100

-100

-50

0

dB Out

Figure 15. Transfer function of a feedback compressor

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