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SSM-2122 Datasheet(PDF) 10 Page - Analog Devices |
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SSM-2122 Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page SSM2120/SSM2122 REV. C –10– V+ LOG AV V– SIGNAL INPUT SIGNAL OUTPUT 160k Ω IN |IIN| RECIN 39k Ω 1.5M Ω 1k Ω CAV1 V– +VC –VC 2200pF 47 Ω 36k Ω 36k Ω 100pF 12k Ω CONOUT V– 200 Ω 200 Ω THRESHOLD 160k Ω 39k Ω 1k Ω CONOUT V– FC ≤ 20Hz 36k Ω 5.6k Ω 36k Ω RIN1 V+ V+ LOG AV IN |IIN| RECIN 1.5M Ω CAV2 V– FC = 5Hz RIN2 36k Ω +VC –VC 36k Ω 200 Ω 200 Ω 12k Ω 2200pF 47 Ω DOWNWARD EXPANDER Figure 14. Dynamic Filter with Downward Expander DYNAMIC FILTER WITH DOWNWARD EXPANDER A composite single-ended noise reduction system can be realized by a combination of dynamic filtering and a downward expander. As shown in Figure 14, the output from the wideband detector can also be connected to the +VC control port of the second VCA which is connected in series with the sliding filter. This will act as a downward expander with a threshold that tracks that of the filter. Although both of these techniques are used for noise reduction, each alone will pass appreciable amounts of noise under some conditions. When used together, both contribute distinct advantages while compensating for each other’s deficiencies. Downward expansion uses a VCA controlled by the level detector. This section maintains dynamic range integrity for all levels above the user adjustable threshold level. As the input level decreases below the threshold, gain reduction occurs at an increasing rate (see Figure 15). This technique reduces audible noise in fade outs or low level signal passages by keeping the standing noise floor well below the program material. This technique by itself is less effective for signals with predominantly low frequency content such as a bass solo where wideband frequency noise would be heard at full level. Also, since the level detector has a time constant for signal averaging, percussive material can modulate the noise floor causing a “pumping” or “breathing” effect. The dynamic filter and downward expander techniques used together can be employed more subtly to achieve a given level of noise reduction than would be required if used individually. Up to 30 dB of noise reduction can be realized while preserving the crisp highs with a minimum of transient side effects. +20 –30 –40 –50 –60 +20 –30 –45 –60 –75 Figure 15. Typical Downward Expander I/O Characteristics at –30 dB Threshold Level (1:1.5 Ratio) |
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