allison_r, what kinds of sounds are you expecting it to pick up?
AGC (ALC) has certain disadvantages you need to be aware of, in particular the time constant of the gain control and it may not work well in some situations. AGC doesn't fix the sensitivity at a constant but correct level, it adjusts it dynamically to try keeping the output level constant under different input sources.
At it's worst, you amplify all the unwanted background noise then get a dead moment if a loud noise is heard, then the background noise resumes. That probably isn't what you want. There may be other methods to prevent overload which let you keep optimized gain but we need some idea of the range and abruptness of the sound over a period of say a few seconds.
Brian.
Many portable cassette players employ agc when recording. When playing back speech on a typical player, a silent stretch results in background hiss growing in volume over a period of about 3 seconds. A sudden sound causes drop to normal volume in 1 or 2 tenths of a second.
Circuits probably are on the internet.
Thanks for the extra info allison_r.
It sounds like what you need is a wide dynamic range rather than AGC. Sounds such bird song which are intermittent in nature can cause real problems to an AGC circuit but how you use the recording afterwards determines whether it is suitable. If you are only analyzing the sound spectrum or communication between species the quality isn't particularly important but if you wanted it to 'sound good' I wouldn't recommend AGC is used. As I see it your options, from best to worst are:
1. A compressor on the mic signal before recording and an expander during playback. This is similar to how Dolby noise reduction works. It gives a more faithful playback while keeping the louder sounds from overloading the recorder.
2. A fast acting AGC circuit (~0.25 seconds) to give a reasonable compromise between level output before 'breathing' (background hiss rising and falling) becomes objectionable.
3. A clipper circuit. This lets you run at higher gain all the time but sets a ceiling loudness level to prevent overload. It does the job but the clipped sounds may be noticably distorted.
Option 1 is most complicated electronically, option3 is cheapest.
Brian.
Is there an advantage if you were to make simultaneous recordings, one high volume, one low volume? Or do you already have some sort of duplication? It's making a large bet if you place it all on a sole recording for three days.
This idea may already be in your block diagram/ schematics. Create simultaneous soft and intense signals coming from two circuits. Choose in real time which to record. Your detector needs to act at some large threshold voltage.
For a more elaborate version, consider a programmable gain op amp.
The gain ratios are simple to adjust.
The methods above are not necessarily easier to build than the method you are considering, nor do they necessarily provide a more usable audio recording.
Noise has nothing to do with distortion.Both AD743 and AD797 are suggested in the diagram. AD743 lists a 2.9 nV/√Hz at 10 kHz while AD797 lists 0.9 nV/√Hz at 1 kHz.
Since I'd be using this in the 3-45 kHz range, does this mean that the AD743 has less signal distortion at upper frequencies (hence the reference to 10 kHz instead of 1 kHz), or are they equivalent?
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