If I'm understanding correctly, you're saying to put a 5V zener...
namely the micro controller must be above 8MHz. The mega is 16MHz -- do you think this would be enough to run both two fast fourier transforms
Also, adding onto software variable sensitivity -- would it be possible to implement the same thing in hardware as well using a digital potentiometer in my voltage divider?
This gizmo claims that it can operate as a limiter, and accommodate 60 dB on its inputs with only about 0.5 dB change on the outputs. http://www.onsemi.com//pub/Collateral/SA575-D.PDFIt's available in a through-hole mount for something around $5.50 from Mouser, though the through-hole version is marked "end-of-life." If that's true, you should be able to get the a flat output response over a voltage input range of 1000. Using the "+10 dB ~ twice as loud" rule of thumb, that's an apparent loudness range factor of 64. That might do it.
1) The amp that the audio will be tapped off from, is the output driving the speaker with reference to common ground or is the amp using some kind of bridge circuit (common in lots of audio driver ics) where neither speaker lead is referenced to ground?
2) Using a compander like the SA575 will help regulate the levels and although it has a dynamic range of 60dB or so, what is the maximum input in dBm?
3) Keep in mind that bass frequencies use a lot of power to reproduce and although one might think the amp is only being run at say 10% power (5 watts), music passages with lots of bass could produce peaks of 10 or even 20 watts of output power.I would use an audio transformer with a primary Z of around 10K connected to the amp and a secondary Z of around 600 ohms feeding a voltage divider circuit, some clamping diodes and possibly the SA575 after that.The transformer would not only overcome the potential problem of the amps output not being referenced to ground, but the 10K:600 ohm ratio would work in stepping down the voltage as well.A series capacitor between the amp and one leg of the primary of the transformer will act as a high pass filter to attenuate very low bass frequencies that could cause core saturation.
... automatic gain control amplifiers, I assume this IC's advertised automatic level control is the same thing?
"...voltage input range of 1000." Do you mean over a range of 1000dB, 1000V, 1000x, or what?
I have drawn a schematic based on the diagram in the data sheet and attached. Does it look correct?
The amp I am using is a TEAC A-H01 Stereo Amplifier/DAC. I couldn't find anywhere whether it references common ground, so it would probably be safer to assume that it doesn't. Not sure if it's useful, but my speakers are based off the Overnight Sensation MTM design (https://sites.google.com/site/undefinition/overnightsensationmtm)
The data sheet for the SA575 (http://www.onsemi.com/pub_link/Collateral/SA575-D.PDF) does not mention input specifications (as far as I can tell), but the highest input they mention in ALC mode is +9dB, after which things start to get a little messy. It would not be hard to get the input voltage under this level. I couldn't find any mention of maximum input power.
I was under the impression that voltage was the main concern here, as the speakers are relatively low-impedance so little current will likely be flowing through the SA575 (though I'm not sure what it's input resistance is).
Is it more important to pay attention to power output from the amp then? How about in reproducing the original line-level signal--since bass frequencies take more power to drive the speakers, would they be misrepresented in the tap of the speaker-level audio signal as they would have higher voltages?
Also, wouldn't using a high-pass filter to attenuate the low frequencies eventually produced an incorrect spectral analysis as the low frequencies will not be represented? I could adjust in software if needed, but the lower frequencies are definitely important for visual effects.
Quote"...voltage input range of 1000." Do you mean over a range of 1000dB, 1000V, 1000x, or what?A range of 1000x. If the datasheet's claims are accurate, you should be able to design a circuit with that will deliver a constant AC voltage at the output while the input signal varies by a factor of 1000.
How low do you want to display?You need to make a trade off between possible core saturation and possible slight inacurate display of low frequencies.this is the price to pay when tapping off from a high power point (amp output) instead of using a line level feed.Also, how much low frequency content does music actually have?By that I mean, how important is it if say frequencies below several tens of Hz are attenuated by several dBs?
I have been reading about audio transformers, and I think I have a pretty good idea of what you were saying to do. If I'm correct, the transformer will just isolate my circuitry from the amplified signal, and in the process allow me to set a ground reference. However, I'm not sure exactly what effect the primary/secondary impedance has. I'm thinking that the voltage won't change from primary to secondary coils, but the impedance on the input will be high to prevent current from flowing to my circuit. Am I thinking about this correctly?
Power level ratings are maximum at lowest frequency rating, power can be increased slightly at higher frequencies.
Below is a link to the type of transformer I'm referring to:(plenty other places sell them)http://www.henrys.co.uk/PA/100v_line_transformers.htmlThe smallest one is fine.Whichever one you decide on, first make sure about the frequency response as they are two main types of audio line transformers, those optimized for speech (you don't want those) and the other type has a greater frequency response specifically for music.Just remember, the mere fact that you are going via a transformer (due to the amps outputs not being referenced to ground), will mean that the low end of the frequency response will always be worse than if you had a direct connection to a low level, high impedance output.