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Thread: Droid source voltage test (and a few signal generator apps)

  1. #11
    Founding Member 86mr2's Avatar
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    Quote Originally Posted by earthtodan View Post
    What does the dock out do?

    There is the possibility that the distortion I'm measuring is the input limit on my computer's sound card, not the output limit on the devices. I'll have to test for that somehow.
    I am quite suspicious that is true. The levels of distortion you measured should have generated a bunch of complaints about iPhone sound quality.

  2. #12
    Founding Member earthtodan's Avatar
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    I went in to my control panel and turned down the level of the mic input. Then I re-tested with my Droid. At max volume, no harmonic distortion!

    I have edited my previous posts to remove the graphs showing distortion. Looks like the Droid can be played at max volume. I'm glad to see this, because I haven't heard anything disturbing when I use it at max input in my car, so my ears agree.

    My first clue was how the THD products suddenly sprang up out of nowhere, like they hit a ceiling. I know that a non-sinusoidal waveform is actually just a sum of many sin waves, so I thought, maybe those odd-order distortions are what clipping looks like. However, my second clue was that the oscilloscope shows a clean sine wave all the way up to the top. It's a pixelated screen, but it ought to show a dirty waveform if those distortion products are really there.

  3. #13
    Senior Member cvjoint's Avatar
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    Wow, that makes the results even better. A processor like the Ms8 can't take more than 2v anyhow from what I've seen.

  4. #14
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    Quote Originally Posted by earthtodan View Post
    I measured my roommate's iPhone 4S (He's a mac, I'm a PC). For the sake of consistency, I used the same 0dB 1kHz test tone.

    This is the max voltage using the phone's MP3 player. At .989V RMS one-sided, it is 1.978V RMS total, which is pretty impressive.



    Deleted faulty THD measurements.
    That looks a little high, by about times the amount actually. Could it be that publshed rms analog output voltages are one sides rms? That would correspond with what all other idevices in this range have (ie 1 volt RMS). As well as the limitation that the battery voltage brings which IIRC can be no more that half its DC rating. 1 volts is also what the input meters on my soundcard's mixer show when I send an rmaa calibration signal into it. I trust the card ratings and the iphone 3gs is ~-6dB (half the voltage) below its max input of 2 volts rms.

    Then there's also this......

    NwAvGuy: Testing Methods

    "HIGH-END BENCH DMM (revised 4/15): A surprising number of people are trying to make audio measurements with typical portable DMM’s. And the readings are often grossly wrong without even realizing it. True RMS measurements are not trivial. In effect, the meter has to accurately measure the “area under the curve” and time average it—see True RMS Measurements for more information. This proves to be rather difficult across a wide range of frequencies if you want to maintain reasonable accuracy at high frequencies and not have the reading “hunt” up and down at low frequencies. The fact is, most DMM’s priced under a few hundred dollars that claim “True RMS” are really only accurate around 60hz—i.e. power line frequencies. Some will measure sine waves accurately across the audio band, but many will not even do that. I have a $150 “True RMS” Extech meter--a relatively well regarded brand--that’s off by nearly 6 dB at 20 Khz compared to 60 hz on a sine wave and is a joke above 1 Khz on non-sinusoidal waveforms. And really complex rapidly changing waveforms like white/pink noise or real music drive such meters crazy. To do it right, you need expensive true RMS circuitry and the ability to optimize the sample rate and averaging for the waveform being measured. Good high end bench DMM’s, like the Agilent 344xx series, let you set these parameters. They also read directly in dB. I use a 6 1/2 digit Agilent true RMS bench DMM that's extremely accurate and flat from 10 hz - 100 Khz for exact levels and other measurements. It has resolution down to 0.1 microvolts so it can even be used to measure noise."

    After reading that whole page, I don't think I be wasting my time doing RMAA test anymore.

  5. #15
    Founding Member earthtodan's Avatar
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    Interesting. I tested one of the devices at 400 hz and I think 15khz, and they agreed with the 1khz measurement, but I didn't test it at 60 hz. I also didn't test consistently; I derived the RMS on the iPhone from the RMS/2 measurement given by the DMM, whereas on the Droid I used the peak-to-peak measurement multiplied by .707. I suspect the peak voltage measurement on the DMM is more accurate.

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