On Wednesday, March 14, 2012 7:44:03 AM UTC-4, Spooky wrote: > > You didn't mention your sampling frequency.... Remember Nyquist? To > reproduce any digital signal, you MUST sample at twice the frequency of > the highest frequency you want to reproduce. In this case, you'd have to > sample at a minimum of 34,000 samples/second. HOWEVER..... > Not exactly true. What is true is that frequency components above the threshold appear like, and cannot be distinguished from, frequencies below it. To avoid this confusion, it is common to remove them with a low pass filter before sampling. However, this is not the only way of doing things. It is also quite common to use a filter with selects frequencies above the threshold, and take advantage of the sampling folding them down to a frequency range where they are easier to deal with computationally. Obviously that's more common in communications equipment where you are only interested in a narrow range of frequencies, rather than baseband voice or music.
> > When I analyze the recorded sound, 17kHz frequencies aren't there, its > > like the phone has a low pass filter that eliminates these > > frequencies. > > Yes, the *PHONE* certainly does...but where? The question is, does the > non-phone portion of the Android device have access to the raw audio, or > has the low-pass filter already been applied? Check to see if you can > record frequencies above 5 kHz (DS0[1] is 300--3400 Hz, but since there > are no "brick-wall" filters, it usually extends to about 4000 Hz, and > then we allow a bit extra to make sure). If you can access up to 5000 Hz > or higher, you're getting audio before the low-pass filter. > Before *which* low pass filter? A device intended to be used for both telephony and higher quality audio likely has several. There is probably something in front of the actual ADC (likely on chip) which prevents aliasing in the sampling itself. There may be an intermediate filter in the audio system if any resampling is done, and this may depend on the sample rate requested by the program. And then there's probably one in the GSM or SIP or whatever encoding system to limit to the narrower voice bandwidth of those channels. > -- You received this message because you are subscribed to the Google Groups "Android Developers" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/android-developers?hl=en
