On Apr 22, 11:33 pm, "Ron N." <rhnlo...@yahoo.com> wrote:> You also never answered whether you tried plotting your > data after the type conversion to make sure you didn't > clip or wrap it.I did plot the data, after the type conversion. I find the following (typical data sample length 2048 doubles): 1. a few spikes at the start - most probably due to a large DC offset being introduced somewhere. 2. a large sequence of zeros - about 250 - 300 samples long 3. the sinusoidal curve that one would expect - a little bit noisy, but still okay.
Java Audio question - PLEASE HELP
Started by ●April 13, 2008
Reply by ●April 23, 20082008-04-23
Reply by ●April 23, 20082008-04-23
On Apr 23, 4:18 am, "John E. Hadstate" <jh113...@hotmail.com> wrote:> Even (especially) after reading your later post, I don't > understand what you are saying. From your later post, it seems > that 7 Hz is your bin resolution, so the first bin corresponds > to 7 Hz, the 2nd to 14 Hz, etc. Are you saying that you don't > see a spike near bin 71 (500/7)? Perhaps the amplitude of your > injected tone is too low compared to the amplitude of your > other signal (so the spike is there, it's just lost in the > noise). What exactly is the problem?The problem is, as I mentioned in a posting just before, that when I plot the data, I see a long sequence of zeros, about 250 - 300 sample points long, before I see the sinusoidal signal. There are a few sharp and large spikes at the start, which might be due to DC offset, but I do not understand the sequence of zeros. The total sample size of the collected data is 2048.
Reply by ●April 23, 20082008-04-23
On Apr 22, 11:04 pm, Lew <l...@lewscanon.com> wrote:> > fs = sample rate in Hz > > dT = 1/fs = interval between samples > > N = number of samples taken > > T = N x dT = total time period > > > f1 = 1/T = frequency of the first harmonic in Hz > > So if you accumulated twice as many samples, over twice as long, your > fundamental frequency would be halved? That is not sensible. > > -- > LewThat is precisely the case. Please check the definitions above. Please note that f1 is the first harmonic if I were to calculate an FFT with the collected samples. The fundamental frequency is 500.0 Hz, and stays that way.
Reply by ●April 23, 20082008-04-23
cpptutor2000@yahoo.com wrote:> With reference to all the postings I am sorry I have caused confusion > about what I meant by the 'fundamental frequency' etc., May I give a > simple example? > > For a given signal being sampled for analysis, the number of points > obtained and the time between samples combine to determine the total > length of time taken to examine the signal. The following definitions > apply: > fs = sample rate in Hz > dT = 1/fs = interval between samples > N = number of samples taken > T = N x dT = total time period > > f1 = 1/T = frequency of the first harmonic in Hz > Keeping these points in mind, I have found that f1 in my case is > around 7 Hz. I am NOT measuring 7 Hz. If I were to take an FFT of the > collected data, then I can expect to get peaks that are harmonics of 7 > Hz. That is if I were to detect if the 500.0 Hz signal of the tone > generator was being picked up, I would have look for the 500/7 > harmonic of 7 Hz. I hope I have made myself clear about what I am > trying to do.Pete, It would help if you used the same terms that the rest of us do. In particular, it is important to avoid using standard words in non-standard ways without the sort of glossary you provided here. *Fundamental* applies not to an FFT, but to the longest-period component of a periodic waveform. What *you* have called the fundamental is often referred to as bin width or resolution. If your signal repeats every 2 ms, then its fundamental is 500 Hz. It may have harmonics at 1000, 1500, 2000Hz, etc. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 23, 20082008-04-23
On Apr 23, 6:06 am, "cpptutor2...@yahoo.com" <cpptutor2...@yahoo.com> wrote:> On Apr 23, 4:18 am, "John E. Hadstate" <jh113...@hotmail.com> wrote: > > > Even (especially) after reading your later post, I don't > > understand what you are saying. From your later post, it seems > > that 7 Hz is your bin resolution, so the first bin corresponds > > to 7 Hz, the 2nd to 14 Hz, etc. Are you saying that you don't > > see a spike near bin 71 (500/7)? Perhaps the amplitude of your > > injected tone is too low compared to the amplitude of your > > other signal (so the spike is there, it's just lost in the > > noise). What exactly is the problem? > > The problem is, as I mentioned in a posting just before, that when I > plot the data, I see a long sequence of zeros, about 250 - 300 sample > points long, before I see the sinusoidal signal. There are a few sharp > and large spikes at the start, which might be due to DC offset, but I > do not understand the sequence of zeros. The total sample size of the > collected data is 2048.It could be a data header. Or it could be how long it takes your mic or ADC to initialize. Why not just ignore this header or turn-on transient? Does your sinusoid have roughly balanced positive and negative values? Or do you have a DC offset of your sinusoid?
Reply by ●April 23, 20082008-04-23
cpptutor2000@yahoo.com wrote:> On Apr 23, 4:18 am, "John E. Hadstate" <jh113...@hotmail.com> wrote: > >> Even (especially) after reading your later post, I don't >> understand what you are saying. From your later post, it seems >> that 7 Hz is your bin resolution, so the first bin corresponds >> to 7 Hz, the 2nd to 14 Hz, etc. Are you saying that you don't >> see a spike near bin 71 (500/7)? Perhaps the amplitude of your >> injected tone is too low compared to the amplitude of your >> other signal (so the spike is there, it's just lost in the >> noise). What exactly is the problem? > > The problem is, as I mentioned in a posting just before, that when I > plot the data, I see a long sequence of zeros, about 250 - 300 sample > points long, before I see the sinusoidal signal. There are a few sharp > and large spikes at the start, which might be due to DC offset, but I > do not understand the sequence of zeros. The total sample size of the > collected data is 2048.Look to the acquisition software. It's unlikely that there's a startup-transient that large in the hardware, but there may be an offser between the pointers that write the data and retrieve it. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 23, 20082008-04-23
Jerry Avins wrote: cpptutor2000@yahoo.com wrote:> On Apr 23, 4:18 am, "John E. Hadstate" <jh113...@hotmail.com> wrote: > >> Even (especially) after reading your later post, I don't >> understand what you are saying. From your later post, it seems >> that 7 Hz is your bin resolution, so the first bin corresponds >> to 7 Hz, the 2nd to 14 Hz, etc. Are you saying that you don't >> see a spike near bin 71 (500/7)? Perhaps the amplitude of your >> injected tone is too low compared to the amplitude of your >> other signal (so the spike is there, it's just lost in the >> noise). What exactly is the problem? > > The problem is, as I mentioned in a posting just before, that when I > plot the data, I see a long sequence of zeros, about 250 - 300 sample > points long, before I see the sinusoidal signal. There are a few sharp > and large spikes at the start, which might be due to DC offset, but I > do not understand the sequence of zeros. The total sample size of the > collected data is 2048.Look to the acquisition software. It's unlikely that there's a startup transient that large in the hardware, but there may be an offset between the pointers that write and retrieve the data. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 23, 20082008-04-23
<cpptutor2000@yahoo.com> wrote in message news:a1d43b38-854e-470e-adbf-e514cf982960@a70g2000hsh.googlegroups.com...> On Apr 23, 4:18 am, "John E. Hadstate" <jh113...@hotmail.com> > wrote: > >> Even (especially) after reading your later post, I don't >> understand what you are saying. From your later post, it >> seems >> that 7 Hz is your bin resolution, so the first bin >> corresponds >> to 7 Hz, the 2nd to 14 Hz, etc. Are you saying that you >> don't >> see a spike near bin 71 (500/7)? Perhaps the amplitude of >> your >> injected tone is too low compared to the amplitude of your >> other signal (so the spike is there, it's just lost in the >> noise). What exactly is the problem? > > The problem is, as I mentioned in a posting just before, that > when I > plot the data, I see a long sequence of zeros, about 250 - > 300 sample > points long, before I see the sinusoidal signal. There are a > few sharp > and large spikes at the start, which might be due to DC > offset, but I > do not understand the sequence of zeros. The total sample > size of the > collected data is 2048. >I'll second Ron's motion and add that some file formats that I'm familiar with reserve the first 512-byte block (or more) for "metadata"; that is, special labels that identify things about the data like the sample rate, the number of bits per sample, the endian-ness, whether stereo or mono, the number of channels, etc. Since these things don't often consume all the allocated space, the remaining bytes ("reserved for future expansion" is a favorite phrase here) are often set to zero. As long as identifiable data starts at a constant offset from the beginning of the file and assuming that you don't need the metadata, my advice would be to read it and discard it.
Reply by ●April 24, 20082008-04-24
cpptutor2000@yahoo.com wrote:> On Apr 22, 11:04 pm, Lew <l...@lewscanon.com> wrote: > >>> fs = sample rate in Hz >>> dT = 1/fs = interval between samples >>> N = number of samples taken >>> T = N x dT = total time period >>> f1 = 1/T = frequency of the first harmonic in Hz >> So if you accumulated twice as many samples, over twice as long, your >> fundamental frequency would be halved? That is not sensible. >> >> -- >> Lew > That is precisely the case. Please check the definitions above. Please > note that f1 is the first harmonic if I were to calculate an FFT with > the collected samples. The fundamental frequency is 500.0 Hz, and > stays that way.No, that doesn't make sense. If I recorded a pure 1000 Hz sine wave, that is the frequency whether I take 44000 samples for one second or 88000 samples over two seconds. The frequency isn't affected by how long I record it. -- Lew
Reply by ●April 24, 20082008-04-24
Jerry Avins wrote:> cpptutor2000@yahoo.com wrote: >> With reference to all the postings I am sorry I have caused confusion >> about what I meant by the 'fundamental frequency' etc., May I give a >> simple example? >> >> For a given signal being sampled for analysis, the number of points >> obtained and the time between samples combine to determine the total >> length of time taken to examine the signal. The following definitions >> apply: >> fs = sample rate in Hz >> dT = 1/fs = interval between samples >> N = number of samples taken >> T = N x dT = total time period >> >> f1 = 1/T = frequency of the first harmonic in Hz >> Keeping these points in mind, I have found that f1 in my case is >> around 7 Hz. I am NOT measuring 7 Hz. If I were to take an FFT of the >> collected data, then I can expect to get peaks that are harmonics of 7 >> Hz. That is if I were to detect if the 500.0 Hz signal of the tone >> generator was being picked up, I would have look for the 500/7 >> harmonic of 7 Hz. I hope I have made myself clear about what I am >> trying to do. > > Pete, > > It would help if you used the same terms that the rest of us do. In > particular, it is important to avoid using standard words in > non-standard ways without the sort of glossary you provided here. > > *Fundamental* applies not to an FFT, but to the longest-period component > of a periodic waveform. What *you* have called the fundamental is often > referred to as bin width or resolution. > > If your signal repeats every 2 ms, then its fundamental is 500 Hz. It > may have harmonics at 1000, 1500, 2000Hz, etc.No wonder I was confused. It was a case of, "There's glory for you!" (Carroll, Lewis, /Through the Looking-Glass/, Chapter VI, "Humpty Dumpty", <http://www.gutenberg.org/etext/12>). -- Lew
