Re: "near field" speaker measurement


"Earl Kiosterud" <someone@xxxxxxxxxxx> wrote in message
news:mcNki.12025$g44.9074@xxxxxxxxxxx


"Adam S" <not.valid@nosuchaddress> wrote in message
news:4692cef9$0$6925$afc38c87@xxxxxxxxxxxxxxxxxxxxxxx
Adam S wrote:
Earl Kiosterud wrote:

If I'm on the right track, it might seem that the result of the low-pass filters could
be
examined for non-zero frequency, and even figure out which way the quadrature vectors
are
spinning, and from that figure out which way to correct the latency correction.

Well, looks like there is much I didn't think about. Thanks for pointing it out, but the
maths is starting to get worse. I'd agree with the microphone having a delay relative to
the speaker. I'd have to go back to first principles to work out what happens to H, the
complex ratio of channel A and B. You now introduce three frequencies. The quadrature
reference signal, that of the speaker and that of the microphone.
let
f0 = frequency of quadrature signal
f1 = frequency of speaker signal
f2 = frequency of microphone signal

so you have (f0-f1) and (f0-f2) . I'll have to have a think about this some more.

I turns out that a delay on one channel will indeed effect the H vector, but only its
phase vs frequency relationship while the magnitude of H is always the ratio of
magnitudes of channels A and B.

Using exponent representation of vectors.

Let channel A signal be

A = |A|*exp(i*Wa*(t-Ta) + ¥a)

where
|A| = magnitude of channel A
Wa = frequency of channel A (rad/sec)
t = time (sec)
Ta = time delay of channel A (sec)
¥a = phase shift of channel A (rad)

channel B is similarly

B = |B|*exp(i*Wb*(t-Tb) + ¥b)


ratio of A/B is then

H = |A|/|B|*exp( i*Wa*(t-Ta) + ¥a - i*Wb*(t-Tb) + ¥b)
= |A|/|B|*exp( i*( t*(Wa - Wb + Tb - Ta) + ¥a - ¥b ) )

So you can see from the term (Wa - Wb + Tb - Ta), that if Wa-Wb are not non zero then it
has same effect as adding extra time delay difference. The magnitude of H stays constant.
This is not to say everything is nice and dandy, especially for log frequency sweeps. The
(Wa-Wb) term will not be constant in log sweeps and therefore won't appear as humble
group delay, messing up the true phase information (¥a - ¥b). I think errors become
insignificant with practical delays times and sweep rates.
Hope I'm on the right track saying this.

I theory both (Tb-Ta) and (Wa-Wb) could be predicted and compensated.

Adam

Adam,

My neurons are beginning to complain. Stretched thin! Some questions:

It seems to me that accurate phase data depends on accurately calculating cos(w*t)*A, etc.
If t isn't accurately known, (if there's a latency error), then all bets are off, it would
seem, for phase data. It also seems to me that low-pass filtering should not be necessary
if the latency is accurately known. Can you comment on that?

I don't understand how cos(w*t) can ba used to get a quadrature vector, since w is
changing all the time (linearly, logarithmically, etc.). For constant w, I'm OK with w*t,
but not where w is a function of time. Would it not be necessary to integrate w*t for the
time involved to get the total angular displacement during the latency period? (Or cheat
and use a software buffer!)

I wasn't sure what Ta and Tb are. Are they the latency of the input of the sound card?

Onward.
--
Regards from Virginia Beach,

Earl Kiosterud
www.smokeylake.com


Adam

I said low pass filtering wouldn't be necessary if the latency were accurately known. Later
in the day a neuron spontaneously fired saying "that's wrong," with which the rest of the
neurons agreed. So I retract that statement. There's still the 2f component that the
multiplication of the two sines produces.

I also asked about Ta and Tb. I then realized that they're the latency of the A and B
points. T is time.

You haven't commented on the soundcard's input latency. And I haven't thought much about
it.
--
Regards from Virginia Beach,

Earl Kiosterud
www.smokeylake.com



.

Relevant Pages

  • Re: Real World Significant Sources of (what we usually call) Latency
    ... Well just for giggles and grins, I thought I'd try some round trip latency ... by recording one channel direct, and the loopback output into the other ... channel, and then checking the difference in the recorded waveform. ... right with same results - consistantly 5 ms delay between the two channels. ...
    (rec.audio.pro)
  • Re: "near field" speaker measurement
    ... I turns out that a delay on one channel will indeed effect the H vector, ... phase vs frequency relationship while the magnitude of H is always the ratio of magnitudes ... If t isn't accurately known, (if there's a latency error), then all bets are off, it would ...
    (rec.audio.tech)
  • Re: "near field" speaker measurement
    ... The latency due to sound waves traveling to the microphone isn't compensated for, ... measure latency for the channel connecting to the speaker terminals. ... if the signal source is not exactly equal to the quadrature ... of the low pass filter is actually 1/2 the bandwidth of the sweep measurement. ...
    (rec.audio.tech)
  • Re: "near field" speaker measurement
    ... The latency due to sound waves traveling to the microphone isn't compensated for, ... measure latency for the channel connecting to the speaker terminals. ... if the signal source is not exactly equal to the quadrature ... of the low pass filter is actually 1/2 the bandwidth of the sweep measurement. ...
    (rec.audio.tech)
  • Re: Delay changing channels
    ... Jaime experiences no delay still makes me think that there is something ... going from one channel to the next. ... one to the tuner. ...
    (microsoft.public.windows.mediacenter)