Re: jdelay

thanks, this is great! and with a morning-coffee all those
hypot and
atan2 begin to look familiar... but i'm struggling to
re-engineer this
for()  loop back into an integral ;) - I need to
carefully read
thru your post, probably not today..

Fons Adriaensen wrote:
> On Wed, Apr 11, 2007 at 01:44:29AM +0200, Robin Gareus
wrote:
> 
>> BTW. would it be unreasonable to "fix"
jdelay to work with an amp&mic ?
> 
> It will work via speaker an mic if both are linear
phase (very unlikely
> for speaker) and there are no significant reflections
(echos) in the signal.
> It can't be 'fixed' to work in other conditions - you
need something
> different. But jdelay is meant to measure sound card
latency and not
> delays in arbitrary channels.
> 

not that it had any straight forward audio apps. I was
amazed how much I
could do in just 5 minutes! -

using jdelay as black-box that produces numbers. i can
report success to
use it to measure distances down to cm accuracy. between a
headphone and
mic. walking around in a room.

the current implementation is obviously not tailored for
that, but this
old satellite-idea seems to be versatile... - Do you know
who "invented"
it ?

robin



(sorry for top-posting; seemed appropriate here

>> Does the phase-measurement depend on electrical
characteristics that get
>> "blurred" in mid-air? - Is there
documentation for the algorithm used in
>> jdelay ?
> 
> That will be difficult to find. The algo used is one
developed some 40
> years ago - and still used today - to measure the
round-trip delay (and
> hence distance) between a satellite and a ground
station.
> 
> It requires a transmission channel with a linear phase
characteristic
> (same delay at all frequencies). This is easy to have
on a space link.
> On the other hand it can be made to work with very weak
signals and
> lots of noise, and with delays that are not constant
but a function of
> time. The equipment I designed while at Alcatel will
operate reliably
> with an input S/N ratio of -40 dB (yes, _minus_ forty)
and will provide
> an accuracy of a few centimeters (depending on how long
you measure).
> 
> The algo uses a number of tones with frequencies F0,
F1, F2, ...
> and measures the phase difference between input and
output for each
> of them. Let P0, P1, P2, ... be those measured phases,
expressed in
> _cycles_. Let D (in seconds) be the delay we try to
measure, and let
> , 0 <=  < 1  be the fractional part of x,
i.e. x - floor(x).
> The algo calculates V = D * F0, the delay expessed in
cycles of F0.
> 
> Clearly we will have Pi = { D * Fi } for all i.
> 
> V = P0 
> 
> This is the first estimate. The accuracy depends only
on how good
> the phase measurement is done, but it is of course
ambiguous if the
> delay can be more than one cycle. 
> 
> The other tones will be used to resolve this ambiguity.
We select
> F1 so that
> 
> gcd (F0, F1) = F1 / 8.
> 
> Now consider the value N1 = 8 * P1 - P0. This will be
an integer
> in the range 0 to 7, plus some fraction due to
meassurement error.
> So we set
> 
> V = V + round (N1)
> 
> If the delay can be longer than 8 cycles of F0 then
this is still
> ambiguous. We select F2 so that
> 
> gcd (F0, F2) = F1 / 64.
> 
> Now consider N2 = 8 * P2 - P1. Again this will be an
integer in
> the range 0...7 plus some measurement error. So we set

> 
> V = V + 8 * round (N2)
> 
> We now have a measurement range of 64 cycles of F0. The
other
> tones are used in the same way to multiply this range
by 8 for
> each tone used. until the ambiguity-free range is large
enough.
> 
> In jdelay F0 is Fsample / 8. The '??' are printed when
the distance
> between Ni and the nearest integer is more than 0.2,
which indicates
> that the signal is not reliable.
> 
> 

Fons Adriaensen wrote:
> On Wed, Apr 11, 2007 at 01:27:27PM +0200, Fons
Adriaensen wrote:
>
> some typos...
>
>> gcd (F0, F1) = F1 / 8.
>> gcd (F0, F2) = F1 / 64.
>
> should be:
>
> gcd (F0, F1) = F0 / 8.
> gcd (F0, F2) = F0 / 64.
>
>
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