[Table of Contents]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [ARSCLIST] Dynamic-frequency Range

From: Patent Tactics, George Brock-Nannestad

On 22 October 2006 Don Cox wrote (but it arrived here with an extreme delay 
of several days).

> On 21/10/06, George Brock-Nannestad wrote:
> > ----- if you read Thomas Stockham's "blind deconvolution" paper in the
> > IEEE Proceedings (not to hand, so no precise reference, alas) you will
> > be disappointed to find out where his logic breaks down. Essentially
> > he says (my interpretation) "Caruso was a tenor, but only recorded
> > acoustically. Jussi Bjoerling was a tenor, and he recorded
> > electrically, so the linear distortion to the tenor voice via the
> > acoustic recording process may be determined by measuring tenor tones
> > on corresponding acoustic and electrical recordings. And since there
> > was only one recording horn, you can then apply the inverse filtering
> > to the acoustic sound."
> This is completely different from what I was suggesting. You need an
> impulse signal such as the sharp click from a spark to derive a function
> for deconvolution.

----- actually, it was Michael Gerzon who was the first, in 1974, to apply 
this approach to sound recordings in discussion with Richard Heyser ("A 
Rosetta Stone For Audio?", Journal of the Audio Engineering Society, Vol.22, 
No. 5, p. 251). Gerzon's expression was "the sound of a clap handing", 
playing on the convolution principle.

Unfortunately, the energy available for the deconvolution is not very large 
with just one sharp click, because if there is non-linear distortion (i.e. 
overload from the same sharp click), the calculations do not hold up.

> Simply comparing one tenor with another and adjusting some EQ is not at
> all the same.

----- precisely, but that was Stockham's approach

> +++++
> Imagine an image which has been spoiled by camera movement during the
> exposure (or in the case of an astronomical image, by air movement).
> There will be a certain smudge which is applied equally to each pixel of
> the image. The image is said to be convolved with this "smudge"
> function.
> In reality there will be a complex function involving optical
> aberrations, light spread in the sensor, and movement.
> Provided there is some point source in the image, such as a star, which
> we know should be represented by one pixel, then the function can be
> measured from the actual multi-pixel image of the star. It is then
> mathematically possible to deconvolve the image and remove the
> aberrations and smudging (in practice, to some extent).

----- actually, Stockham's paper cited above also deals with blind 
deconvolution in the optical field, and I believe that his results were more 
valid here

> The nearest audio equivalent to a star is an impulse signal such as the
> sharp click generated by a spark at high voltage. 
> The big problem is that noise makes it hard to derive an accurate
> convolution function. However, averaging from a number of clicks would
> help.

----- no doubt, but any energy left "hanging" (reverb) must have died out 
before the next click

----- a click, such as a scratch in a record may be used to measure the 
transfer function of a pickup-cum-preamp, if the output is analyzed. However, 
also here we have the problem of non-linearity. For this reason sweeps are 
usually used, or more recently multi-tone signals of a known composition. In 
1997 and for a few years I worked in the AES Standards Committee to create 
some standards for a multi-tone signal useful for mechanical sound 
recordings, but the idea did not catch on.
> > ----- ...................................
> > 
> > ----- ...................................
> > 
> > ----- ..................................
> > 
> > ----- finally I should say that if you correct e.g. the Melba distance
> > test recordings by inverse filtering the recording horn, diaphragm,
> > and stylus bar, with headphones you have the most uncanny feeling of
> > her receding and then approaching, even to use a reasonably low voice
> > directly into your ear. The sheer depth of that simple recording is
> > fascinating when it is correctly reproduced. I have no idea if the
> > commercial CD versions permit this.
> I'm not sure what you mean by "filtering" but I suspect it is some kind
> of frequency-dependent approach. Deconvolution is not a form of EQ.

----- I beg to differ, deconvolution is obtaining the ideal filter for the 
linearly distorting "black box" that has done "something" to an impulse 
function. By digital signal processing any approximation may be made to this 
ideal filter, so it is in effect a form of EQ. 

> The Melba distance tests do give a strong effect of her receding on the
> Ward Marston Naxos transfers, on my equipment.

----- this speaks well of both the transfer and your equipment.

Kind regards,


[Subject index] [Index for current month] [Table of Contents]