[vorbis] Low bitrate encoding

[Marshall Eubanks]

"Dr.Joerg Bergmann" wrote:

> On Tue, 9 Jan 2001, Marshall Eubanks wrote:
>
> >
> > This is how the samplers that I am familar with work - it's
> > called accumulation rather than integration, frequently.
> >
> > One detail is what time is reported - the end time, or the middle
> > of the sample.
>
> This is _no_ question, an integral (or accumulation) is one identic
> value for the whole interval. An average is an average, there are no
> special averages for the middle and the end.

You misunderstand. Each sample has a time tag. Sometimes the time tag
is for the end, sometimes for the middle, or for the beginning or some other
time. I have run into problems before when times don't match up
because one time tag is for the middle, another for the beginning.
As I said, this is  a detail.

>
>
> >
> > The other detail is what _analogue_ averaging (AKA low pass filtering) is
> > applied. Generally (but not always) frequencies that are above the
> > Nyquist frequency are low pass filtered.
>
> Thats OK. But sampling itself adds some kind of low pass, an additional
> analogue low pass will only reduce signal/noise ratio. An analogue
> low pass may be (after fourier transformation) described by some
> weighted averaging (weighting function is zero for negative times due to
> signal theory, non-zero for positive values, in general none-zero for all
> future times!)
>

This is backwards. EVERY electronic system that has an analogue step is a low pass filter.
(This includes all microphones, guitar pick-ups, etc.) Generally, the analogue system is
designed so that the electronics do not pass frequencies much above
the Nyquist limit. Sometime, for various reasons (such as using off the shelf components vs
a redesign), they are not.

Frequently, A/D systems also involve a Nyquist limit analogue low pass filter to limit aliasing.
This can also actually reduce noise (& thus increase SNR).

>
> J"org Bergmann, Dresden, Germany
> email at jbergmann.de


--
                                 Regards
                                 Marshall Eubanks

T.M. Eubanks
Multicast Technologies, Inc
10301 Democracy Lane, Suite 410
Fairfax, Virginia 22030
Phone : 703-293-9624
Fax     : 703-293-9609
e-mail : tme at on-the-i.com     tme at multicasttech.com

http://www.on-the-i.com http://www.buzzwaves.com

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