Technical Question (was Digital Talking Book Standard )

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	One part of the NISO standard I read said that players
should be able to allow the user to speed up the recording while
restoring the pitch.  In other words, the players should be able
to deliver compressed speech much like what we presently have
with the variable-speed Talking Book machines and tape players
and the electronic pitch restoration devices which have existed
for several decades.

	One question I have for the group is whether or not it is
possible to even somewhat continuously vary the time base of the
sound cards found in most computers?  I know that most sound
cards can be set to sample at 1 of a number of different rates,
but the rates are still rather fixed at multiples of 8 kilohertz
sampling and multiples of 11.025 kilohertz sampling rates.  The
8-KHZ rate is good for communications-grade audio such as would
be found on 2-way radio and telephone systems while the rates
based on 11.025 KHZ samples can neatly fit in to the 44.1 KHZ
compact disk standard.

	My question is whether or not it is possible to sample at
rates that are deliberately non-standard in order to simulate the
effect of a continuous speed control.

	This would also make it possible to rescue damaged tapes
by recording them at a sampling rate that is off by enough to
compensate for a recorder that is not quite recording at the
correct speed.

	This may sound totally off-topic, but a digital Talking
Book player has to be able to vary its sampling rate in order to
emulate a speech compressor.

	There are actually two flavors of compression which have
been used in the past.  One is to speed up the tape or record and
then run the audio through a pitch correction circuit so it
doesn't sound like "The Chipmunks."  The other compression scheme
is one in which the tape is played at normal speed through a
device that has a second recorder whose tape is stopped and
started very quickly such that pauses longer than a set length
are removed.

	Of course, the pause-removal system was less popular
because somebody had to make the compressed recordings.  The
pitch corrector can be run right wen it is needed and run on the
original recording.

	If sound cards can be made to slide from one sampling
rate to another, then we should be able to have both kinds of
compression on audio recordings.

	In reality, I know that a variable sampling rate is more
than likely going to be a series of small steps, but if they are
small enough, it gives the appearance of continuous variability.

	I have played in the past with the timer/counter device
that controls the pitch of the P.C.'s speaker and that pitch is
set by stuffing a 16-bit number in to a counter that divides a
roughly 1 MHZ clock by whatever is in the counter.  By the time
one is in the audio range, it is very hard to tell the difference
between one step and the next.  I am hoping there is something
similar in most sound cards that one can mess with to get odd
sampling rates.

	I hope some of you experts can please fill in the vast
holes in my knowledge base, here.

Martin McCormick





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