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From: floyd@humu.UUCP (Robert W. Floyd)
Newsgroups: sci.electronics
Subject: Re: FM audio recording?
Message-ID: <424@humu.UUCP>
Date: Thu, 1-Oct-87 14:38:18 EDT
Article-I.D.: humu.424
Posted: Thu Oct  1 14:38:18 1987
Date-Received: Mon, 5-Oct-87 07:19:45 EDT
References: <430@root44.co.uk> <180@usl>
Organization: Naval Ocean Systems Center, San Diego
Lines: 24
Keywords: FM tape
Summary: FM audio was developed for a different purpose


FM recording was originally developed for recording very low frequency
signals on instrumentation tape recorders.  A side advantage was its relatively high dynamic range
(approximately 46 dB back in the 60's).  This was much better than the
26 dB dynamic range of the analog recordings.  The record heads were
optimized to achieve the greatest possible bandwidth at a given speed, and this
was done at the expense of dynamic range.  An additional problem was that
this performance was achieved partially by making the recording heads very
thin, so that they had to be replaced every thousand hours or so.
	The net result was that by 1970 it cost approzimately $10,000 to 
purchase an FM recorder that could achieve an audio bandwidth of 20kHz at
60 inches per second, and achieve a dynamic range about 3 dB greater than
consumer equipment.  The Navy might pay that much for 3 dB, but the average
consumer wouldn't.  In the meantime, many engineers, particularly in the
telephone business, had realized that the information content of a signal
meant for human ears could be compressed since humans were sensitive to
relative, not absolute changes in amplitude.  The best known implementation
of this idea is Dolby, which can be done quite inexpensively.
	In summary, the advantage of FM has been dynamic range.  The cost
	has been bandwidth, which is relatively expensive, in terms of
	heads, tape speed, mechanical complexity, etc.  Since effective methods
	such as Dolby and digital recording have been developed to
	improve dynamic range, it is unlikely that FM audio recording will
	every be commercially viable.