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From: rdp@teddy.UUCP
Newsgroups: net.audio
Subject: Re: Question About Tape Freq Response
Message-ID: <1371@teddy.UUCP>
Date: Tue, 1-Oct-85 15:24:20 EDT
Article-I.D.: teddy.1371
Posted: Tue Oct  1 15:24:20 1985
Date-Received: Sat, 5-Oct-85 07:35:38 EDT
References: <7000007@petrus.UUCP>
Reply-To: rdp@teddy.UUCP (Richard D. Pierce)
Organization: GenRad, Inc., Concord, Mass.
Lines: 45
Summary: 

In article <7000007@petrus.UUCP> copp@petrus.UUCP writes:
>It appears from the advertising (am I too gullible?) that the
>tape manufacturers are almost continuously improving their product,
>at least the high end response.  On one hand this sounds great;
>on the other hand, would you applaud if speaker, cartridge, amp,
>etc., manufacturers kept boosting the highs by an additional db
>every six months?  If nothing else, doesn't this ever-increasing
>high-end response play havoc with Dolby tracking?

Not so. What is normally being improved is the high frequency response
at high recording levels. The low-level response (normally measured at
something like -20 db re 200 nW/cm, which I believe use to be the Dolby
0 reference level) remains pretty much unchanged. If this changed, this
would not only play havoc with noise reduction, but would then screw
up general high frequency response.

Normally, when a deck is set up, the playback amp frequency response is
adjusted by playing back a reference tape. The adjustment is made in
essentially a tape formulation independent manner (that is, the playback
EQ is, for normal iron-based tapes, fixed, and only adjusted to meet an
agreed upon standard. For reel-to-reel decks, there is the NAB and the IEC
playback EQ standards).

Now, inorder to ensure as linear an input to output transfer function as
possible, it is the record EQ that is diddled with to ensure the signal
that finds its way onto the tape matches this playback standard as closely
as possible. This ensures that people can exchange tapes from one machine
to another. 

Now, most of this adjustment is done at a fairly low record level (see above)
at higher levels, two things come into play. First, the ability of the tape
to linearly record signals at high frequencies and high levels is limited. The
tape can only record so much signal, and trying to exceed that results in 
a "soft" clipping effect, and reduced high frequency information. Secondly,
note that whatever signals are recorded at these levels, the noise reduction
system (Dolby, DBX, ANRS) does not function at these levels because it does not
need to. The signals are high enough to mask any noise present. The noise
reduction effects are only present at lower levels.

The above is, admittedly, a gross oversimplification, so I hope that the
cognizetti out there don't start flaming accordingly.

Dick Pierce

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