[Vintage-Audio] What Did They Really Do?

Thu Feb 24 22:28:40 EST 2005

The Teac A-5500 is 1976-1978 and the Teac AN-180 about 1974.

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From: Gerry Steffens <gsteffens at pitel.net>
To: 'Vintage home and professional audio equipment from 1975 back'
<vintage-audio at mailman.qth.net>
Subject: RE: [Vintage-Audio] What Did They Really Do?
Date: Thursday, February 24, 2005 10:16 PM

What is the vintage of the machine?  I believe that undesignated devices are
B but the vintage would help.

Gerry

Collecting & Restoring since 1959
Gerald Steffens P.E.
Oronoco, MN

-----Original Message-----
From: vintage-audio-bounces at mailman.qth.net
[mailto:vintage-audio-bounces at mailman.qth.net] On Behalf Of Duane B.
Fischer, W8DBF
Sent: Thursday, February 24, 2005 9:13 PM
To: Vintage home and professional audio equipment from 1975 back
Subject: Re: [Vintage-Audio] What Did They Really Do?

Gerry, 	

I have a high end Yamaha single cassette deck from 1991 that has Dolby A, B
and
C. buttons on it. The others have Dolby B and/C.	

What about the Dolby on the Teac A-5500 reel to reel deck (their top of the
line
consumer at $800 then) or the Teac AN-180 external Dolby and Equalization
units?
Not marked as Doby B or C.	

Duane W8DBF	

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From: Gerry Steffens <gsteffens at pitel.net>
To: 'Vintage home and professional audio equipment from 1975 back'
<vintage-audio at mailman.qth.net>
Subject: RE: [Vintage-Audio] What Did They Really Do?
Date: Thursday, February 24, 2005 10:02 PM

First Dolby A was never available in the home market place as far as I know,
only in commercial equipment as in radio stations for example.  It was also
far more expensive.

Dolby B was the simplest of the noise reduction systems and its effect could
potentially be partially overcome with tone controls.  Tape hiss starts at
some frequency and the magnitude of it raises as frequency increases up to
more or less a plateau.  This rise begins at about a fixed frequency, say 6
khz.

If one made hiss a fixed magnitude signal that begins at 6khz and is 10 db
in magnitude.  You want to record a fixed 10khz signal on tape that is 15 db
in magnitude.  Without Dolby B you would have a signal on tape that
approximates 10 db of hiss and 15 db of sound.  So that is what you get
during playback or a signal to noise ratio of 15 to 10 or 1.5 to one.

With Dolby B you boost all signals above 6khz by an amount, we will assume a
flat 3 bd for this example.  Assuming no losses, you now have the same 10 db
of noise and 18 db of signal.  Clearly if you listened to music boosted this
way it would sound tinny and harsh because of the amplified highs.  So now
during playback you cut the signal above 6khz by the same 3 db.  You now
have 7 db of noise and again 15 db of sound.  The signal to noise ratio is
now better than 2 to one.  Hence you hear less noise.  With a properly
calibrated Dolby B system there should be no difference in sound with
quality equipment or in the worst case one should be able to compensate with
an equalizer.

Dolby B operates on this principal.  And, if a recording is recorded with
Dolby B you can use tone controls to cut the highs some and it is still
listenable (it might not be balanced or original but you can still listen to
it).  Think of walkman devices with Dolby and car stereos without it in the
70s, using tapes made on a home deck with Dolby B.

Dolby C, like DBX, was an encoding boost system and then decode on playback
system.  DBX accomplished the noise reductions through compression and the
decompression as I recall.  Both were more complex and required far more
calibration, record to playback to work and sound good.  Such calibration
was in the quality of the equipment.  Both of these sounded lousy to anyone
if you attempted playback on a machine without the proper decoder.  I have
been through the encoding algorithm for each at one point but don't remember
them off the top.  Also they were not conducive to simple explanations like
B.  I might have my thoughts on these two reversed, don't remember.

Dolby B effect can be created by using an equalizer in the record circuit.
One can boost highs according to a plan and then use a second equalizer to
cut the same frequencies by an equal amount in the playback circuit (I even
had on that had a switch to throw it from record circuit to the playback
circuit and invert the boost set in the record circuit - so you just pushed
a button on the equalizer to move to playback and un-boost).

Gerry

Collecting & Restoring since 1959
Gerald Steffens P.E.
Oronoco, MN

-----Original Message-----
From: vintage-audio-bounces at mailman.qth.net
[mailto:vintage-audio-bounces at mailman.qth.net] On Behalf Of Duane B.
Fischer, W8DBF
Sent: Thursday, February 24, 2005 7:50 PM
To: Vintage home and professional audio equipment from 1975 back
Subject: Re: [Vintage-Audio] What Did They Really Do?

Great stuff Gerry!	

Which raises the burning issue: What is the difference between Dolby A, B,C
and?

I have never cared for Dolby, maybe my keen hearing due to highly focused
listening always noticed the loss in the higher frequencies. Maybe my
equipment
was good enough that I had minimal noise to begin with, so what could be
gained
by using Dolby was not a factor in my case. I did notice a reduction of tape
hiss and the like, but also the clipping off of the crispness of the high
notes.
That always bothered me.	

Duane W8DBF	

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