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From: brown@nic_vax.UUCP
Newsgroups: net.video,net.analog
Subject: Re: How does Beta HiFi audio avoid head switching transients
Message-ID: <106@nic_vax.UUCP>
Date: Mon, 11-Mar-85 00:50:25 EST
Article-I.D.: nic_vax.106
Posted: Mon Mar 11 00:50:25 1985
Date-Received: Tue, 12-Mar-85 21:11:36 EST
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Organization: Nicolet Instrument Corp. Madison WI
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> 
> 	Can anybody explain to me how the Beta HiFi system avoids
> corrupting the stereo hi fi sound signals when it switches from
> one spinning video head to the other (which should produce some sort
> of phase and amplitude transient every 60th of a second in the FM sound
> subcarriers). ?

I have a VHS Hi-Fi system, the JVC HRD-725U.  I also contain the service
manuals.  The edition date of the one that I have is dated February, 1984.
Unfortunately, they did not give the description on what it done about
the head switch-over.  But I will quote what is printed:
	A detailed description on the holding circuit for compensation of
	FM signal dropout and on the head switching signal generator circuit
	will be given later.
One could infer two things from this statement.  The techniques of how they
do it are a secret for now.  But, it sounds as if during head switchover
they repeat a portion of the signal.

> 	This transient is invisible in the video because the switch
> between heads is in the vertical blanking interval where the machine
> generates sync anyway.  Audio, on the other hand, is continuous

The above statement is partially true.  On all helical scan systems, the
video head switch-over is done 6.5 horizontal lines (+/- .5) before the
vertical equalization pulses start.  Roll down the picture using the
vertical hold and you will be able to see it.  If your set has a vertical hold.

A side note on Hi-Fi systems.  If you read the consumer video magazines and
have seen the add from Sony, you have noticed that they say that their system
is better than the VHS system.  I won't debate that here.  But I will tell
about what Sony will not tell the consumer.

As you know, or will know, Sony puts their FM carriers between the color
sub-carrier and the video sub-carrier.  It is done using the same heads
that the video uses.  On the VHS system, since there isn't enough room to
add the FM sub-carriers to the video sub-carriers, separate heads are used.

In Europe, the Beta Hi-Fi system from Sony was not released as it was in 
the states.  The reason?  Since the PAL and SECAM systems are of wider
bandwidth, the FM sub-carriers will not fit within the video sub-carriers.
Sony's solution?  Use the same technique that is used for VHS, i.e., separate
FM audio heads.  So much for Sony's idea being better.

Mr. Video

The above statement was not a paid political announcement by the VHS camp.