A Closer Look at Hi-Fi Video
One of the facts of life about writing for magazines and
newspapers is that things are ephemeral. Sure, a number of enthusiasts file their old
copies of favorite magazines for future reference, but mostly an article is trashed as
soon as the next issue hits the stands. On a website like this, however, readers can check
the archives, and discover material that was written months or years ago. When they take
the time to comment on what they've read, it sometimes takes a while for me to figure out
what they are talking about.
Case in point: Recently Kip C. Wells e-mailed me about some
comments I had made about VHS Hi-Fi recording and its predecessor, Beta Hi-Fi. He quotes
me as saying, "Perhaps rashly, Sony claimed that while the technique could be adapted
to Beta, there was no way it could be applied to VHS. Predictably, VHS Hi-Fi appeared
shortly thereafter with all the same benefits, even if achieved in a slightly different
manner." After some searching, I discovered he was referring to a piece entitled "Hi-Fi VCR's Hidden Audio Capabilities," which
appeared in June, 2002.
The point of Mr. Wells's letter was to describe in some
detail the precise differences between the two systems. It's a very good primer on how
these improbable techniques work, even if a bit technically heavy for some, so with his
permission, I thought I'd share it with you.
"VHS Hi-Fi and Beta Hi-Fi work using frequency
modulation, but the main difference is how these formats actually recorded the audio to
the tape. This difference is actually quite significant and gave VHS Hi-Fi and Beta Hi-Fi
very distinct advantages and disadvantages.
"NTSC Beta Hi-Fi relies on frequency multiplexing,
inserting an AFM [audio frequency modulation] subcarrier between the luma and chroma
undercarriers in the video signal. This is advantageous in one way because, since the
hi-fi and video are on the same track, you only have to factor video tracking for both
video and hi-fi. But Beta Hi-Fi does suffer from a 60Hz switchpoint noise that manifests
itself as a 60Hz hum, especially if the video hits any bright points. The problem is
exacerbated if the tracking is not quite right and/or if the hi-fi tape was recorded on a
different machine.
"Also, pre-Beta Hi-Fi decks would have problems with
slight video noise in the picture because of the AFM subcarrier. Newer non-hi-fi Beta VCRs
were designed to factor the hi-fi signal and not process it with the video track in the
video process circuits.
"VHS, both PAL and NTSC, does not have sufficient
bandwidth in between its luma and chroma undercarriers to accommodate an AFM subcarrier
within the video signal. But perhaps the idea for JVC to make VHS Hi-Fi a reality
unwittingly came from Sony.
"PAL Betamax also has the same problem as VHS when it
comes to adding the AFM subcarrier to the video signal: insufficient bandwidth between the
luma and chroma undercarriers. What Sony did in this particular case was use a different
process for recording AFM onto a PAL Beta cassette: depth multiplexing.
"In depth multiplexing, two extra heads are installed
in the helical drum. On recording, the audio track is recorded first on the tape. Then the
video track is layered on top of it. Since the audio and video carriers occupy different
frequency ranges, the audio and video tracks do not interfere with each other on the
magnetic tape. Because of the rather interesting quality of magnetic tape in how
information is recorded on it, a higher-frequency video track would be located closer to
the surface of the tape while the lower-frequency hi-fi track would be located further
away from the surface of the tape. Reading the depth-multiplexed signal would be
relatively easy, as all the video and hi-fi sections would have to do is deal only with
the carriers they were designed to work with, within their specific frequency ranges.
"Turns out that the process used for PAL Beta Hi-Fi
also works with VHS, and could also explain how JVC was able to develop a VHS Hi-Fi
prototype in short time. The advantage with VHS Hi-Fi is that the 60Hz switchpoint hum is
lower than [with] NTSC Beta Hi-Fi. The disadvantage is that with VHS Hi-Fi, tape-wear is
slightly accelerated, as the tape would be scanned with two additional heads for hi-fi.
"Also, because you have separate audio and video heads
in the helical drum, hi-fi tracking may not always be inline with the video tracking,
which can be greatly pronounced when playing hi-fi tapes recorded on a different hi-fi VCR
at a lower linear speed. What may be a good tracking setting for the video may not be so
great for the hi-fi, and vice-versa."
Thanks for sharing that background.
...Ian G. Masters
ian@mastersonaudio.com
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