However; it is plauged by a problem...head switching.
My understanding is that the video heads jump between audio and video duty; since the audio is recorded "under" the video. If you play the tape on the same unit that recorded it; most of the time...you were fine.
Pop that tape in another HiFi machine and you get the same problem I always had with pre-recorded movies; ticking. You'd get this audible ticking in the audio; which I've always understood was due to the rapid switching of head mode.
There is no sharing of duty for the heads - except in NTSC Beta-HiFi.
VHS Hifi machines have two video heads, and two audio heads. The reason there are two heads for each is the simplified loading. The tape can only wrap 180-degrees around the head drum. To get a continuous recording, two heads are required that switch on and off alternately. C-format reel-to-reel recorders do a nearly 360-degree wrap of tape around the head drum so do not need to double up on head count. (*)
With the half-wrap of tape around the drum, there is an instant where the heads change over (both the audio heads and video heads). If this is not perfectly aligned time-wise with the physical condition of the head leaving the surface of the tape, you get a momentary loss of signal, 25 or 30 times a second, with the consequent buzzing. With video it doesn't matter, since it can be timed to coincide with the vertical blanking period, when no video needs to be recorded anyway.
Having two heads in use alternately allows another trick called azimuthal recording: the diametrically opposite heads can have a different gap azimuth. This allows the two adjacent tracks written diagonally onto the tape to be so close as to even overlap slightly, leading to very efficient use of tape, and making 'LP' and 'SLP' modes workable.
NTSC Beta specifications have relatively widely spaced luminance and chrominance subcarrier sideband frequencies. There is enough space between them to put in the two audio FM carriers, and record Hifi audio with the video heads. All 4 carriers (luma, chroma, left , right) are recorded by only two heads - but the carriers are recorded simultaneously, there is no switching or cycling of function.
PAL Beta, and all VHS flavours have closely spaced luma and chroma carrier sidebands, so an additional pair of heads is required to make a depth-multiplexed recording, where the audio is deep, and the video overwrites it, only erasing to topmost layer of video signals. The audio heads read 'through' the video recording on playback.
Machines that claim 4 video heads do so because they need 2 extra heads for noise-free long-play, or noise-free still-frame. They still only use 2 video heads during a normal recording.
(*) VHS-C does a real piece of magic: The head drum is physically smaller (42mm) than the standard VHS drum (62mm), and the wrap angle is 270 degrees. Using 4 video heads and 4 audio heads, and a head drum speed 1.5x faster than normal VHS, it can make a recording that is physically identical to a standard VHS recording, including the hi-fi audio.
