A VHS player was originally a rotating cylindrical drum spinning on an axis at a canted angle to a video tape pulled through a tape path. It had a seperate erase head before the drum and seperate audio head and speed signal control head after the drum.
The erase head simply "cleaned" the tape of any magnetic patterns before the tape was used by the drum heads to lay down new video information in angled or tilted tracks across the width of the tape. Each track represented one scanline across the width of the Television picture.
After the scanlines were recorded to the tape, the audio head would "skim" a small width at the edge of each scan line for its use to store sound information across the Left side of each scanline, and the Control track "skimmed" a small width at the opposite edge for its use to store tracking information across the Right edge of each scanline.
On tape this meant the very Top edge of the tape had audio information, and the very Bottom edge had control tracking information.
This didn't effect the picture much because normally those edges on the Left and Right of the Television image are hidden away under a varying sized bezeled picture frame around the central viewable image on the Television.
The audio track was mono (not stereo) in the first VHS standard. Dual tracks and HiFi were two different standards that would come much later. And by most relatable audio standards quite low in frequency bandwidth.
The control track was a timing signal which when played back acted as a feedback signal to the tape path drive motors to servo-regulate the speed of the tape as it moved through the system, so that scanlines and signal arrived at the playback Television at the correct rate in order to regenerate the video signal. If the signal was too slow, drive electronics sped the tape up, if too fast, drive electronics slowed it down.
Frame rate tracking, indexing or accuracy were never part of this Control Track mechanism.. certain specific manufacturers replaced this track with their own variation to encode extra information and provide either their own version of a frame or location tracking system.. or re-implemented the Society of Motion Picture and Television Engineers (SMPTE) frame accurate time code on the control track.. but this was rare and non-VHS standard.
Basically the VHS "standard" was feature-less and made to be cheap and easy to implement across many manufacturers and vendors. Broadcast quality producer level features were reserved for much more expensive and purpose built equipment.. or, non-VHS (non-Home) video equipment.
DV (for Digital Video) would later re-think and cross pollinate ideas from Broadcast features and Home video features to create a new "incompatible" video standard that would be low cost enough to be accessible to the Home video market.. mostly by way of the "Camcorder".. but it was not VHS... even though it did borrow some of its ideas to achieve its goals.
The confusion often led consumers to assume that DV was an upgrade or improved version of VHS, when actually it offered video with a different set of goals and compromises. In some ways better, others worse and definitely not media compatible.
Originally there were two video heads on the spinning drum. The M shaped tape path wrapping the tape around the drum allowed one head to complete one scanline copy to the tape angled from top to bottom along the length of its travel path. the next head would begin its traversal as the last head left the tape path and rotated out of contact around the backside of the drum.
So the minimum number of VHS player video heads was "two".
VHS had a specific tape speed, meaning the size of the video heads were fixed to optimize the size of the magnetic track based on this speed. The inital speed was called "SP"
When longer length video recordings were made possible by changing the tape "speed" the size of the video heads had to be changed to make the width of the magnetic tracks smaller.. so (two) additional heads were added for "LP" (Long Play).
And then additional heads might be required for "EP" (Extended Play).
The LP tape speed dropped out of favor and choice became SP or EP, even if SLP was advertised.. instead of changing tape speed.. the length of the tape was increased per cassette for SLP.. so tape speed was the same for SLP as EP... so in the end only two sets of head sizes were normally included developing into the (4-Head VCR as a consumer staple).
A "Flying" Erase head was also added to the drum so that the point at which an "Editing" Cut or Insert could be made closer to the actual point at which a video was stopped or "frozen" when playing and then engaging recording from a second VCR. This was a "Prosumer" feature rarely used by most people.. but made "Linear" editing near real time during playback on a VCR used for both playback and recording from other decks possible.
Previously the former Erase head was not on the drum and offset far enough that at the point where a frozen frame to new recorded video might overlap, or include "magnetic" bleed through of signal from a previous recording or random noise on the tape with a pattern.. leading to chroma aberations at the insert point. By moving the erase head closer to the actual recording head this problem could be minimized.
So that added a (5th) possible head to the VCR (not counting the original Erase and Audio and Control heads that were "not" on the drum)
Finally "offical" stereo was added to VHS, by [deep] recording an opposite angled, slanted set of audio tracks at a different frequency and magnetic strength to the video tracks. This minimized crosstalk between the signals and a bandpass filter could be used to further reduce the perceived "noise" in the video signal from the audio signal in the central portion of the tape normally used only for video signal.
Although this made stereo possible, at near CD quality.. it also introduced a perceived "buzzing" or possible interference when electrically switching from one head to the opposing audio head on the drum. Further bandpass filters were used to attempt to reduce the "noise".. but circuitry degradation over time meant the buzzing could increase over the years with older equipment. A technique some people used was to switch off the stereo track and fall back on the (mono only) track recorded for backwards compatibility (unless specifically used for different content like alternative languages, or narration it was a duplicate of the stero track) at the edge of the tape.. which would not have any switching-buzz noise.
The "mono audio track" is also sometimes called the "Linear audio track".. chosing between them is a good thing, [mixing] them is usually a bad thing (if they are backwards compatibility duplicates of the same sound track) primarily because the two are physically located at different points along the tape path..any imperfection (which is very common) will introduce a slight difference or signal delay.. that manifests itself as a (tunnel) echo effect in the audio when both tracks are being played in [mixed mode].. over time on older equipment or older tapes this effect increases.
[Mixing] the stereo and mono tracks did have a purpose however, if they contained different content.. for example, an orchestral music content recorded on the stereo tracks, and a speaker, dialogue or narration content recorded on the linear mono track. In this way it was use as a simple audio "mixer" setup, and allowed for post-production with a single vcr, often called ADR- "Automated Dialog Replacement" in the film industry.. its also known as a "looping" or "loop session" recording to improve the sound quality of dialogue. Today however this can be acomplished with much greater ease in computer software mixers for working with sound and video.
The legacy effect of "mixing" stereo and mono tracks sources that contain the same content however is not recommended.
So adding two more drum heads brought the total on the drum up to (4 + 2 + 1 = 7) for the video, audio and flying erase head.. and if LP is actually supported (6 + 2 + 1 = 9 heads) .
In the end 4 + 2 +1 was more normal and advertised as 4-head plus HiFi audio plus a "Flying Erase head".. if the product was a high-end model intended for limited Insert Linear video editing between two or more VCRs.. also called "decks"
