VIDEO CAMERA 1995

 BootLog.co.uk

HomeSoftwareArchiveTop TipsGlossaryOther Stuff

FEATURE

 

HEAD

HEAD COUNT

 

INTRO

Four heads good, two heads bad, or so we are sometimes led to believe, but what is the truth behind the number of heads in VCR and camcorders, and how many do you really need?

 

COPY

We’d better begin by looking at how a VCR works. Long story short: video signals very complicated, contain lots of information, only way to squeeze them onto tape is to have fast ‘writing speed’, i.e. tape head passes quickly over tape. However, uneconomical to have fast moving tape -- cassettes would be too big --  so head made to move as well, by mounting it on spinning drum, wrap tape around it and make head scan across tape in a series of diagonal stripes. The head-to-tape speed on a PAL VHS VCR is a brisk 4.85 metres per second, though the tape moves at a crawl, just 2.34 centimetres per second. This technique is called helical scanning and is the principle used on all domestic and most professional video tape recording systems.

So far so good? It is possible to record video signals using just a drum with just one spinning head but this would involve the tape being wrapped almost completely around the circumference of the head drum. Several early professional and broadcast video systems used the so-called ‘omega wrap’ (after the shape of the omega symbol) but tape threading is complicated and it puts a lot of strain on the tape.

VHS and all domestic VCR formats have a minimum of two recording heads, mounted 180 degrees apart, on the opposite sides of the head drum. This means the tape only has to be wrapped around just over half the circumference of the drum (VHS uses an ‘M’ wrap, after the shape of the tape path), making deck design simpler, and cheaper to build. The heads are tilted at slightly different angles to one another, so that adjacent tracks will only be read by the appropriate head. So what’s the point of any extra heads?

Two head systems are fine for recording and playback at a single fixed tape speed, because each head can be made to accurately follow its own tracks, but what happens when the tape speed changes? This problem arises when the tape is stationery (still frame) or moving faster or slower than normal (slomo, fast play and picture search). Because the tape speed has changed, (or stopped), the path described by the spinning heads will be different too, they can no longer accurately follow their own recorded tracks and cross over onto adjacent tracks on each pass; that produces the characteristic noise bars and picture instability that was common on older machines (and cheaper VCRs today).

It is possible to have passable still frame on a two-head VCR, using heads with extra-wide head gaps, so they pass over more of each track, however, on budget machines the picture is often noisy and unstable, moreover trick-play facilities -- if it has them -- are usually poor quality on LP recordings. Several VCR manufacturers have used 3-head systems specifically to overcome the problem of wobbly still frame on budget single-speed VCRs. The third head is positioned next to one of the main ‘A’ and ‘B’ heads (normally the ‘A’ head). The third or ‘B1’ head is only used during still frame, where the ‘B’ and ‘B1’ heads scan the same track, the image is stable but there can be a loss of detail in the picture.

There’s still a few 3-head machines around but these days most mid-range, twin-speed VCRs have four video heads, or rather two double-gapped heads, each pair built onto a single ferrite chip. The extra heads are optimised for freeze-frame, slomo and LP operation. Further improvements to picture stability and extra trick-play facilities are possible using a variety of techniques, including digital field store memories, elaborate picture processing circuits and precision deck mechanisms.

When it comes to VHS-C and S-VHS-C camcorders it gets even more complicated. They have miniature head drums (41.3mm diameter, as opposed to 62.5mm of a standard VHS head drum); to ensure format compatibility the tape has to be wrapped 270 degrees around the drum’s circumference, the drum rotational speed is increased from 1500 rpm to 2250 rpm, and four heads -- mounted at 90 degree intervals -- are needed for video record and replay, in what is known as an A-B-A-B configuration. VHS-C camcorders with LP facility may have 8 heads (4-pairs). It doesn’t stop there, several top-end machines also have a flying erase head, for clean insert editing, which makes a total of nine video heads, and we haven’t finished yet. But before we move on it’s worth saying that the 8mm (and Hi 8) format was designed from the outset as a 2-head system, with a small (40mm head drum), there are exceptions, but we’ll save those for another day .

That just about covers the video heads, but VHS VCRs and camcorders with stereo hi-fi sound systems need yet another pair of heads on the head drum. They’re positioned 180 degrees apart, offset from the video heads by 138 degrees. The audio heads precede the video heads, recording the stereo audio information, deep in the tape’s magnetic layer; a fraction of a second later the video heads come along and record over the top of the audio signal, in the upper strata of the magnetic coating. During replay the audio heads read the audio signal through the video information.

The extra hi-fi heads takes the head count on a typical stereo VHS VCR up to six, (for the sake of brevity we’ll ignore all the static recording heads...), a stereo VHS-C/S-VHS-C camcorders could -- in theory at least -- have as many as eleven heads, though we haven’t actually come across one with as many as that, yet!

So, to sum up, most 2-head VCRs are single-speed mono machines, the really cheap ones usually have naff still frame, noisy picture search and usually poor LP performance, the better (i.e. more expensive) ones from the well-known Japanese and European manufacturers, usually have wide-gapped heads, and can have quite reasonable still frame facilities. Three head VCRs are usually single-speed mono machines as well, but should have good still frame reproduction. Four head VCRs should have good still frame and reasonable LP picture quality, often with  advanced trick-frame facilities as well (multi-speed replay -- slomo, fast play etc.). Stereo VCRs generally have six heads on the drum (4 video, 2 audio); edit VCRs with flying erase have an extra head, taking the total to 7. Super VHS VCRs and camcorders generally have precision heads that are also used for standard VHS recording and playback. Hope you paid attention, we’ll be asking questions about this later...

---end---

Ó R. Maybury 1994 1410

 

 

 

 

 


 

[Home][Software][Archive][Top Tips][Glossary][Other Stuff]


Copyright (c) 2005 Rick Maybury Ltd.

admin@rickmaybury.com