GB2165986A - Video recorder/players - Google Patents

Abstract

The disclosure relates to a video recorder/player having control means for selecting both fast and slow forward and rewind of the recording. The particular field being viewed is monitored and the device is provided for searching for a required field including means to assess the time required to reach the field from the current field. in order to minimise the time to reach the required field means are provided to select only fast movement if the assessed time is greater than a predetermined time with means to re-initiate operation of the control means if the recording does not reach the required field after the initial search. Means are provided to increase the predetermined time to a longer period so that eventually slow advance or rewind is selected to bring the recording to the required field.

Description

SPECIFICATION Improvements in or relating to video recorder/players This invention relates to video recorder/players and in particular to those which use tape as the recording medium.

The problem this invention is addressed to is: "If we are at some point tl on the tape, how do we get to point t2 as fast as possible? Clearly the fastest way is to use Fast Forward (if tl is before T2) or Rewind (if tl is after t2).

However tape position TIC value-see note below) can only be readin Play, Scan and Review modes. We therefore need a model which allows us to get to a point just short of, or just after, t2 so that we can enter one of these modes until t2 is reached.

Written into each field of the tape (in the non video part) is the Transport Indicator Counter (TIC). This is 0 for the first 50 fields, 1 for the next 50 and so on. Thus, since there are 50 fields per second, the TIC represents the time (in seconds of play time) from the start of the tape.

MATHEMATICAL MODEL Details of how the model was derived are given in Appendix 1.

The relevant formulae are as follows: Tf= [ V(at2+fl)-V(atl +ss) ] /Vf+df Tr= [ V(a(tm-t2)+fi)- V(a(tm-t1 )+fl) ] /Vf+# T=required FF (Tf) or RW (Tr) time in secs. to get from TIC value tl to TIC value t2. The Tr formula is used when t1 > t2.

V=rotational speed in FF (Vf) or RW (Vr) modes in revs/second.

a= sown. See below for w and s. This is a constant for a particular class of VCR (VHS, Betamax, etc.).

ss= (r/w)2. See below for r and w. This is a constant for a particular tape size (E180, etc.) within a particular class of VCR (VHS, Betamax, etc.).

t= TIC value in secs. This is in effect the playing time in secs from the start of the tape to the point in question. tl and t2 are used to represent the current and desired tape positions whilst tm represents the highest TIC value on the tape.

a delay in secs between selecting the desired mode and the start of tape motion assuming the VCR was in stop mode to begin with. Af and br are for FF and RW modes respectively.

s= tape playback speed in metres/sec. This is a constant for a particular class of VCR (VHS, Betamax, etc.).

r= spool radius in metres. This is a constant for a particular tape size (E180, etc.) within a particular class of VCR (VHS, Betamax, etc.).

w=tape thickness in metres. This is a constant for a particular class of VCR (VHS, Betamax, etc.).

It may seem that the formulae in the previous section can simply be applied repetitively until the desired tape position is reached. There are however a number of approximations which would lead to the tape oscillating if this approach were followed; for example the asumptions that V is constant after the delay of a seconds and that the tape can be determined to be precisely at a particular point.

To overcome the problem of oscillation this invention provides the following solution.

FF or RW mode is applied for the appropriate time as calculated from the formula. This is repeated until the tape is within x seconds (x defined below) on either side of the desired point.

The value x is set initially to about 100 seconds (depending on VCR model) and increased by 5 seconds each time an FF or RW attempt is completed. When the tape is within this range of the desired point, Scan or Review mode is applied until the desired point is reached. In practice, this technique invariably results in one application of FF or RW followed by one application of Scan or Review.

Before the formulae can be applied the parameters need to be evaluated. This is done immediately after the Delivery System memory map is downloaded from the video tape.

(Note however that if we allow the flexibility of different tape sizes, then the tape size will have to be specified by the author and stored in the memory map.) When the system is entered initially it: (1) Establishes VCR type and model from the Intelligent Cable (and, if necessary, tape size from the memory map).

(2) Uses this to obtain from lookup tables values for a, ss, Jf, br and tm.

(3) Notes current TIC value (tl) and select FF mode for 20 seconds, noting the new TIC value (t2). FF rotational speed is then derived from:~ Vf= [ A/(at2+ss)~A/(at1 +fl ] / [ 20 - (5f ] (4) By a similar process to that in (3) calculate the RW rotational speed, Vr (tf is the total tape playing time): Vr=(V [ a(tm -t2) +fij - V [ a(tm-t1 ) +ss])/(Tr-#r) (5) Stores each of the derived values as system constants.

The relationship between elapsed time and tape movement in fast forward and rewind modes is complex. To ease consideration we break the relationship into two components: ~the relationship between number of revolutions of the supply or take-up spool and tape movement. This depends only on the geometry of the tape cassette, the spool diameter, the length and thickness of the tape.

~the relationship between elapsed time and spool revolution. This is dependant upon the VCR model being used, in particular the power of the motor(s), the acceleration and deceleration characteristics and the time taken to load and unload the tape from the cassette.

Consider a tape of thickness w winding round a take-up spool of radius r. After each revolution the effective radius is increased by w and the displacement x of the tape after n revolutions is given by:

=2irm+::wn(n-1) . (A1.2) 2;rm+::wn2 . . . (A1.3) Solving this for n: n = #[x/w#+(/r/w)]-r/w . (A 1 .4) Thus the number of revolutions n(x,y) to get from x to y is given by: n(x,y) = #[y/w#+ (r/w)2 ] - V [ x/w7i+ (r/w)2 ] . (A 1.5) Now let t1 and t2 be TIC values corresponding to x and y.If the linear playback speed is s we have: x=t1.s and y=t2.s Setting a=s/wn: & fl=(r/w)2 yields: n(x,y) =n(t1 ,t2) = V[at2+ss]- #[at1+ss] (A 1.6) In equation A1.6 note that a and ss are constants whose values depend only on tape thickness, spool radius and the playback speed.

Note that t1 and t2 relate to the elapsed time from the start of the tape and we are considering tape winding onto a take-up spool (i.e. fast forward). To consider rewind we must start with an empty supply spool and work backwards. In this case the times t1 and t2 must be subtracted from the total playing time of the tape.

The speed at which the spools turn during rewind and fast forward is not exactly constant and depends in a complex way on the load imposed by the tape. For the purposes of this analysis we ignore this effect and also the acceleration and decceleration of the tape. We cannot, however, ignore the fact that the tape does not start to move for some time after the start of the operation. In the case of both VHS and U-Matic VCRs this time amounts to several seconds and is the time required to unload the tape from the playback drum.

Referring to formula A1.6, let us assume that it takes Tf seconds to get from TIC ti to TIC t2 in FF mode. If the (assumed constant) rotational speed is Vf revs/sec and the delay time is #f seconds, then:n(t1 ,t2) =Vf(Tf~Af) Substituting in A1.6: Vf(Tf-Sf)=V [ at2+fl ] -V [ atl +fli . . . (A1.7) Vf=(#[at2+ss]-#[α1+ss])/(Tf-#f) . . . (Al .8) Tf=(#[α2+ss]-#[αt1+ss])/Vf+#f . . . (A1.9) By a similar process we can derive the formulae for RW mode as follows: (tf is the total tape playing time) Vr=(V [ a(tm-t2)+fl ] - #[α(tm-t1)+ss])/(Tr-#r) . (A 1.10) Tr=(#[α(tm-t2)+ss]-#[α(tm-t1)+ss])/#r+#r . . . (A1.11) Tape speed, s=0.02339 metres/sec.

Playing time, tm= 11143 secs.

Spool radius, r=0.0127 metres.

Radius of full spool, R=0.04267 metres.

Tape length, x (=s.tm)=260.63 metres.

If the full spool is made up of n turns of thickness w then:nw=R-r=0.02977 metres Substituting in A1.3: 260.63=2,:n(0.0l 27)+,rn(0.02997) So, n=1498 turns w=2/105 metres.

Finally, we can calculate: For VHS machines: a=s/w7r=372.05 For E180 tapes: fl=(r/w)2=403225

Claims (1)

1. A video recorder/player having control means for selecting both fast and slow forward and rewind of the recording, means to monitor the particular field being viewed and means to search for a required field including means to assess the time required to reach the required field from the current field, means to select only fast movement if the assessed time is greater than a predetermined time, means to reinitiate operation of the control means if the recording does not reach the required field of view after the initial operation and means to increase the predetermined time to a longer period so that eventually slow advance or rewind is selected to bring the recording to the required field.