GB2155682A - Synchronisation by image shift monitoring - Google Patents

Abstract

Synchronisation by Image Shift Monitoring is a method for synchronising any two devices that carry an audio track. An identical signal contained on both items is fed to opposite sides of a stereo monitoring system, such as loudspeakers or headphones. The 'precedence effect' which causes the brain to detect positions of sounds in a stereo field, is used to determine a synchronised state when the image of the synchronising signals appears central. As this image shifts to one side or the other it is corrected by a variable speed control connected to one of the devices. The figure shows the technique applied to the sync tracks 8 and 11 of an eight-track recorder and a sub-mix recorder respectively. <IMAGE>

Description

SPECIFICATION Synchronisation by image shift monitoring Synchronisation by Image Shift Monitoring is a method whereby separate recordings can be synchronised using a common to both sound source arranged in a particular manner so that a stereo image is formed. When the image shifts to one side or the other an appropriate amount of adjustment returns it to a correctly synchronised state.

There are synchronising systems available today, generally for matching sound to picture in film and video recording. There is also a method for locking in synchronisation two multitrack sound recorders to make available extra tracks. This is usually done with two twenty four track recorders, with one track on each machine used for synchronising, thus making available forty six audio tracks. Another reason for linking two machines together is to save tape wear and so decrease the risk of quality deteriation.

With this method applied to Pop music recording, the first stages of the recorded tracks would generally contain the drums, spread over many tracks to allow different elements of the kit to be treated and balanced at the reduction mix stage. The other basic rhythm instruments would also probably be recorded onto the first tape. These tracks would now be roughly mixed into a stereo pair and dubbed onto the second twenty four track recorder at the same time copying a synchronising track. The vocal and instrumental tracks would now be recorded onto this second machine using the rough mix as a guide. These recordings sometimes require the tape to pass over the heads many hundreds of times. When it comes to reducing all of the recorded tracks down to a stereo pair for record production, the two machines are locked in synchronisation.

The rough guide mix of the original tape isnow ignored and replaced by the original tracks which are still in pristine condition.

The cost of these synchonising devices is several thousand pounds and would not be a practical investment for an eight track studio for example, where the cost of the synchroniser would be greater than the purchase of a sixteen track recorder.

The system devised is of low cost and highly suitable for eight track or even four track studios wishing to increase their track potential. It could also be a useful addition to sixteen and twenty four track studios where extra tracks are required to prevent track bouncing. (The method of copying one or more tracks across to a single track sometimes adding a live recording at the same time). This would avoid the drawback of not being able to rebalance at a later stage, as by then the original tracks would have been erased and reused.

The available tracks if this sytem is adopted are as follows: 4 track recorders can be increased to 13 tracks.

8 " " ,, ,, ,, ,, 57 16 ,, ,, ,, ,, ,, ,, 241 24 ,, ,, ,, ,, ,, ,, 553 If a maximum of one stage of track bouncing is used then the available tracks increases to the following.

4 tracks can be increased to 16 tracks.

8 ,, ,, ,, ,, ,, 134 16 ,, ,, ,, ,, ,, 1411 24 ,, ,, ,, ,, ,, 4946 It would be unlikely that this system would be used to this degree, especially with the larger track formats, but in practise a combination of straight recording with a sub mix being made for instruments or voices that need to be layered.

Another advantage of this system over other synchronisers, is that in practically all cases, one of the already recorded tracks can be used as a synchronising track, rather than having to set aside a track that has a synchronising tone recorded on it. A very important factor for small track studios, where every track counts.

THE METHOD A suitable recorded track is selected for synchronising. It should be one that plays through the whole piece without too large a gap, where synchronising information would temporarily lost.

The gaps can actually be several seconds long, because the system uses the human brain, with its high degree of complex decision making to make any adjustments necessary when synchronising information is recovered. A rough mix of the music recorded so far, is made as a guide, on one track of a two track tape recorder, preferably digital. On the other track is recorded the track selected for synchronism. These two are copied onto a fresh piece of multitrack tape, on seperate tracks. The new recordings are made in the normal way on the other available tracks, using the guide for tempo, pitch and artistic feel. When complete, the recordings are mixed together, at the same time monitoring the guide track to help judge the balance of the individual parts. Effects such as automatic double tracking, echo etc, can also be added at this stage.This allows for a high degree of concentration on one section of the music, rather than having to balance everything and add effects to all of the parts at the same time while doing the final reduction mix. After rehersal, this mix of the new recordings is dubbed onto one track of a two track tape recorder. The other track has the synchronising track copied onto it. If a digital recorder is used, such as the type that record onto video tape, then the recordings can be made in stereo. The synchronising track is transfered to the analogue track, which although lacks quality due to the slow tape speed, is quite good enough for synchronising. If while copying the music tracks there are any gaps, the faders of the mixer are brought down. This results in a perfectly clean and noise free tape being produced, which has either music or complete silence on it.

This new tape is now transferred to the original master tape, and synchronisation is achieved in the following manner.

The synchronising track on the sub mix is sent via an amplifier to one side of a pair of stereo headphones or speakers. The synchronising track on the multitrack master is sent by the same process to the other headphone or speaker. A variable speed control is connected to one of the recorders. When the two machines are started together from the same place on the synchronising tracks, a phenomenom occurs when they are synchronised. The identical signals appear between the two speakers, or in the middle of your head if using headphones. If the image shifts to one side or the other, it is brought back by varying the speed control. See Fig. 1.

This is caused by the Precedence Effect, also known as the Haas Effect (after the Dutch resercher who evaluated it). It is caused by the brains ability to detect the difference in time at which a signal reaches one ear and the slight delay caused by the thickness of the head, and the time it takes for that same sound to travel to the other ear. This is the main way in which stereo images are formed. If the signals reach both ears at the same time, the image appears to be central.

Because this sytem relies on the fact that both signals coming from the two sources must be identical, it follows that there is only one place where they are in synchronism. This is why practically anything can be used for synchronising. Even a steady drum beat. If the drum pattern is repetative and the two tapes are started a bar apart, it won't synchronise even though the beats appear to be identical, there will be minute differences in speed, volume, attach and decay of the signals.

In order to allow time for synchronisation to be established before the music is transferred, I have found it advisable to record approximately one minute of appropriate synchronising signal to preceed the music on the master tape. A most suitable signal for this purpose is a spoken count. Counting has the added advantage of showing exactly how much time is left to establish synchronisation before the music starts. White noise superimposed over the count is also very good in that it gives a contant image that appears to float about in the stereo field, and is easy to keep central.

The actual speed variations are in fact very small. It need only be of concern that synchronisation was briefly lost, or the speed varied to such a degree that a pitch change might be audible, if the image goes too far to one side, and suddenly parts, leaving an audible hole in the centre. This is the point at which the signals have gone out of phase and maybe too far out of synchronisation. This depends entirely on the material being recorded and an experienced operator will determine whether to abandon that particular take.

An example of the synchronising method using an eight track recorder and a digital sub mix recorder is given in Fig. 2. No's 1 and 2 represent the new tracks being transferred. 3 to 7 are other tracks available or already recorded upon. 8 is the track containing the synchronising information. 9 and 10 are the digital stereo tracks previously mixed from the eight track sub mix, and 11 is the analogue track containing the synchronising information. Signals from 8 and 11 are fed via amplifiers to opposite sides of stereo headphones, and the master eight track recorder has the variable speed control.

It is also possible to visually monitor the effect using a centre zero microamp meter as in Fig.

3. 1 is the synchronising signal from the sub mix and 2 is the synchronising signal from the multitrack.

It is best to arrange the headphones or speakers in such a way so that the variable speed control needs to be turned in the opposite direction to where the image is shifting, thus 'steering' the synchronising information in the stero field in a logical way.

POSSIBLE FUTURE DEVELOPMENTS It should be perfectly feasable to synchronise pictures on video recorders with this method, for post production picture and sound mixing.

It should also be possible to send a music backing track across the world by telephone, using digital information, probably at a reduced speed (for band width reasons). The artist could dub his voice or instrument onto another track and these two tracks could be sent back by two separate telephone lines and the new track dubbed onto the master tape using the backing track for synchronisation.

Claims (4)

1. Synchronisation by Image Shift Monitoring is achieved by the running of two recorded mediums, which have an identical information track on each, and set up so that one track plays in one stereo positioned speaker or headphone, and the other track plays through the opposite speaker or headphone. In a synchronised state, the image appears to be central and shifts to one side or the other of the stereo field if synchronisation starts to drift. This is corrected by varying the speed on one of the recorded devices, the control being wired in such a way that the image is 'steered' back to a central and thus synchronised position.

2. Synchronisation by Image Shift Monitoring as claimed in Claim 1, where in general an already existing track can be used for synchronising rather than having to produce a specially recorded information track.

3. Synchronisation by Image Shift Monitoring as claimed in Claim 1, can use an infinitely variable signal for synchronising, which only allows synchronisation to be established in the correct place. This enables synchronising points to be found even if the programme is started other than from the beginning.

4. Synchronisation by Image Shift Monitoring can be visually monitored as illustrated in Fig.