The recorders are data storage systems and fall into three categories:
Despite the onslaught of the digital recorders over the past decade the analogue recorders are still hanging in there primarily because engineers still believe that the analogue sound has not been surpassed by the digital medium, and quite rightly so. The top of the line 2" analogue recorders are still being used and sold but primarily by the perfectionists and dedicated audiophiles who will probably have a digital system with the analogue being used for bass, electric guitars and drums. Vocals and the rest being covered by digital. Some engineers still insist on mixing down to 1/2" stereo analogue masters and some mastering studios actually transfer their digital masters via an analogue recorder to soften the harshness of the digital top end etc.
Meanwhile the home recording enthusiast is very likely to have a 1/2" sixteen track or a 1" twenty four track etc. The important aspect of the analogue recorders is their need for regular maintenance and servicing. If you are an owner of a analogue recorder you must have the alignment tape required for correct alignment of the transport and electronics. This is a tape with pre-recorded signals that determine:
The above are the variables in head alignment in an 8 track analogue recorder. The head alignment is executed in three dimensions through the variables:
All variables are really self-explanatory from the diagram and the head block on which the heads sit has the appropriate adjustment screws to line the head up. What is required is the correct alignment tape for the tape format and speed. These tapes can be purchased from their manufacturers.
Before you let your precious alignment tape near your recorder you must degauss it:
As the magnetised tape travels through the tape path the magnetic flux on the tape progressively is transferred to the metal parts such as the tape guides and heads. This build-up if left unattended will induce magnetism onto these parts and they will progressively erase the high frequencies on the tape as it passes. Therefore regular degaussing of these parts is a necessary maintenance procedure. A degaussing tool is required for this operation.
It is common practice to wrap a layer of insulation tape around the head of the degausser so that the metal doesn't damage your heads if you accidentally touch them. The technique here is to first turn off the tape recorder. If you don't you'll blow out all your circuits!! Then switch on the degausser with the head 1m(3ft) away from the recorder. Slowly bring the degausser head up to the play head and move the degausser slowly down and back up the head then slowly draw it away around a foot, then do the same to the next head and the next if there is one. Then using the same technique do the tape guides, rollers etc. Finally draw the degausser away from the machine around 1m(1ft) and switch the degausser off. Your machine is now degaussed.
The heads on a recorder acquire a build up of tape oxide after constant use so it is necessary to clean the heads and the tape guides regularly, like twice a day. Cotton buds and Isopropyl Alcohol (available from most chemists and drug stores) is the most common method. The main thing is to regularly check your heads to make sure that there is not too much build up. If there is you should recheck your head alignment or transport alignment. Too much tension on the tape can cause oxide build-up because of the increased pressure on the head. Another cause is worn heads so if you suspect worn heads get your machine checked by an expert. This is an exaggerated version of what happens:
Alignment tapes come in three types according to their calibration. They can be either CCIR, NAB or IEC alignment. CCIR is recognised as being the British or European standard whereas the US is NAB. IEC only applies to the speed of 30ips. When you purchase an alignment tape you must know what your machine is calibrated to. Secondly you must know what speed your machine uses and if more that one speed is available you will need to purchase an alignment tape for each speed. These standards apply to the pre equalisation curves that a recorders have. Basically they boost the high frequency onto the tape and then reduce it back again on playback. This improves the signal to noise ratio. Each of the types, CCIR, NAB or IEC, have different EQ curves.
Secondly alignment tapes are recorded at a specific flux level measured in nanowebers. The first ampex alignment tapes were at 185 nw which meant that if you aligned your playback head to zero and then your record head to zero you would be recording a flux density of 185 nanowebers. Tapes then came out at 250 and then 300. You must check what level your alignment tape is at and what the capability of your machine is. The new tapes are capable of recording a higher flux than the old and the level has been going up an up over the years as tape manufacturing improves.
Simply the bias is a high frequency (typically between 150kHz and 280kHz depending on recorder type and model) signal that is added to the record signal to compensate for the irregularities in the ability of tape to hold flux uniformly. It is an adjustment in the record side calibration.
You will need an oscillator capable of producing frequencies from 50Hz to 16kHZ for this procedure. Some consoles have a tone generator built in. Your machine will also have 1,2 or 3 cards for the record, play and bias adjustment.
OK so the machine has been cleaned and degaussed, now you can put your precious alignment tape on the machine.
The first thing to do is to check the tape path and confirm that the tape moves freely within the guides etc. Now you are ready to check the playback alignment. You must first check the azimuth. Take a playback signal from your two outer tracks, 1 - 8, 1 - 16, 1 - 24, and bring them up on your console at equal level panned centre. Now playback the 10 or 16kHz section on the tape and adjust the azimuth on the head so you get the highest reading. This assures that the head azimuth is correct and that the phase relationship between your outer tracks is the same.
Now play back the 1kHz tone and adjust the playback level to zero. If you wish to add more flux to tape because you have the latest tape you may wish to put 3db more level onto tape. In this case you should line up the playback reference to -3db. Now play back 10kHz and adjust the playback highEQ on your record card to read as close to zero as you can get or -3db if adding higher flux. Now playback the low frequency tones like 100Hz and line these up to zero (or -3db) or as close as you can get. Now recheck your 1kHz tone and do all the other tracks the same. You are now ready to align the record head. Remove the alignment tape and store it in a safe place away from anything magnetic like speakers etc.
Now put on a new roll of tape which you have labelled Record Test Tape. Keep this tape with your alignment tape for further alignment sessions. Put the tape on and put all the tracks into record. Roll tape and hit record and select playback on all of the tracks. You must now adjust the record level so that you get around 0db level played back. Now you are ready to check the Bias level. The adjustment for this will be on your bias card. Record a 10kHz tone and switch the machine to playback and adjust the bias level control. You will notice that as the signal rises it reaches a peak and starts to drop again. If not you must find this area. Line it up to the peak and then keep increasing it until it drops by 3db. This is called overbiasing by 3db. Do this for each track.
Now you must check the azimuth of the record head which is done by recording 10kHz onto your two outer tracks and playing them back through the console like we did before adjusting the azimuth so that you get the highest reading.
Now that the bias and azimuth are calibrated you can start the frequency response calibration starting once again with 1kHz followed by the high and low frequencies and adjusting the record card controls. A good machine should give you a flat response + or - 3db from 50Hz to 15Khz.