RECORDERS

 

The recorders are data storage systems and fall into three categories:

  • Analogue Tape
  • Digital Tape
  • Digital Hard Disk.
Analogue Recorders

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:

  • Head alignment
  • Transport alignment
  • Frequency response and level structure

So let's look at the Head alignment factors:

 

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:

  • Azimuth
  • Zenith
  • Wrap
  • Skew

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:

Degaussing

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.

Cleanliness.

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

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.

Bias

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.

Alignment

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.

 

Digital Recorders

If you have moved to the new digital recorders and have just read the previous rave you must be breathing a sigh of relief as the digital recorders don't have frequency response or bias and azimuth adjustment. All they do is record 0s and 1s, albeit really fast at 48kHz/sec.

So lets start with some basic knowledge of digital sound. What does it mean when they say that a track is recorded in 16bit digital at 44.1K.

Bit Rate

Digital sound is made up of words of 0's and 1's and 00, 11, 01, 10 are the four possibilities in a two bit word. A three bit word can be made up with 000, 111, 001, 010, 100, 101, 011, 110, which means there are eight possibilities. You see - 2 bit gives 4, 3bit gives 8, 4 bit gives 16, 5 bit gives 32, and so on. Now if we were to use the bit words to express volume with a four bit word we could give 16 different values for volume. So the higher the bit rate the more accurate the resolution be it volume, digital pictures or digital sound. So 24 bit digital sound has more resolution and accuracy than 16bit digital sound.

Sampling Rate

Digital sound is produced by sampling a sound (or should I say the electrical version of it) in real time and expressing it in bit words. Once you start sampling or recording digital sound a clock starts and progressive samples of what the sound is are taken. The rate at which the samples are taken is called the sampling rate.

The drawing above shows a wave of a sound being sampled. If the time in the drawing is 1 second, then there are 6 samples (the last one is the first in the next second) of the sound in one second or a sampling rate of 7. So obviously the higher the sample rate the more accurate the resolution. So when we say that the sound is 16bit, 44.1Khz it means that the sound is being sampled at 44.1 thousand times a second and it is being measures with 16 bit accuracy. In the above waveform the sampling volume levels given would be 0,2,2,0,-2,-2, Not a very accurate version of a simple waveform. But 44.1kHz, now that's fast, or is it? Lets look at sound in seconds.

In this chart you can see the relationship between the sampling rate and the waveforms it's sampling. 1kHz will have 44.1 samples taken of each of it's waveform as its oscillating at 10,000 waveforms a second. 100Hz will have 441 samples taken of each of its waveforms. But 10kHz will have 4.41 samples taken of each of it's waveforms. Now look at the first waveform we drew. In that drawing we took 6 samples of the waveform and got an amplitude reading saying 0,2,2,0,2,2. imagine how inaccurate 4.41 samples are of a complex waveform. That is why digital high frequencies sound harsh!! The industry has constantly denied this factor and even gone to the extent of saying the hear can't distinguish between a square wave and a sine wave above 7kHz. Pigs Bum.

At a sampling rate of 96kHz you get 9.6 samples of a 10kHz wave and believe me, you can hear it.

In an article by Rupert Neve, I read recently, he said that we should aim for 24bit resolution and 192kHz sampling rate if we want to equal the quality of high quality analogue recording. We will get there. DVD is already up to 24 bit 96kHz sampling so we are on the way. But if your 16bit, 44.1kHz CD sounds bright, consider what makes it bright and you will see that it's a false bright created by the high frequencies sounding like square waves!!

Why 44.1kHz Sampling Rate?

Why not 44, or a nice round number like 50. When the first engineers were inventing digital sound they had worked out the on/off, 0/1, idea and needed a way to represent it. The idea came to use white dots on a TV screen where a white dot was on and a black dot was off. Neat. So you record it like a video picture on a video recorder. That was fine, but the engineers had been caught out before. What about PAL (the European video standard) and NTSC? (the American and Japanese standard.) They weren't going to get caught up in that again, no way, so they configured a number that was compatible between the 528 line NTSC and 625line PAL and the number was 44.1kHz. Just a piece of useless info you might want one day!

What you can see from the above is how the digital recorders were developed. They were Beta Video Recorders with an external processor and digital audio had arrived. The beta video became the DAT and the DAT became the ADat and the D88 and they are all basically video recorder decks. The ADat used a SuperVHS deck while the D88 used a High8 deck. The basic SuperVHS deck was pretty awful and the ADat of today is a completely rebuilt deck. It's a shame the world chose to make the VHS deck the standard because the Beta Decks were far superior. Market forces don't always give the best outcome. Did you know that when an ADat or D88 records on a new track it plays the bit stream off the tape , mixes in the new track, and records it again. Now that's worth thinking about.

Hard Disk Recorders

Now we are entering a new era in recording with the advent of the computer. Now hard disk drives can store gigabytes of information and retrieve it at high speed. The Pentium 11, 350mHz computer I am writing this on will play back around 13 - 16 tracks of stereo digital audio in real-time at 16 bit 44.1kHz. It will also process it in real-time so I can add EQ, Compression, Reverb and Effects, I can cut and paste it, I can timestretch and pitch change it, I can even put it in tune. It cost under $US-2,000 and will do my Internet, keep my tax records and play games as well. The hard drive can be backed up onto CD with my CD Burner. At the time of writing the new 1gig processors are being released and my local dealer is offering 27gig hard drives for around $300. As the telephone lines get better and the Internet gets faster and faster I will soon be able to play with a guitarist in London , a Bass player in the US, a drummer in Africa in real-time wow! where is it going to go?? Anyway back to reality.

The first units were stand alone units that had hard drives built in and all the companies were there with some kind of model. Meanwhile the programmers were busily rewriting their computer sequencer programs (Cubase and Notator) for the Macintosh platform and Cakewalk was writing theirs for Windows. Dididesign was developing ProTools and computer based hard disk recording was born. Now they all have computer based hard disk recording programs and due to market forces again they were all rewritten for Windows.

I won't go into the pros and cons of all the current programs, there are lots of qualified people able to discuss them all and I suggest you read the reviews suffice it to say that this is where the next generation of studios will be heading. In fact I would suggest that most of you reading this will have some kind of computerised hard disk recording already and are as excited about the future of this technology as I am. They are already offering 24bit 96kHz sampling and the new generation effects units are getting better an better. For the price of an ADat you can get a whole studio!!!!!

I say no more