Characteristic Properties Of Magnetic Analog Tape Recording

Introduction

Characteristic properties and sound of magnetic tape recording are still required attributes even in today's digital age and looked after by sound engineers throughout the whole world. That's why can still see multitrack (and 2-track) analog tape recorders practically in every top notch recording studio. But the whole approach has changed throughout the years at least in one important thing.

Nobody disputes, that the signal processing and editing capabilities of audio recording software are far more versatile and comfortable to use, than working with the actual analog tape recorder - that is why they are the preferred working environment. The initial problem with digital audio recording from the early era - low resolution, which resulted in a cold sounding recording - eventually resolved with the advent of the quality of digital audio recording. Today's quality of digital audio recording is so high, that if we use high quality converters with stable clock at high resolutions (96 kHz/24 bits or even higher), we do not loose practically anything from the original sound. So, the digital audio recording is now very true to the original.

But the analog tape recording isn't just a recording. It is not exactly true to the original sound, it is much more than that. It is actually a very sophisticated signal processor capable of adding desirable qualities to the sound. And this is the one and single most important reason, why it is still being used today. In order to use that advantage, it is a common practice to record to analog tape first and than transfer the tracks to Pro Tools or whatever software you use. This way you get the qualities of both - analog sound and endless editing capabilities.

Why do we love analog sound

There are many aspects to why we actually love analog sound.

First of all, the analog sound and analog tape recording is the whole history of music (and not only music) recordings, our first-ever encounter with high quality audio recording of any kind - the fact we simply can not deny. We actually learned the audio recording is to be supposed to sound this way. And since we also liked that sound in the first place, there was not any reason to change that. In fact, any change and deviation from that sound would make us feel, like there is something wrong with that.

In the golden days of analog a lots of imperfections of analog tape recording were not perceived as a significant problem. When the first digital technologies came out, many of the sound engineers, musicians and producers could not wait to get their hands on it and so, the first boom of digital started. But after a short time they also started to realize, that some of the aspects of analog tape recording, those imperfections digital was supposed to fix were actually very desirable, highly musical attributes and their sudden absence was perceived as if something was missing there. Thus people would start to talk about cold digital and warm analog.

Of course, this had a lot to do with the fact, that digital technology was not at an appropriate level at that time. Today, of course, the digital is so good, that these things are just a matter of personal preference. But the fact is, even in spite of many advantages of digital audio recording technology, the analog tape recording has never been fully abandoned. It is probably interesting to say, that if the digital had been invented first - the analog would have had no chance, but... thanks to the history, the situation is different. The analog sound is the reference of how digital is supposed to sound one day.

Characteristic Properties Of Analog Tape Recording

1. dynamic compression and signal distortion - tape saturation

2. modulation noise

3. tape noise

4. head bump

5. tape speed

6. wov and flutter

Tape Saturation

In digital audio recording the signal level is represented by binary values. There is an exact maximum value which can not be exceeded. Digital audio recording is able to react to increased signal level in a totally linear way. There is a maximum recording level which can't be exceeded and any attempt to record a hotter signal will result in overload and signal distortion. The transition to the state of overload is all sudden and the resulting sound is typically very unmusical and therefore unacceptable.

In analog tape recording the signal level is represented by the amount of tape magnetization. The higher the signal level the more magnetized the tape gets when running through the tape head. Imagine a magnetic tape as a group of miniature magnets, turning themselves according to the strength of the magnetic field they are exposed to. Certain value (signal level), which can not be exceeded exists also here and is characterized be the state where all of the tiny magnets are already turned in that one particular way. But what makes an important difference here is the fact, that this transition to overload and distortion is not sudden but very gradual and smooth. So, in a certain span of signal level we have almost linear response. Than we reach a signal level where the amount of magnetization starts to drop and so the signal becomes compressed. As the signal level and compression rise further the signal also becomes progressively more and more distorted, because the signal peaks get rounded. Both of the signal halves are rounded in the same way, so we get mainly odd harmonics in the distorted signal, especially the third harmonic. We refer to this process of getting compression and distortion from analog tape recording as to tape saturation. And if we use it in healthy amounts and do not overdo it, than it can be very cool especially on the rock drums, guitars and bass. The resulting sound is subjectively richer and more compact. For different instrument a different level of saturation is desired - typically the most saturation being used for drums and percussion. The amount of saturation is simply controlled by controlling the recording level.

For some technical reasons (which are beyond the scope of this article) there is a loss of high frequencies when "reading" the signal from the tape and so, there is always an equalization in order to compensate for this - higher frequencies are amplified. Because of this those higher frequencies will go into saturation earlier. Therefore tape saturation is frequency dependent with the high frequencies being compressed and distorted first. While the signal distortion in the low frequencies can be distracting it can be the right opposite for the higher frequencies. The reason for this is probably the fact, that while the majority of harmonics for the low frequencies fall within the audible spectrum, it is only true for the first few harmonics for the higher frequencies. To sum it up - tape saturation is the most prominent and most desirable quality of analog tape recording and the reason number one why this technology is still in use today.

Modulation Noise

Modulation noise is very specific. It only exists when there is an actual recorded signal on the tape and it is being added to it during playback. Its level corresponds to the level of the recorded signal and so it is a very live, ever-changing element. It is hard to the describe, nonetheless, it is an inseparable element of analog tape recording and probably one of the sources of the analog warmth.

Tape Noise

Tape noise on the other hand is the biggest problem of analog tape recording and it is mostly a negative factor. Its level can be reduced significantly by using a noise level reduction system. The most recent of these systems (Dolby SR) have a signal to noise ratio around 100dB, which is even better then theoretical dynamic range of 16-bit digital audio recording. Also, this tape noise is much less obtrusive, than the quantization noise of digital...

Except these most typical properties there are many more.

Head Bump

The typical wave in low frequency characteristic - so-called head bump - is caused by the actual design of tape heads. And because each manufacturer has it own unique design, each type of tape recorder has its own low frequency response, which also means, that each type of tape recorder has a different sound.

Tape Speed

Tape speed affects frequency response and noise level. The higher the speed, the higher the frequency range and less noise. However, for tight sounding lows, lower speed was often preferred. Professional standard is 15 inch per second, while 30 inch per second being often used for mastering.

Wov And Flutter

During the transport of the tape through tape head a gentle wov and flutter may occur. If the tape recorder is properly aligned this is only very subtle, practically inaudible, but still existing element contributing to the analog warmth.

Tape Medium And Bias

Each tape medium also differ in its sound qualities and sometimes very significantly. In general, in the golden days of analog tape recording, there was less dynamic range.

Under normal, unadjusted conditions - the dynamic range of analog tape recording would be non-linear in lower levels. Because of this phenomenon all analog tape recorders use bias - a sine wave signal at a frequency significantly higher then the audible spectrum (100kHz typically used), which is being added to the recorded signal and filtered out during playback. With proper bias adjustment a very linear response can be achieved in the whole dynamic range up to the point where the saturation begins. Lower bias will extend dynamic range and boost high frequencies and vice versa. A very slight higher or lower bias adjustment is often used by recording engineers and producers to get the desired sound. Tape bias is typically adjusted by recording and playback of different test tones (100Hz, 1kHz, 10kHz) so that the level of the tone being played matches the level of that tone before being recorded.

Conclusion

A we can see, there are many aspects to analog tape recording - not only positive, but also negative. And those mentioned here are only the most important ones, not to mention all the other very subtle things involved in the whole operation of analog tape machine.

With digital we got rid of many imperfections of analog. Gone are the hours of work spend on tape adjustment, we no longer have to spend money on analog tape medium. But we also got the "imperfections" of digital now, which means we lost some of the desirable qualities of analog tape recording. And so we have to say - the analog tape recording is not objectively better or worse, just different. And we can choose what we use according to desired results. We can use any of them and even both combined, so we get the best of both worlds.

Analog Or Digital

Which one do you prefer?

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