Everyone Wants In The Club: Analog Tape II
This is the second part of a series of posts about the challenges that confront designers of analog tape machines. What is thought by a select few of passionate “reel to reelophiles” to be the ultimate format for delivering high-end audio presents a lot of difficult mechanical and electrical problems. The following is a discussion from the users manual of my Nagra IV-S, on of the greatest analog tape machines ever devised.
Amplitude Modulation Noise
Record a continuous sine signal and reproduce it. Its amplitude ought to be perfectly constant, but in reality it varies by a few percent, thus producing an amplitude modulation that is heard as a noise. The amplitude fluctuations are due to several reasons, as the irregularities of the tape, the tape edges and an imperfect magnetic layer:
Irregularities of the Tape
The magnetic layer of the tape is not perfectly homogeneous; its structure and thickness are not quite regular and this is an important source of the amplitude modulation noise.
Tape Edges
The poor condition of the tape edges can also cause amplitude modulation. It is for this reason that the width of the playback head is slightly less than that of the tape.
Asperity or Drop-out Noise
When some foreign substance, such as a dust particle or any heterogeneity of the magnetic layer of the tape moves across the recording head, the particle lifts the tape off the head. The amplitude of the useful signal drops and this is audible as a noise. That noise can be notably minimized by cleaning the tape, by increasing the specific pressure of the tape on the head and by over-biasing:
Cleaning the Tape
Experience has shown that drop-out noise is worse with certain European tapes with a matte back and it appears that particles from the back loosen themselves and contaminate the magnetic layer. Sound engineers have adopted the habit to scrape the tape in a very simple an efficient way using the QRAC (a tape cleaning blade) accessory before the tape reaches the heads when doing high quality recordings with new tape, this phenomenon occurring principally only when new tape is used. With certain tapes, this cleaning operation has been very pronounced effect in reducing the drop-out noise.
Increasing the Specific Pressure of the Tape on the Heads
While a tangible increase of the tape pressure increases head wear, it also presses foreign substances into the tape – due to its elasticity – and this reduces drop-out noise.
Over-biasing
When over-biasing the tape, we operate on that part of the bias/efficiency curve where a reduction of the bias causes the efficiency to increase. Lifting off the tape reduces the bias, and the increase efficiency compensates partially for the drop-out of the recording field. This method has been used for many years with NAGRA recorders operating in Europe, where the use of matet back tape is of long standing. With 3M tape No. 206, the improvement is not sufficient to justify the drawbacks in over-biasing, but nevertheless, the operation point is still on the falling slope of the bias/efficiency curve. Those recorders adjusted for 3M tape no. 138, and with which tape No. 206 is used, operate with under-bias, which means the drop-out noise becomes very audible.
Group C: Head Magnetization Noise
A recording head through which DC flows, will record a DC signal on the tape, and this signal cannot be played back, since conventional playback heads do not permit DC reproduction. This signal should be inaudible in any case. The DC signal is, however, a source of modulation noise, and the latter is perfectly audible.
DC is not even necessary for being the cause of this problem: an asymmetrical magnetic bias, a magnetic interference field, or permanent magnetization of the heads or of the shielding have the same effect. The earth’s magnetic field in particular is sufficient, unless the head is well-shielded.
In a recorder like the NAGRA IV-S – unless the oscillator has a breakdown – the bias symmetry is one order better than is necessary whilst the shielding of the recording head is “just good enough”, allowing for accessibility. The earth’s magnetic field, being essentially vertical in our latitudes, can cause an increase of the phenomenon when the NAGRA is turned into a vertical position.
Moreover, when operating in the RECORD, NO LIMITER position, transient signal may magnetize the recording head sufficiently to produce an audible noise. A sound of sufficient amplitude (saturation), decaying progressively, will fortunately demagnetize the recording head. The shielding, too, can be magnetized; a phenomenon particularly observed on recorders that have been shipped by air. It is therefore necessary to demagnetize them from time to time. Another effect of a magnetized head is noteworthy: It records a DC signal. As long as the tape is normally transported, only the modulation noise during playback is heard, but, when the tape is accelerated from stop to normal running speed, a “pop” can be heard. It marks the differential of the recording of the DC signal with respect to the accelerated time. This effect can be used to check whether the recording head is magnetized or not.
From this discussion, it should be obvious that analog tape recording has a large number of imperfections. In the next installment I’ll put the charts from the manual that show the dynamic range and frequency response of a first generation tape. The fidelity potential of analog tape pales in comparison to a well done high-resolution PCM digital file.
Interesting. However, my take on this is that the distortions introduced by continuous (analogue) recording of the sound is less objectionable to our hearing than digital discretization of the sound; we are used to listening to continuous waveforms in real life. Therefore, in my opinion, variation in speed, or modulation effects will be much more forgiven by our minds than discretization effects.
Dave, thanks for the comments. This is the first time that I encountered the term “discretization”, although I’m pretty sure that I know what you’re referring to…the discrete sampling of a continuously variable analog waveform into amplitude samples at a particular sampling rate. Of course, there are known errors and challenges with any format and PCM digital has its unique problems (jitter, quantization error, aliasing etc), but I know that what comes out the end of a high-quality DAC is once again a continuous waveform without any hints of “discretization”. I’m interested in what detrimental effects you’re describing. The process of digitizing and reconstructing analog waveforms produces fidelity that far exceeds the specifications AND sound of any analog format available.
Dave, the analog output of a DAC is no more discrete than was the analog input to the ADC, back up the chain. It is as analog and continuous as analog can be. It is much purer analog than any output from any known analog storage medium. That is the weakness of so-called analog audio: the storage mediums are unsatisfactory. A DAC is the best way to create an analog waveform.
Mark, thanks for the reply. When an analogue waveform is digitized it is discretized, ie converted from the continuous to the discrete. And, what I’m getting at is that, in my opinion, the human brain can differentiate between a continuous waveform which is an analogy (analogue) of the real real thing, and a waveform that has been discretized (and then re-constructed to be continuous).
That, again in my opinion, is why analogue recordings, when played back through analogue equipment, are preferred by many.
BTW thanks for the FTP info, I look forward to listening to the recordings on the FTP site.
I support you and all others that prefer the sound of “analog” recordings. The notion of “discretization” being the cause I may disagree with but that not really the core issue. Just because you prefer the sound of analog doesn’t necessarily make it “high-resolution audio”. That’s my point. And I hope you would agree. The specs just don’t measure up.
TBH I didn’t say I preferred one or the other. I listened to the Stravinsky digital vs digital to analoge to digital files, and have to admit the analogue version introduced a veil to the sound. However a fair comparison would have been to listen to the same session made using analogue equipment, on analogue equipment, and then the digital version made from the analogue tape and recorded back to analogue; I suspect there would have been some “veiling” introduced again.
On a side note I listened to the “Say You’ll be Mine” CD vs BD mix: to me these mixes were opposites of extremes, on the one hand the CD mix was loud and highly compressed, on the other the BD mix was too soft and sapped the life out of what was a “rock” track. I would say that although the CD mix was aggressive it did capture the music style, ie “rock” driven by the drummer. The BD mix sounded tame because, for one reason, the drums (or at least the kick drum) was too far back in the mix. In the end I preferred the CD mix because it got where the music was coming from.
So, I suppose, ultimately it doesn’t matter how the music is recorded, or in what definition (“standard” or “high”) what’s important (to me anyway) is that the emotion of the musical event is accurately captured.
Very well said. We do spend a lot of time talking about specs and formats instead of listening to great music.
To the Stravinsky comparison…the microphone preamps outputs were sent to the inputs of the 96 kHz/24-bits Sound Designs records AND to the inputs of the Nagra IV-S…same signals to to different format recorders. If I could A|B these for you in the studio (as oppsed to going back to digital so you could have a file to downloads), you would hear the same thing. Even the artist and his friends noted the difference. “How come there’s so much noise?”, was their comment. The digital was much preferred.
Ali Isabella is a recording that I didn’t record but did mix in 5.1 and stereo. The “Nashville” version was heavily mastered to the normal commercial standards and it possesses all of the things that note. When I played both for the artist and her mother, they loved the more open and more natural sound. This is exactly why I want my new iTrax 2.0 site to offer options…that way we can all get it the way we want it.
As well as offering surround, maybe you should offer different versions of the mix; the CD mix, but without the compression.
That’s the plan but it seems the artists and labels don’t want options…I’m working on it.