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.
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.
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.