First, let me state that I'm not against analog recording. I have been using analog tape machines for over 30 years and still have my NAGRA IV-S machine and QGB 10 1/2" reel adapter in my studio (I just noticed a QGB went for over $5K on eBay…but I cannot part with my machine). However, I continue to read on other forums that analog recording is the "brass ring" or ultimate standard which all other recordings should be compared… including HD PCM digital.
Nothing could be further from the truth. Even some very well respected audio engineers insist that analog tape machines deliver a superior reproduction of music. I do agree that it produces its own sonic signature that is preferred by some engineers/producers (T. Bone Burnett among them) and consumers, but their preference couldn't possibly be because the playback fidelity of the recording matches the acoustic reality of the original musical instruments/voices. Analog recording just cannot produce the same level of fidelity (dynamic range and frequency response) that a well-done HD PCM recording can.
Take a look at the printed specifications as listed in the NAGRA IV-S INSTRUCTION MANUAL: Note the frequency response curve. It clearly shows that at 15 ips (inches per second), the highest frequency that can be captured on a master tape begins to drop quickly after 20 kHz. This is for the master tape, the one used at the time of the original recording. Each analog copy to copy degrades the SNR by an additional 6 dB. We consumers never get to hear the original source tape. The normal production process was/is to record to an analog multichannel machine (which BTW has less than half the track width of my NAGRA and therefore 6 db less SNR, mix down to a 2-channel stereo machine and then finally to a master. This is then cloned using analog to analog methods (another 6 dB of SNR loss) and the vinyl parts cut. The point is that these numbers are best case examples.
NAGRA IV-S Frequency Response Graph at 15 ips
The next image is also from the manual and shows the dynamic range of the NAGRA IV-S. The very best that it can do isNAB 72 dB at 15 ips using NAGRAMASTER EQ. I'm written on this site that 12-bits of word length (using PCM coding) is enough to fully capture the dynamic range of a well done analog recording. Note that another 1.5 dB can be achieved by having wider tracks on the heads. If you use Dolby SR or another noise reduction system, the number can jump up substantially but it will never equal plain old 16-bit PCM digital and the amount of signal processing applied during Dolby SR encoding and decoding is ridiculously high. I used to do these alignments at the studio where I first started engineering records. Another interesting specification on this list is the crosstalk and phase fluctuations that occur between tracks on an analog tape machine. These are not issues with HD PCM digital encoding (not that PCM doesn't have unique issues of its own…I will write about those as well).
NAGRA SNR Specifications
Part I of this article is largely taken from the NAGRA IV-S INSTRUCTION MANUAL. It is full of specifications and charts...some of which I will quote and reproduce here. I know many will dismiss the information, specifications and charts out of hand. They want nothing to do with specifications. Their usual defense goes something like "just listen and use your ears" or "analog is just warmer and more emotional". However, the facts don't lie. Of course, music recordings, music production and music reproduction are more than just specifications just as they are more than pure emotions and feeling. However, there are certain baseline facts that cannot be refuted. Imagine if someone was to say that a video image at 720 x 480 pixels running 15 frames per second was HD? There is value is establishing technical standards/specifications and I believe that the world of music recording and delivery is way overdue for some of the rigor that is applied to video.
Here's the discussion of noise from the manual.
Section 5 of the Nagra instruction manual is entitled, "Noise added to a Signal by a Tape Recorder". It describes the types of noise that are invariably as part of audio recording using analog tape. Below is the section in the manual:
Like all other elements of an electroacoustic chain, a tape recorder adds noise to the useful signal. Manufacturers of both equipment and tape are constantly preoccupied with the problem of reducing noise to the lowest audible level.
To distinguish between the different sources the noise are classified into the following groups:
Background Noise: It is present whether a useful signal has been recorded on the tape or not.
Modulation Noise: It is apparent when a signal is recorded on tape and is proportional to the signal.
Head Magnetization Noise: This is a modulation noise, caused by a DC signal recorded by a magnetized head. While it is therefore a modulation noise (B), it is continuously present (A), and therefore is classified separately (C).
Group A: Background Noise
In a high quality recorder, background noise is essentially due to the tape and the recorder manufacturer cannot do much about it, except by making it less annoying by raising the recording level (distortion compensating circuits of NAGRA IV-S) or by using recording standards that attenuate the noise (NAGRAMASTER standard).
Group B: Modulation Noise
Modulation noise is a specific shortcoming of the magnetic recording process and it can be caused in several ways, especially by tape vibration noise, amplitude modulation noise and head magnetization noise.
Tape Vibration Noise
The tap is not transported in a perfectly regular manner; its speed varies (wow, flutter and vibration) and the frequency of the variations may be quite high. The frequency of a sine signal of constant level is therefore modulated by speed variations. Wow and flutter are well known subjective experiences, whereas a high frequency vibration of the tape simply make the recording sound dirty. As the noise disappears with the signal, it is obviously a modulation noise.
The problem is very serious with equipment of simple design, having no mechanical filters or - worse - having pressure pads on the heads. The tape vibration noise in the NAGRA IV-S has been rendered negligible.
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 matt 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 Tap 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 matt 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.
I find this information very informative and enlightening. I hope that readers will insist that digital download sites provide accurate information regarding the provenance of any track being listed for sale. If it came from an analog source, it will be limited in its dynamic range AND frequency response. An HD container will provide the best possible playback of the source tape (even safety copies etc), but it is NOT THE SAME AS A NEW HD RECORDING and doesn't deserve to be identified as an HD track.