Low Frequency Analog Recording Phenomenon

[Victory Pete]

Years ago I noticed something strange while recording kick drum and bass guitar. It seemed the levels were a bit higher on playback. I put in a constant 40hz signal and the level stayed the same on playback. It seemed to me it was the initial attack of the kick and bass notes that caused an inrush of signal that accounted for the level difference. My theory was that at such a dynamic attack the signal acted like a brief DC current in the heads which after all are coils with inductance. According to the Inductive Reactance formula 2 x Pi x Fr x L = Z (resistance) if frequency goes down so does the resistance and the current may increase. Does anyone have any experience with this "Alleged Phenomenon"?

VP

PS It was most noticable with LED segmented meters than VU needles. My LED meters have peak hold.

 

[mshilarious]

Attack is HF, not just LF. I think the Blackmer page I linked before might have some insight--if you go shifting HF around you can change the peak level of a signal even if the levels at each frequency don't change. Going back to your square wave tests and my Excel chart, note that the peak level of the fundamental sine is higher than the peak of a perfect square at the same fundamental frequency. If you screw with the component sine waves of the square (amplitude or phase) you will consequently change peak level.

How to Bias Analog Tape Recorders — Blackmer Design

There are also time delay dispersion effects in the magnetic recording process that do some rather odd things to the time relation between low and high frequencies. As an experiment, fly an impulsive signal, like a kick drum with a good sharp high frequency edge, a strong deep bass thud and a clean, well defined start time, from your Pro-Tools (or whatever) editor into an analog tape recorder. Now fly the reproduced tape signal back into Pro-Tools and try to line it up with the original. Something funny happens; when the two high frequency energy peaks line up there is energy appearing well before the main spike in the tape processed signal.

VU meters have a slow attack (200 or 300msec off the top of my head) so they will not catch stuff like that, by design.

 

[Beck]

The phenomenon is called, “Low frequency Head bump.” The center frequency varies from head-to-head based on design characteristics. 40 Hz is pretty low. You might try between 60 and 100 Hz and you’ll typically see a +2 to +3 dB increase on playback for a steady tone, but it’s common for kick drum to make this even more apparent and might be more than 3dB on transients.

 

[Victory Pete]

The phenomenon is called, “Low frequency Head bump.” The center frequency varies from head-to-head based on design characteristics. 40 Hz is pretty low. You might try between 60 and 100 Hz and you’ll typically see a +2 to +3 dB increase on playback for a steady tone, but it’s common for kick drum to make this even more apparent and might be more than 3dB on transients.

When I calibrated my machines I noticed instead of a bump at low frequencies there was a "Hole". When I ran the frequency sweep it started off at 0db around 50Hz then dipped down below 0db and then back up after 100 Hz. I will try some more tests but it definitely seems to need a "Fast Rising" low signal to come back hotter. That is why I came up with my "DC inductance" theory. For a split second isnt a kick or bass signal similiar to DC? And wouldnt that cause a surge of current in the coil in the record head?

VP

 

[Tim Gillett]

With head bump (or contour effect), if you try to reduce the bump, you can end up with a "hole" somewhere else in the bass. You have to trade off.

It illustrates the problem of recording with the direct analog method. We are forcing the tape to be linear across a very wide range of frequencies or wavelengths, which it struggles to do. Not only head bump but head losses, gap losses come into play.

Think of the HiFi sound on VHS tapes. It used a high carrier frequency and frequency modulated it like FM radio. The frequency response was wide and ruler flat.

Then think of digital PCM audio. It also uses a single carrier frequency. Ruler flat response but unlike FM modulation, much better signal to noise ratio. And of course no wow or flutter, and very low distortion across the entire spectrum.

Tim