[sobriquet9]

The tape delay methaphor confused me. Tape recorders do not record DC or other frequencies much lower than, say, 20 Hz. So that circuit would run into one of the rails just as quickly as the previous cirquit without DC feedback.

[mrob]

It must be an FM encoded tape, which can record DC. There were upgraded versions of both VHS and Betamax with FM audio support (although I expect in practice the inputs were AC coupled).

[sobriquet9]

FM audio does not carry DC either. PLL in the receiver/decoder will eventually catch up with the constant frequency shift.

[ddingus]

It can carry frequencies much closer to DC.

The above mentioned FM upgrade to VHS, called HI-Fi VHS I believe, had excellent audio recording capabilities. I used it to archive albums back in the day, and when I play some of the warped ones back today, the speaker cone can be seen moving where it would playing the original vinyl.

[sobriquet9]

But not DC itself. Even if it goes down to 1 Hz, the circuit in question will saturate in a few seconds, or be in saturation when initially turned on and never get to zero as there is no DC feedback.

[ddingus]

Yes, I did not mean to imply otherwise.

The way I have always come to understand this boils down to what modulation actually is. DC is not modulation. Anything above DC can be, or maybe technically is and must be managed or accounted for. Frequency drift might be one example.

DC is basically continuous wave and the signaling, if any is intended, is basically limited to turning the wave on or off, or the wave just being present.

The VCR was basically reproducing very low frequency waves. One could see the cones moving in and out with the rotation of the 33 RPM vinyl.

The only other analog device that did something similar was an old vacuum tube reel to reel running a very high tape speed. And the effect was not as dramatic.

Both of those devices could basically archive a CD respectably. The VCR added about 3db and the reel a bit more that I was not in a position to measure.

The most striking things about that ancient reel to reel device were that it was rack mounted with some other gear, EQ, pre-amp, amp, and some other things, and how much better it could really do when using pretty great tape at crazy speeds. The highs would start to roll off pretty high, 18khz or so. The usual was 10 to 12.

That was a very fun old piece of gear. Left over relic from the 60's available for curious high schoolers to tinker with.

[mrob]

Who says you have to use an analog PLL decoder? FM audio tape is clearly capable of storing DC because you can store a constant sine wave at a different frequency to your carrier frequency. Maybe you'll decode it with software defined radio software.

[sobriquet9]

Even when you decode with an SDR, you still need to adjust the receiver frequency to match that of the transmitter, as reference oscillators might have slightly different frequencies that can also slowly shift, e.g., due to changes in temperature. In case of a tape recorder, tape speed during recording and playback can also be slightly different due to mechanical factors.

[mrob]

Sync everything to GPS. Record a harmonic of the carrier on the same tape, or just record the carrier itself on a parallel track. Have the SDR use this reference tone to undo any tape speed fluctuation artifacts. You can bring the deviation from DC so low that your comment no longer has any relevance to tape machines and just becomes a pedantic comment that mathematically perfect DC doesn't exist in physical reality.

[sobriquet9]

Tape machines predate GPS.

A metaphor should be referring to something common, not something that can be made in theory to fit a different set of requirements, but does not really exist.