Some basic information about HIFI DIN sockets

There seems to be little information about the use of DIN sockets in hifi and the technical background,.

I regularly mess with all kinds of vintage hifi and audio stuff, but I have a particular soft spot for compact cassette decks and tape recorders, and a side effect of this fondness is that I'm somewhat frustrated by RCA cables. Let's say the obvious: stereo RCA cables in their "modern" form are a blatant bodge. The plug can only carry one signal channel (since it was designed before stereo showed up and became the norm in home audio), so let's just use two of them. Yeah, maybe, if you're just putting something together quickly for a tech demo, but this limitation adds a lot of unnecessary complication to hooking up tape recorders in the form of a frankly ridiculous number of sockets and plugs¹.

If you're not familiar with DIN connectors - and there's a good chance you aren't, photos of them with the question "what's this?" pop up fairly regularly in vintage audio subreddits - they were Europe's solution to connecting various bits of consumer audio to each other (they were also used for computer keyboards, MIDI equipment and a whole host of proprietary purposes). Much like RCA, the design had to be altered to work with stereo, leaving some annoying quirks behind for all future generations. Nonetheless, as far as the physical plug and socket goes, I think it is a much more well rounded design. In particular, it has been designed with room for two-directional connections from the start precisely to simplify the connection of recorders.

Isn't that just so much nicer than the tangle of RCA cables you need to achieve the same result? A single connector plugged into a socket and it's done, no need to fiddle with additional plugs or paying attention which channel is which. I certainly thought so, but then I actually tried using them and ran into some problems.

Pinout

The first hiccup with what I've outlined so far is that playback-only equipment like tuners or turntables have no need for the recording connections, so naturally sockets on those (and the corresponding sockets on an amplifier) would have some pins unconnected. This becomes a problem when you run into some home made cables that also saved the unnecessary effort and expense of wiring up the unused pins². With no markings to set them apart, it's anyone's guess which one of these two interconnects will be appropriate for hooking up a tape recorder

It was of course a trick question, neither of these will work. Turns out that while most tape recorders use the same pinout, there isn't actually a standard layout for DIN audio sockets and apparently tuners tend to follow a different quasi-standard that is effectively mirrored. This again isn't a problem as long as the input socket on the amplifier uses the same pinout, but if you wanted to, for example, plug a tape deck into a tuner input for playback, you'd need a special crossover cable that has its "wires crossed". One of the cables pictured above is like that, but there's no way to tell which one it is just by looking.

Levels

So far I've been mostly complaining about dodgy home-made cables and none of the above should stop me from hooking up my tape recorder with a properly wired cable. So I made myself one from quality components and plugged it into a Sony reel-ro-reel I have. That's when I found that I can't use one of the main features of this three-head tapecorder, because there's no playback through the DIN output during recording. The reason wasn't a mystery for long either, turning the input level up with the monitor switch in the "source" position (i.e. input connected to output instead of tape heads) results in a nasty feedback squeal.

Up until this point, I had just assumed the signals in DIN connectors were more-or-less equivalent to line level signals used on RCA connections. It's generally reasonable to treat them as such, plugging line outputs into DIN sockets on amps usually works and in some cases DIN and RCA connections are wired in parallel. Inputs on tape recorders however, are a different story. Let's take a gander at the manual of that Sony tapecorder:

First let's have a look at how line level inputs (labelled "AUX" on this unit) work. The 60mV sensitivity of the line input may seem odd at first, since line level signals are usually an order of magnitude higher, around 600mV, but this setup is fairly common on tape recorders. The numbers specified here are maximum levels, the actual level on an output is lower than that pretty much all the time, sometimes by quite a large margin. Most music isn't a continuous wall of sound after all, and if you factor in variations in output levels across different units, the often wildly varying loudness of the recordings themselves, inputs being more sensitive makes sense. If the input signal is coming in too hot, you can simply turn the record level down (reducing sensitivity) a bit until it is just right. But this way it's possible to make use of the full dynamic range of the tape even if the source is a relatively quiet recording.

Looking at the specs for the DIN socket ("REC/PB") on the other hand will reveal, that it has a sensitivity of 17mV while the output is basically line level (my limited reearch suggests most DIN outputs are in the 200..600mV range). And this isn't the most extreme example I've encountered, the manual for a Fisher cassette deck I have has a sensitivity of 0.1mV(!) for the DIN input. Considering the already higher chance of crosstalk when all signals travel through the same cable, it's hardly surprising then that all three tape recorders I've tested turn the DIN output off during recording to prevent feedback. To make all of this more confusing, looking at the schematics of some amplifiers with DIN sockets for tape recorders I found that the output pins are connected through fairly high value resistors (around 500 kiloohms) in order to reduce signal level (even signal coming from other DIN connectors) to something within the same order of magnitude as the sensitivity of these inputs. So the signal is attenuated severely before passing through the cable to the tape deck where it then has to be amplified back (usually by passing through some part of the microphone preamp circuitry), which seems like a good way to add some noise to your recordings. I can only speculate on the reasons for this arrangement - if you know more about this topic please do get in touch - but all this means that DIN cables are fairly limited in their usefullness by the signal levels used in them, which is kind of a shame.