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Why won't the TI-99/4A cassette port work with my PC sound-out port? Here's why ...

TI-99/4A TL;DR: the OEM TI cassette cable tape-audio-1-input jack is wired incorrectly for equipment available now. Leave pin 9 floating and run pin 3 to CS1 plug sleeve and things magically work. You can cheat by amplifying the signal a lot (and expect it to have issues from time to time) or fix your wiring. Your choice.

Problem description:

Under most circumstances, when wired straight into either the headphone or line-in jack, the signal heard from the TI speaker is low. Really, really low. Much, much lower than I remember it being. And, thus, the TI doesn't see the signal at all. Plugging the microphone jack in sometimes helps, but it's still twitchy.

If you screw around with the cable (ground pin 4, plug the mike jack into headphone out and the audio-in jack into line out, pull the plug out about three millimeters, and so forth), suddenly the incoming signal volume dramatically increases and the TI sees it. That's ... not right, it's very wrong, so I looked deeper.

I've spent two days poring over schematics (the official TI schematics, the SAMS schematics, with an eye towards Thierry's cassette port description) ... and I think I found the problem.

Let's start with Thierry's tape description. This description appears to be the same description that is reproduced throughout the net, but it's slightly wrong (or, at least, cryptic). It looks like this (omitting the ASCII DB9 diagram):

# I/O Use
- --- -------
1 > Cass 1 motor control
2 > Ditto (negative)
3 > Output to tape 1 or 2 (neg)
4 > Audio gate
5 > Output to tape 1 or 2
6 > Cass 2 motor control
7 > Ditto (negative)
8 < Input from tape 1
9 < Ditto (neg)

According to the schematics, that's mostly correct. There's one thing that's glaringly missing, though -- a ground connection.

The way Thierry's diagram should read is thus:

# I/O Use
- --- -------
1 < Cassette 1 motor control (switch input)
2 > Cassette 1 motor control (switch output)
3 x Ground (connected to cassette 1 and 2 microphone sleeve)
4 > Audio gate (unused, signal not present on cable)
5 > Audio output to both cassette 1 and cassette 2
6 < Cassette 2 motor control (switch input)
7 > Cassette 2 motor control (switch output)
8 < Audio input from cassette 1
9 < Audio input from cassette 1, connected to ground through RC filter

Note pins 8 and 9. 8 goes to the tip of the 3.5" jack -- that's the signal. 9 should be grounded, according to the TRS convention where tip and ring carry signal, but sleeve is always grounded.

This would be why I was never seeing signal without the mike line connected to something; there is no ground continuity between the recorder and the console except through the microphone connector.

I say again: If the microphone connector isn't connected, there is no ground connection. The TI depends on the microphone jack ground connection to ground the headphone/line-out jack.

I haven't done the math to figure out what band that RC filter on pin 9 is passing, and the schematics have different opinions about what else is wired into that part of the circuit. Maybe cassette 2 was originally supposed to have output capability, and there was a last-minute cost-cutting measure

The fix is simple: connect the sleeve of the audio-in jack to pin 3, as it's supposed to be grounded anyway.

That, however, is going to also ground the ring on the cable jack, and depending on how the audio source is wired, that may cause issues/damage the ring audio source. There are two possible fixes here: replace the male mono jack on the cable with a stereo mono jack (and leave the ring floating), or build a work-alike interface.

I chose the latter, mostly because my TI cable is extremely beat-up and therefore has questionable electronic reliability, but partially because it's a cleaner solution.

I took a small project box and used a nibbler to cut a hole for a female DB9 connector. I then installed two female stereo 3.5-inch jacks. Pin 5 on the DB9 is connected to the tip of one of the 3.5-inch jacks (microphone), pin 8 to the tip of the other (audio-in), and pin 3 to the sleeve of both (ground)

You can wire in cassette 2 and/or motor control (I'm doing the latter, as the display shield on my BeagleBoneBlack has capability for this), but that's the magic signal fix.

Electrical engineering bits that the uninterested can skip:

What IS going on with that circuit hanging off of pin 9? page 23 of this tech ref makes it clear that it's an RC filter. The QI circuit on page 30 makes it look like it's boosting the impedance (which might actually work, but I don't have a QI). The SAMs looks very much like the RC filter variant, except it has another resister in series before the filter that's probably boosting impedance.

Why hasn't anyone noticed this problem before? I believe that it's because the problem can be mitigated by throwing enough amplification at the problem. You'll see recommendations on forums like AtariAge to run the signal through amplified speakers, thence through the speakers' headphone jack. Others recommend using the line-out jack only, or the headphone jack only, at specific OS-side volume levels.

Each method succeeds to varying degrees, depending again on the hardware that's being used. None address the root issue: the TI cable isn't grounding both plugs, and worst-case is feeding audio backwards into the RC filter on pin 9.

It's that simple. You don't need to mess about with specific volume control settings or specific brands of USB audio dongles: it's a TI design flaw that assumed that the cable would a) always be plugged into a mono source and b) always have the microphone plug connected.

So, in summary, here's the behavior summary as I see it:

1) The TI cassette cable has mono plugs,
2) The audio-in plug therefore shorts the ring (right channel) and sleeve (ground),
3) The TI cassette cable connects the microphone sleeve to console ground,
4) The TI cassette cable connects the audio-in sleeve to ground via (at least) an RC bandpass filter,
5) If the TI cassette cable microphone plug is not plugged into the sound source, there will be a ground loop through the audio-in sleeve,
6) Behavior of the system beyond that point depends very highly on the quality of the sound source design,
7) Anecdotally, adding a large amount of amplification (and possibly equalization) to the signal overcomes the ground loop.

From an electrical engineering (and, thus, physics-derived) standpoint, here's how I see it:

1) There are at least three different versions of the circuit schematic for the goop connected to the audio-in sleeve, which in turn means ...
2) ... there will be variance in what works for each user, depending on the goop in that particular console and the sound source design,
3) The incoming signal is amplified by a pair of op-amps to +5VDC in the console, so significant signal amplification should not be necessary,
4) In general, ground loops are bad and should be avoided if at all possible,
5) In general, significant signal amplification is wasteful and should be avoided if at all possible.

So, here are the possible fixes (in ascending order of desirability) as I see them:

1) Run the signal through a large amount of amplification to overcome the ground loop,
2) Plug in the microphone jack and use a mono-to-stereo plug converter to isolate the ring from the sleeve on the audio-in jack,
3) Replace the mono-intended cable with a stereo-intended cable wired per the first post in the thread.

The first one works for some people, but is electrically incorrect and will not work in all cases. The second and third are functionally equivalent.