The OEM TI cassette cable is wired wrong! (or, how I spent today)

[ckoba]

So I've been trying to unravel TMA-1's signal-not-found issues (as detailed in another thread), and I found something interesting.

 

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.

 

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, and so forth), suddenly the incoming signal volume dramatically increases and the TI sees it. That's ... not right, it's very wrong, and I hope I didn't break anything.

 

I've spent the past 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. 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 was no ground continuity between the recorder and the console except through the microphone connector.

 

So as I see it, the fix is simple: connect the sleeve of the audio-in jack to pin 3, as it's supposed to be grounded anyway. I mentioned that I'd built a quick interface when I couldn't find my OEM TI cable a few days ago; I reused those parts and put them in a project box.

 

I've got pin 5 connected to the tip of the microphone plug, pin 8 to the tip of the audio-in plug, and sleeves of both plugs connected to pin 3. That's it. 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.

 

What exactly is going on with pin 9 is an open question. I haven't done the math to figure out what band that RC filter 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. I don't even know how this was working in the first place, but I know this -- leaving pin 9 floating and grounding the sleeve of the audio-in jack makes ALL of the bogons go away.

 

I do hope this helps someone else, because I burned a good ten hours running signals, reading schematics, and replacing the wiring on my keyboard once it broke off (another story).

 

(what IS going on with that circuit? page 23 of http://www.mainbyte.com/ti99/man/ti99_tech.pdf 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. Can someone with more analog clue than I possess explain what TI was trying to do when they tied that into the audio-in ground line, because it had to work at some point)

Edited January 2, 2016 by ckoba

[+Schmitzi]

Hi ckoba,

do you have a pic of this OEM-cable ? Are there any numbers on it ?

thx

schmitzi

[ckoba]

do you have a pic of this OEM-cable ? Are there any numbers on it ?

 

It's PHA2000 -- the official TI two-cassette cable. I don't have any pictures right now, and I'm too tired to go upstairs and take any. Take my word for it -- it's the stock TI cable, TI logo on the connector and everything. I have two working consoles, both exhibit the same behavior, and maybe this is why everyone treats cassette operations as if they were black magic.

 

The problem isn't the cable per se, I guess, it's what the console is doing on pin 9. The console may be expecting the cassette <-> console ground connection to be through the microphone line, and/or the mono plugs on the recommended tape deck could have had its two connections to tip and ring rather than tip and sleeve (so pin 9 would be grounded at the deck rather than getting an audio signal).

 

This can be fixed console-side by cutting the trace from pin 9 on that DB9 connector and soldering a wire between pin 3 and pin 9. That way you could continue to use the TI cable, I guess, but then you'd have to deal with the tip/sleeve-versus-tip/ring/sleeve issue.

 

Do you have any pertinent comments regarding my analysis of the circuitry surrounding pin 9?

Edited January 2, 2016 by ckoba

[+Schmitzi]

ah OK. If PHA2000 then don´t need a pic. I asked because I read about "3rd party", as I have some 3rd-party-cables too.

 

I remember an issue 1 year ago, when I "restarted", but I cannot say what it was in detail.

Was an overwhelming TI-me, 2 be back to the TI with so much infos and doings.

But now I will check if I have this (double)-cable, to have a look

Sorry, I have no more relevant infos on your issue.

[Stuart]

If you've got a stereo extension cable you could hack about, might be worth fitting a mono socket on it and making sure that tip and sleeve of the stereo plug is connected to tip and sleeve of the mono plug. Sound like currently you've got connections to tip and ring so you're losing one side of the signal?

[ckoba]

Sound like currently you've got connections to tip and ring so you're losing one side of the signal?

 

No, I've got it fixed. I built a small box that handles signal and ground properly, and have detailed a workaround for those that want to use the original TI cable.

 

It seems like the cable is expecting to see signal on tip and ground on ring (because it expects to be plugged into a mono jack), but it's seeing signal on both tip and ring and no ground connection at all. That other side of the signal isn't being lost, it's being fed into what the console thinks ground should be (along with being shorted to the source's ground, but impedance seems to be saving us there) and is cancelling out the signal.

 

I think. That's the only explanation that makes sense -- maybe old tape recorders picked up signals from tip and ring, because for mono use ring and sleeve were the same. Anyone feel like taking oscilloscope measurements on an old GE tape deck output, or tearing one apart and seeing where the tines on the headphone jack were placed respective to the jack body?

 

You're right, this could be mitigated by putting a mono-to-stereo 3.5" adaptor between the OEM jack and the audio plug, but that probably wouldn't handle what was being fed from the console to the sleeve via pin 9.

 

In any case, the TI cable a) shorts one channel to ground (which should only attenuate the signal on modern electronics, but could kill that channel's amp on older gear) and as a consequence b) doesn't properly ground the audio input.

 

This is bad, and feels like a design flaw (or, assuming that TI assumed that what was coming in on pin 9 was *only* ground, a very nasty shortcut). This may well be why tape transfers are finicky and erratic.

 

From an electrical engineering standpoint, given the cables that are used and the equipment those cables are plugged into, I recommend either:

 

* building a tape interface box wired per my first post, or

* cutting the trace on pin 9 and jumpering pin 3 to pin 9 on the console per my second post, because

* the TI console isn't following TRS spec on the audio-in line.

 

Thanks for asking, though.

Edited January 2, 2016 by ckoba

[simbalion]

Hmmm, I never had any issues when I used tape programs on my TI-99/4a years back. Of course, I was using an original, clunky TRS-80 tape recorder back then. Might have to try again and see what happens with the NOS Texas Instruments tape recorder I found a few years back.

[Omega-TI]

BITD some of the guys (that used non-TI cassette decks) purchased a little adapter that hooked on to the TI cassette cable. It resembled a 35mm film canister with a little pigtail on one end and a jack on the other IIRC. To be honest it's been so long I don't know if it was something to do with the remote jack or the audio line. Does anyone else remember better than I? If so, would one of those.. or a REMAKE of one of those help him out?

[+Ksarul]

They advertized them in 99er, but I'm pretty sure they were for the Remote Jack. . .

[ckoba]

Might have to try again and see what happens with the NOS Texas Instruments tape recorder I found a few years back.

 

Oh, neat. If you dig that up, could you do me a favor?

 

* plug a stereo 3.5" male-to-male plug into the earphone jack,

* ditto for the microphone jack,

* check tip, ring, and sleeve for continuity between each of the plugs. In other words, see if you've got a short between the tip of the earphone plug and the microphone plug (you shouldn't), the ring of the earphone plug and the microphone plug (you probably will), and again with the sleeve (I think you won't).

 

I'm starting to be convinced that the mono audio-out jack of cassette players of the time had its two pickups right next to each other, so audio would be picked up at the tip and ground would be picked up at what was then part of the sleeve (and thus grounding the connection between player and console) but is now the ring (which puts an audio signal into what should be ground, which heavily attenuates the signal based on what I think the TI schematic is doing).

Edited January 3, 2016 by ckoba

[Opry99er]

I don't know if you happened to watch that video I posted a couple days ago. I took my cassette interface white cable, plugged it directly into my PC and transferred files that way. No messing around, no jiggling cables, no ground issues.

[ckoba]

I don't know if you happened to watch that video I posted a couple days ago. I took my cassette interface white cable, plugged it directly into my PC and transferred files that way. No messing around, no jiggling cables, no ground issues.

 

I did see that video, but I did not make the connection to this. I skimmed it briefly just now looking for bits that would correlate to this.

 

At about 1:50, you're plugging the white wire into the headphone-out on your amplified speakers, not directly into the PC. Electrically that's not the same thing, and that amp is going to be putting out a much stronger signal than the ports on your PC, I think, and impedance may be coming into play.

 

That having been said, the comparative volume between the console "beep" and the program read sounds right. You've got a good ground connection between the console and the speakers, or the speaker amp is loud enough to drive the console.

 

... do you have the one-cassette or the two-cassette cable?

[+OLD CS1]

I don't know if you happened to watch that video I posted a couple days ago. I took my cassette interface white cable, plugged it directly into my PC and transferred files that way. No messing around, no jiggling cables, no ground issues.

 

I did the same with output from my phone. I had to double-up on the output from the phone (using both channels) on the single input as the line-out from the phone is lower than the headphone output. I suspect the same would be from the cassette recorder -- the TI is exploiting the built-in amplifier circuit of the cassette deck.

[Opry99er]

 

I did see that video, but I did not make the connection to this. I skimmed it briefly just now looking for bits that would correlate to this.

 

At about 1:50, you're plugging the white wire into the headphone-out on your amplified speakers, not directly into the PC. Electrically that's not the same thing, and that amp is going to be putting out a much stronger signal than the ports on your PC, I think, and impedance may be coming into play.

 

That having been said, the comparative volume between the console "beep" and the program read sounds right. You've got a good ground connection between the console and the speakers, or the speaker amp is loud enough to drive the console.

 

... do you have the one-cassette or the two-cassette cable?

 

 

It is the double cable.

[ckoba]

 

I did the same with output from my phone. I had to double-up on the output from the phone (using both channels) on the single input as the line-out from the phone is lower than the headphone output. I suspect the same would be from the cassette recorder -- the TI is exploiting the built-in amplifier circuit of the cassette deck.

 

Okay, now I think I see why this works for most people -- lots of amplification overcomes the signal drain through the gunk connected to pin 9. If the white jack is connected to a mono source, then the gunk connected to pin 9 works as TI intended (at the very least an RC filter, plus maybe messing with impedance, depending on the schematic you're looking at).

 

If the white jack is connected to a stereo source, you've got a fight between the other half of the stereo signal and the sleeve ground. The gunk hanging off of pin 9 is seeing a faint echo of the "real" signal on pin 8 (because you've now got a ground loop), and since pin 8 and 9 are connected via a cap and a resistor (at least), the real signal is going to be seriously attenuated, thus not giving the pair of op-amps much to work work.

 

So this works for most people because they're throwing a lot of amplification at the problem to overcome a design that didn't expect a signal to be coming in a ground line.

 

Interesting.

 

So, if you don't want to or can't double-amp the signal, try a modified cable/interface box. See if that lessens the need for heavy amplification. I'll bet things will work better if you do.

[ckoba]

It is the double cable.

 

Thanks. Important to know that we're testing like-for-like.

 

As I said just above, this works for most people purely by the power of overamplification overcoming what appears to effectively be a ground loop, per https://en.wikipedia.org/wiki/Ground_loop_%28electricity%29. I don't think TI had stereo 3.5" jacks in mind when they were designing this in the mid-seventies.

 

I truly believe that the right thing to do is to keep any signal from reaching pin 9 ( https://en.wikipedia.org/wiki/Phone_connector_%28audio%29#Mono_and_stereo_compatibility, and also the fact that any true ground connection is going to be on the mike line via pin 3).

 

Like I said, I hope this research and analysis helps someone else from spinning their wheels trying to get a working tape connection ...

Edited January 3, 2016 by ckoba

[+OLD CS1]

I will give this a shot with my custom input unit I made to connect my phone. Much later, mind you, as I already have too much to do this weekend with it being end-of-year (try not to get behind on billing, kids, as it just adds up!)

[Opry99er]

Thank you for researching and working through this issue. I have never had an issue loading from tape, so it never occurred to me to study the pinouts and what-have-yous.

 

Only times I have ever tried to load from PC were successful, but each time it was through the amplified out jack on my powered speakers.

[ckoba]

Thank you for researching and working through this issue. I have never had an issue loading from tape, so it never occurred to me to study the pinouts and what-have-yous.

 

No worries, this is the sort of thing that I was trained to do. Sure wish I could re-use some of those EE brain cells for something else, because I so very rarely get to actually use them at this point in my career arc.

 

When I got back into TI about a year ago, I thought it odd that I couldn't "OLD CS1" unless the microphone plug was also connected, but never really gave it much thought until I had to build a temporary cable (I got ahold of a PEB and a HxC, so I don't usually use tape anymore). I tell you, I had several WTF moments while reading the schematics ...

 

... anyway, glad to help.

Edited January 3, 2016 by ckoba

[ckoba]

I will give this a shot with my custom input unit I made to connect my phone. Much later, mind you, as I already have too much to do this weekend with it being end-of-year (try not to get behind on billing, kids, as it just adds up!)

 

Be wary of the phone screwing with the signal equalization. At United Fruit we played games pre-DAC (and not tunable in customer OS builds) to optimize the sound experience for people using United Fruit headphones (mostly screwed with the bass); the tape signal is frequency-modulated so you want the signal to be unmolested.

Edited January 3, 2016 by ckoba

[+OLD CS1]

Sure wish I could re-use some of those EE brain cells for something else, because I so very rarely get to actually use them at this point in my career arc.

 

Ah, management, are ye?

 

Be wary of the phone screwing with the signal equalization. At United Fruit we played games pre-DAC (and not tunable in customer OS builds) to optimize the sound experience for people using United Fruit headphones (mostly screwed with the bass); the tape signal is frequency-modulated so you want the signal to be unmolested.

 

 

It was a pretty messy assembly (and looking at the picture now, notice where I get my ground!)

 

 

But it worked out

 

http://atariage.com/forums/gallery/image/13491-junky-mobile-phone-to-cs1-demonstration/

 

I built this apparatus to make it easier.

 

[ckoba]

 

Ah, management, are ye?

 

It was a pretty messy assembly (and looking at the picture now, notice where I get my ground!)

 

Nowadays, when I have a gig, I'm either team lead or project manager, yeah. I prefer the former.

 

That "messy setup" is correct. You're not mixing right-channel audio with the TI audio-in ground, so no ground loop and you're good-to-go.

 

Here's mine, for what it's worth:

 

It (and the USB sound converter) sit on top of the PEB under the desk, and that thing on the right is a BeagleBoneBlack with a LCD screen emulating the tape recorder interface.

[Fredrik Öhrström]

I created a tool to test ckoba's hypothesis that a stereo signal in both left and right

gets pushed symmetrically into "Input from tape 1" and "Input from tape 1 (neg)"

thus noticeably reducing the differential, ie the volume.

 

Point your browser to: http://nivelleringslikaren.eu/ti994a_basic_test/

 

Press "Play Type". You can now choose to generate a wav file with:

MONO

STEREO same data in both left and right

LEFT only left contains data, the other is zero.

RIGHT only right contains data, the other is zero.

INVERTED same data, but left is the inverse of right.

 

Press "Play in Speaker" to play directly.

Press "Download WAV" to download a wav for further experiments.

 

I have not yet had time to actually test the differences on my real TI.

 

If the hypothesis is correct, then the best choice would be LEFT or INVERTED.

I.e. the right is silent and does not interfere, or the right is the inverse and amplifies instead of diminshes.

 

I suppose a lot depends on the hardware inside the computer/phone that generates the output.

 

What happens when you push a mono-plug into a stereo line/headphone output? The headphone output

ground should be shorted with the right channel, thus the right ought to go to zero, irrespective of what is played.

But I am sure there is hardware out there, where this might not be true and the right channel actually takes

(fully or partially) control over the ground, and gives the result that ckoba describes.

 

Likewise, on the input side, ie the computer mic input. Older PCs have proper mono mic inputs, newer

USB dongles have stereo mic/line inputs. My old laptop easily hears the mono-output from the TI, just plug it in

and record.

 

A USB audio dongle works, but I cannot push the mono-plug completely into the mic input of the dongle. If I do,

it seems like it shorts the right channel to ground, the dongle is very unhappy and only zeroes come out when recording.

If I leave a millimeter or two, it does not short circuit, or the TI tip ends up on the right channel ring, and it records.

 

(The default headphone/mic output/input of my MacBook refuses any hardware that is not exactly

conformant to mic detection, the TI is not one of them. I tried. :-) )

[Fredrik Öhrström]

Tried it on my real TI with three different hardwares. When playing in right speaker only, it is dead silent,

which indicates that the right channel is properly shorted to ground or ignored. Though this is based on

my ears only, not a measurement with an oscilloscope.

 

Other than that, there is no difference.

The output from my MacBook headphone works nicely, the volume has to be at least 75% of max.

The output from my Eee PC with linux headphone is not powerful enough, it always fails.

The output from my USB dongle headphone works nicely, the volume has to be at least 75% of max.

 

//Fredrik

[+OLD CS1]

In my experience, the output of the TI cassette cable to a stereo deck is always left channel only. When I started trying to read my old tapes into my laptop to work on old BASIC programs in Classic 99, I also created freshly output wavs to load into the TI. I found that, dutifully, the headphone output of the laptop was attenuated for human ears which the TI did not appreciate. I took the $3 USB audio card from my Amiga and used it and the output was loud enough (and clear enough) to be understood by the TI. I do recall I had to do some equalization in VLC when I played it, I think I used the "Reggae" preset.