[Coco] Coco Cassette interface help

[Gene Heskett]

On Tuesday 28 February 2017 16:01:16 Dave Philipsen wrote:

> > On Feb 28, 2017, at 2:38 PM, Gene Heskett <gheskett at shentel.net> 
wrote:
> >> On Tuesday 28 February 2017 14:25:52 RETRO Innovations wrote:
> >>> On 2/28/2017 1:05 PM, Dave Philipsen wrote:
> >>> All you want to do is amplify the signal that is coming from the
> >>> CoCo cassette out. Use a capacitor to de-couple it from the
> >>> comparator circuit. It won't matter if there's DC bias on the
> >>> output of the op amp because you will effectively remove it with
> >>> the cap.
> >>
> >> True, and I will give it a shot, but I'll continue to search for a
> >> way to make the comparator idea work alone.  Amplifying the source
> >> only to get it "over the bias" of the comparator just seems wrong
> >> to me :-)
> >>
> >> Still, I already was thinking of your idea, in the context of using
> >> an LM386 to amp the signal brute force to get it to work.  So, I
> >> agree it's a workable solution.  Thanks for the suggestion.
> >>
> >> I am also going to see if I can feed some known signals into the
> >> comparator to see if I have messed up the design.
> >>
> >> Jim
> >
> > Jim, as an elderly CET, I would simply put in a pot on the
> > comparators bias input, and adjust it to about .6 volts plus or
> > minus a few millivolts, and data recovery should then be fine since
> > theres no tape with its huge hiss level involved because all of the
> > $20 tape recorders sold in the day for up to $80, used a strong
> > magnet to erase the tape, and the weaker magnet as recording bias.
>
> I think most recorders only had a single erase head but they also
> incorporated a hi-frequency component on the erase head to 'bias' the
> tape as content was being recorded.  I believe that helped with the
> recording of higher audio frequencies.

I have never seen a cassette deck selling for under a hundred dollar bill 
that had a bias generator. Among other reasons it shortened the battery 
life in addition to costing money for the build parts. I am not saying 
it doesn't exist, but back in the day I owned quite a few, always 
looking for that magic twanger that just worked.  They were hard to 
find, and after the divorce in the mid-80's (so my kids couldn't tear it 
up) I wound up using a Teac TR-22, a 1/4" bidirectional reel to reel 
which did a bulletproof job using only 1 quarter track at 3.75"/second.  
And I still have a Sony bidirectional metal tape cassette deck that made 
good tapes. But its now around 27 years old and always had a hum that 
was endemic to that model as I took it back and got another twice 
because of the hum. I was PO'd, and that may well be the last Sony 
product I ever bought. I certainly swore off their music cd's when they 
put a computer drive bricker on them to combat piracy. That was the 
straw that killed this camel. 
>
> > Someone said that the coco's output should be fed to the microphone
> > input.  No, at 1.2v ptp, it will seriously overdrive the mic
> > circuitry which is designed to work with just a few millivolts of
> > input.  Fed to the line input, the waveforms will be far more likely
> > to be put on the tape as usable signals.  The "line" input is
> > designed to handle a line level that is from 0 to plus 4db on the vu
> > meter, if the meter is a std vu meter. 0db being .707 volts RMS, is
> > 0.99687 voltswhich is quite close to the 1.2 volts ptp the coco's
> > output of .
>
> Yes, I erroneously mentioned the MIC input but it's actually a
> consumer line level input on the tape deck whose signals are still
> considerably lower in amplitude than an un-loaded speaker output from
> the same deck.
>
> > Very few consumer grade vu meters are anywhere near that well
> > calibrated however.
> >
> > And true AC bias recorders are much better in the hiss department,
> > that single change being good for at least a 20db improvement in
> > signal to noise during the playback because of the loss of the tape
> > hiss.  And because there is no residual magnetism left in the head,
> > the tape should play flawlessly for at least 100x the play passes
> > you get out of the cheaper machine, they erase a wee bit of the
> > signal everytime you play it.
> >
> > Quick test to see what sort of a cassette machine you have. With
> > power off, press the record button while watching the head under the
> > cover. If you see the heads move, put it down and go look at the
> > next higher priced one until you find one that does not move the
> > heads in and out. That one should have an AC bias generator for
> > recording.  And you should have just solved all your poor
> > record/playback of the coco's data output. BTDT folks.
>
> Mmmmm. I'm pretty sure all cassette decks needed to move the heads
> into place due to the fact that the magnetic tape is recessed into the
> shell of the cassette.  There were some really cheap decks that
> actually used a permanent magnet to erase the tape and thus were not
> able to bias the tape during erasure.
>
> > Cheers, Gene Heskett
> > --
>
> Dave


Cheers, Gene Heskett
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