[Coco] Coco Cassette interface help

[Gene Heskett]

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.

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 .

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.

Cheers, Gene Heskett
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