[Coco] Coco Cassette interface help

[Dave Philipsen]

> 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.

> 
> 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