"Real time" tape counter experiments

[20tajk7]

Always wanted to know how does works the real time tape counter used on most mid to high end decks.
These counters are not totally real time counters as they just uses the reel pulses and are not synchronized with any clock.
They are relatively precise when using C60, C90 or large hub C46's but can be quite wrong when using short or less standard tape lengths.

After a long brainstorming I've deducted that on a full length of tape the take up reel pulses count number is the same as the supply reel pulses count number but the counting rate decreases over time on the take up reel and the counting rate increases over time on the supply reel.
So how to get a linear counting rate that can be easily translated to one sec pulses ?

I got a cassette deck with a "real time" counter (Kenwood KX-7030) and connected the reel pulse signals to a small PLC (Chinese copy of a Mitsubishi FX1N) monitored by a computer.
Had to make a buffer circuit (2N2222, 100K on base, 4K7 on collector) as the signal from the tape deck is 5V and the FX1N needs 12V.


X0 input is the supply reel and X1 is the take up reel.
The program stops after one minute, I write the values on a spreadsheet then reset the plc for another cycle.
Done that 24 times to finally found out that counting the pulses of X0 OR X1 gave the most "linear" value of +/-139.5 counts per minutes (mini 134, maxi 149).
So I have broke a part of the secret of these counters !

 

[J!m]

I figured they’d run off the capstan, which is (should be) constant speed…

 

[vince666]

interesting project there!

and there are also some other kinds of time counters, i.e. in my Pioneer 737.
the times goes on only while you are in play or rec mode but not in fast winding mode.
But these are useful only, i.e., while recording since they are like a sort of stop watches and, being so, they are very precise and unrelated to the cassette hubs/reels.

 

[20tajk7]

My Teac V-80 also has that kind of stopwatch counter.
Will test with another program by using pulse edge from the reel signal to get more counts and more precision.

 

[J!m]

I haven’t seen Sam around here for a while but my Tascam has a real time counter and he would know how it works. That one is pretty good and works Ff and REW too; you can go to programmed set points and it works pretty well. But not perfectly of course. Probably within 3/4” of set point on the tape- a second or two variance.

Similar deviation over the length of a C90, using play. I’ve dubbed CD to tape and the CD counter is not in alignment with the tape deck, but does align with reality.

 

[vince666]

of course, counters which rely on pulses from the hubs are directly influenced by the diameter of the totally wound reel of tape and the tape thickness.

i own, i.e. some later Emtec CSII cassettes of c60 lenght but where they used a tape which is more thin of the usual c60...
and the real time counters estimate noticeably more time than it actually is.

 

[20tajk7]

Tried with rising & falling pulses, of course more counts but also more differences between beginning, middle and end of tape.
Also tested with a C60 cassette, the boolean equation X0 + X1 still gives the best results but the beginning and end of tape are still imprecise.

 

[borchee]

Oh, bloody hell, it seems I deleted my own post while editing (posting additional info) - idiot! I didn't know this could be done.

What was it again...something along these lines...

The advantage of counting pulses of the reels and computing the time based on differences in angular speed/rpms/gear ratio in real time, is that the counter works at playback/recording and at fast winding...regardless of starting it at the beginning of tape or anywhere.

The disadvantage is that the counter rather isn't in sync with a clock. This is a good indication as long one uses only one deck, or several that count with same (in-)accuracy, and cassettes of the same brand/type/thickness. If one notes/writes down the beginning of a song, it can be quite accurately found.

I built myself a real time calibration tool/PCB based on the service manual.

As the tape gets packed tighter at playback/recording than at fast winding, the counter time gets altered.

My "experiments" playing a C90:
- previously rewound...counter shows 46:28
- previously played...counter shows 46:17.

Stopwatch clocks in at 46:57, cassette is a Sony HF-S90, deck is Revox H1.

Here the edit:

Tried it with Revox B215 also to see how the counter was set:
- previously played cassette (same one)...counter shows 46:18, so basically the same as H1
- 17 clicks of the calibrator result in 46:57 - exactly as the stopwatch...I got lucky here, it usually is within 1-2 seconds difference compared to the stopwatch.

Perhaps I'll do some moere cassettes and post counter and stopwatch results. Currently a TDK MA90 is doing the rounds.

 

[pioneercollector]

Very interesting, I was not sure how certain machines did the dark art of 'real' time counting, and i was going to mention my Studer A721. but that was covered by the Revox thread ( I am sure it is the same method? )

I wonder what my AIWA XK-S9000 uses as a model? it does seem to be 'fairly' accurate...

I tend to wind the tape to the end and look at the counter and use that as a rough guide as to what is going to fit ..... or not!

PC

 

[borchee]

...I am sure it is the same method?...

Yes, same thing. The only difference is the Studer has the calibrator on board (left front field with two holes marked -ADJ+), the Revox B215 not - well, mine does.



The TDK MA90 is through:
- counter 48:53
- stopwatch 47:17... even longer than the Sony.

So, no universal counter calibration, unfortunatelly - the two tapes are of different thickness, I suppose.

 

[overmodulated]

Interesting thread, and I have one real time counter and it is pretty accurate counting minutes and seconds. Underneath the counter it even says minutes and seconds. Never more than one minute off at the end of a 90 minute tape. Very handy to have that way, you can remember where certain songs are, or just when you're recording/playback will be done,