How to make a Speed Calibration tape

[Beck]

Heh heh...
I was desperately trying not to, but since it occurred against my will, I was thinking about thinking as an unfortunate circumstance of life.

Then I was "thinking" about who would be the best to ask to bring to life what I was thinking about, and I've figured, these guys:

But I think in your case ZEE... "Wit happens"

 

[evm1024]

Nice work so far, Ethan.

Now the proof off concept test:

(First, if you don’t have one make your own speed tape with a 3kHz signal to get your machine back in the ballpark to where it was before the following test).

Next, turn the speed adjustment to an extreme direction to produce an unknown speed. Then make another tape at the unknown speed and use it to bring your machine back to where you believe the machine is now… roughly 15 ips.

~Tim

Tim, You are a hard sell....

How about I use the variable speed control and make a tape at something other than 15 ips. Then I'll count the cycles on it and calculate what freq it should read at 15 ips. Then adjust the vari speed to make the transport play back at that calculated frequency. Then I'll put my MRL in the transport without touching anything and see if the 1 KHz ref tone is 1 khz.

How would that work for you?

Regards, Ethan

PS Not tonight....

 

[Beck]

Tim, You are a hard sell....

How about I use the variable speed control and make a tape at something other than 15 ips. Then I'll count the cycles on it and calculate what freq it should read at 15 ips. Then adjust the vari speed to make the transport play back at that calculated frequency. Then I'll put my MRL in the transport without touching anything and see if the 1 KHz ref tone is 1 khz.

How would that work for you?

Regards, Ethan

PS Not tonight....

No rush on my account, Ethan. I'll be here.

The only problem I see is that we know where 15 ips is on the BR-20... it's with the speed control in the middle.

~Tim

 

[evm1024]

On the BR-20....

15 ips is when you turn off variable speed (and set the deck to 15 ips) provided the speed is correctly calibrated. When you turn on varoable speed you can use the speed control to adjust the speed Plus or Minus 14% or so.

Side note: PLL (Phase locked loops) depend on their reference clock to lock the controlled device to the reference. If the reference is off the PLL will be happy to lock to it.

OK on to the test....

I took my MRL out and ran it through the BR-20 at 15 ips (variable speed off) and measured 1.006 KHz for the 1KHz tone. THis is 0.6% off of 1KHz. Now we do not know if it is the MRL, the Deck or both that is not exactly 1.000 KHz.

Next I turned on variable speed and turned the speed control all the way to the left (which slows down the deck). The MRL 1 KHz tone now plays back at 845 Hz. THis is 16% slower or so.

I just kinda set the dial on the signal generator and ended up with a 650 Hz sine wave tone.

I then recorded that 650 tone onto the 25' of tape at the unknown speed. I then played it back without changing speed and got 650 Hz off the tape.

Next I set my counter to cycle count mode and counted 15363 cycles on the tape.

Calculating 300 inched / 15363 cycles = 51.21 cycles per inch.
15 inches of tape per second times 51.21 cycles per inch gives us 768.15 cycles per second.

So the tape should read 768.15 Hz when the tape is going 15 ips.

I then played the tape back and with the counter in freq mode adjusted the variable speed control until the counter read 768 Hz.

Lastly without changing anything I put the MRL on the deck and listened to the 1 KHz tone. The counter shows it as 1.0046 KHz

Perhaps my speed calibration tape is more accurate than the MRL tape. In any case we are talking 1/10th of a percent difference.


Regards, Ethan

PS I've been thinking about how to make calibration tapes for azimuth and fluxivity....

 

[Dr ZEE]

You've been warned!

... I've been thinking about how to make calibration tapes for azimuth and fluxivity....

Oh, Boy...