How to make a Speed Calibration tape

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evm1024

evm1024

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(Chalk this up to a brain fart if you want to)

I was daydreaming on the way to work this morning and was thinking on how to make a speed calibration tape.

The problem is that no matter what the speed of your deck if you record a 1KHz tone you will get that back. One needs to know the frequency recorded for the tape to be useful.

Many digital counters have a count pulse mode where the total number of pulses are counted. Perhaps some of the software counter have this. This gives us an absolute value that does not depend on some time calibration in a counter and can be used to our advantage. So....

At 15 ips 25' of tape will pass under the heads in 20 seconds. And a 1KHz signal will put 20 k cycles on that same 25'.

SO if we measure out 25' of tape and cut it as close to exactly 25' as we can get, then put leader tape on it (enough so that the deck speed will stabilize) and then record a (close to) 1KHz tone we will have a speed standard. How so?

Set up the counter in total count mode and then play the 25' tape. It will start counting pulses and total them for you. If the tape has 20,000 pulses then you have a 1 KHz tone on it. (20,000/20) If the tape has 21,500 you have a 1.075 KHz tone (21500/20) etc.

With a known signal on tape you can use an accurate frequency counter to adjust the transports speed.

The count of pulses is most accurate measurement that you have in this process. Getting 25' of tape is a little harder. 1/8" off and you have 125 pulses error which gives 0.625 % error.

Errors in the freq counter are a bit harder to quantify. The poor mans way to calibrate a counter is to get a radio that is capable of receiving WWV (the time standard) and setting an oscillator to beat against its carrier (listening to WWV at 10 MHz and setting your oscillator to match means that your osc is at 10 MHZ) Then adjust your counters master oscillator to have the counter read 10.00000 MHz. Or some such....

Of course once you have adjusted your deck to be 15 ips you can set a 1 KHz tone on it and measure 20,000 pulses for a better reference tape.

Like I said, Just day dreaming. I may have to try this out sooner or later.

Regards, Ethan
 
I'd hate to read a transcript of one of your night dreams...

It may be a poor man's method but you still have to have the frequency counter...

So let me see if I get it...

You have a known length of tape and you record on that a known frequency (call it 'k' for known)...then you count the cycles which gives you the actual frequency recorded to tape (call it 'a' for actual, which, as you said, will always come back off tape as it went in, but the proof is in the cycle count...), and then you...:confused:...how do you use that to adjust the transport's speed? I understand the concept but I'm trying to wrap my head around the formula you'd use to get it in spec...divide a by k and apply the resulting percentage (call it 'm' for modifier) to k (call it 'n' for new) and then dial the transport speed up or down until the frequency coming off the 25' strip of tape is equal to n?

So:
  1. a/k=m
  2. mk=n

:confused:
 
cool day-dreaming thoughts :cool:

BUT! if you write lines like this:
evm1024 said:
If the tape has 20,000 pulses then you have a 1 KHz tone on it.
Click to expand...

...you may confuse day-lights out of a reader :D
There's no such thing as JUST XkHz tone on a tape, there's only - X kHz tone on a tape that is recorded exactly at X ips (speed). But you know that :)

I guess the "system" can work as "speed calibration", but the "trick" would be to record 1KHz tone from a trusted well calibrated source onto that special 25-footer :), then rewind, then play it back and "calculate total pulses" with the counter, then adjust speed up or down depending on the "counting" result, then rewind 25-footer again and re-record 1kHz and then rewind again and then play-back again and "calculate total pulses" and keep doing this untill you get as close to 20,000 total pulses as you can get on play-back counting, and then!!!!!!!!!!!! you are all set. Now you deck is "speed perfect", so now you can simply record 1kHz on a brand new a minute-long or so tape and lable it as "My Speed Standard Tape" :D

am I missing something? :)
 
Yes, something missing.

If you have a known tape length and a known number of pulses you can calculate the frequency at any given speed. This is the key.

So if you have 19,000 pulses on 25' of tape you have a (19000/20) 950 Hz tone on the tape >>at<< exactly 15 ips. (15 ips does 25' in 20 seconds)

Then you adjust your decks speed so that the tone measures out at 950 Hz.

In effect you are figuring out the 15 ips tone freq.

--Ethan
 
The actual frequency of the tone that you record does not matter (due to your recording speed being unknown). Known length which is known time at 15 ips and known number of pulses gives you freq at a specified speed. Then you adjust the decks playback speed to match the calculated speed.

Ethan

PS I have ahd dreams at night where Harrison Ford and I were having an adventure together. Snakes, Airplanes, spys and all.


sweetbeats said:
I'd hate to read a transcript of one of your night dreams...

It may be a poor man's method but you still have to have the frequency counter...

So let me see if I get it...

You have a known length of tape and you record on that a known frequency (call it 'k' for known)...then you count the cycles which gives you the actual frequency recorded to tape (call it 'a' for actual, which, as you said, will always come back off tape as it went in, but the proof is in the cycle count...), and then you...:confused:...how do you use that to adjust the transport's speed? I understand the concept but I'm trying to wrap my head around the formula you'd use to get it in spec...divide a by k and apply the resulting percentage (call it 'm' for modifier) to k (call it 'n' for new) and then dial the transport speed up or down until the frequency coming off the 25' strip of tape is equal to n?

So:
  1. a/k=m
  2. mk=n

:confused:
Click to expand...
 
Or you can buy a Flutter and Speed Test tape from MRL.

http://home.comcast.net/~mrltapes/pub570.pdf


(from the MRL PDF)
----------
SPEED MEASUREMENT
The IEC Standard 60 094-3 [5] recommends measuring the "tape speed" by reproducing a recording of an accurately recorded wavelength. When we record the MRL Flutter and Speed Test Tape, the recorded wavelength is ±0.1% of the true value with a tape tension into the head assembly of 0.8 newtons (N) for 6.3 mm tape width (1 newton = 3.6 ounces = 100 grams force); 1.3 N for 12.5 mm width; 2 N for 25 mm width; or 3 N for 50 mm width; and a tension increase at the reproducing head of about 25 % due to the friction of the tape over the other heads and guides.

Note that the reproduced frequency, and therefore the apparent speed, depend on the tape tension. For instance, if a ¼ inch MRL Flutter and Speed test tape is reproduced on a constant-torque reproducer such as an Ampex 350, set on the "Large Reel" position, the tape tension will normally be 2.5
newtons instead of 0.8 newtons. Because of this high tension, even tho the tape is travelling exactly at the correct speed, the reproduced frequency will be about 3145 Hz, so the tape speed will appear to be 0.17% slow.

The IEC Standard 60 094-1 [4] calls for the speed to be its standard value ±0.2 % for professional systems, and ±2 % for home systems. Because the tape is a plastic medium which changes length during storage under tension, we can not guarantee that the recording that you receive from MRL after
storage and temperature cycling will be better than ±0.5 % of true wavelength ("speed"). Other factors which limit measurement accuracy are discussed in [6].

For very accurate tape speed measurements, we recommend using a large pulley having an accurately-known diameter. This method is not mentioned by the IEC Standard 60 094-3, but is standardized by the NAB [7], and described in [6].

Some transports use such a pulley for tape playing- time measurements. For example, the Studer A-80, which we use to record these Flutter and Speed Tapes, uses a pulley whose circumference is exactly 190.5 mm (7.50 inches). The output of the sensor on that pulley is brought out to the back-panel
Remote Control Connector (pin 36 = Return, pin 34 = "Clock"), as a 1 Hz square wave, 0 or 12 volts. We connect this through a 10:1 frequency divider to a period counter. Thus we can make a very accurate tape-speed measurement—one count in a 10.000 s period measurement is 0.01%.
----------
 
I have ahd dreams at night where Harrison Ford and I were having an adventure together. Snakes, Airplanes, spys and all.
Click to expand...

I'd be more interested in a dream where you and Harrison Ford relap heads.

Or you can buy a Flutter and Speed Test tape from MRL.
Click to expand...

But that's no fun...
 
Dang. Youse guys are really making this complicated.

All you do is record a verifed 1K tone on the recorder to be speed ckecked and then play it back and check the frequency with the counter. If it varies much, the recorder is wacked.


Perhaps I missed somethin.
 
pianodano said:
Dang. Youse guys are really making this complicated.

All you do is record a verifed 1K tone on the recorder to be speed ckecked and then play it back and check the frequency with the counter. If it varies much, the recorder is wacked.


Perhaps I missed somethin.
Click to expand...

I don't think he is worried about "variations" in his recorder's speed...rather if his recorder is even at a standard/accurate speed to begin with.

1kHz at 13ips is still 1kHz at 13ips...but that recorder would NOT be running at an accurate speed compared to the 15ips standard.
 
Recording 1k at 17.46ips will play back at 1k. The trick is to find out if the deck is running at 15ips (or whatever speed you are setting it for...maybe you WANT it to run at 17.46ips) and get it adjusted to 15ips. That's what Ethan is presenting here...same reason MRL sells speed test tapes...otherwise it'd be a sham.
 
But you know...unless you are going to be playing back tapes recorded on other decks...I don't think 1-2 ips up/down will make a whole lot of difference, as long as the deck is steady at whatever speed it's running.

Right...?
 
That's right...and it should be stable at whatever speed its running, at least as stable as the deck is spec'ed to run. If it has a quartz oscillator for instance that oscillator will be in charge if it is at 14.8ips or 15.3ips or whatever. This is not the same case if there is a pitch correction function that is enable on the deck though...
 
$100 plus shipping.....

I'm looking on how to get 2% or better speed calibration without spending $100. Not looking for 0.5% or flutter.

Of course if you don't care what speed you run then anything will work. Same for azimuth and operating level....

Mostly this was an idea on how to check your speed and adjust it closer if needed.

--Ethan
 
Amount of periodic pulses on a known tape distance (length) gives you physical period of a puls.
So, basically if you can "measure" (figure out, that is) the physical distance between periodic pulses on tape, then you are all set :)

How about this, then:

Ethan Theorem:
"Blayback frequency for a desired tape speed is a function of physical distance between periodic pulses, pre-recorded on tape."

:D :D :D
 
miroslav said:
I don't think he is worried about "variations" in his recorder's speed...rather if his recorder is even at a standard/accurate speed to begin with.

1kHz at 13ips is still 1kHz at 13ips...but that recorder would NOT be running at an accurate speed compared to the 15ips standard.
Click to expand...


Phased lock loop motors should run at 15 ips. If not, why care if it's running at 14.95 ips as long as the wow and flutter is acceptable ie a stable frequency count over about 1 minute.
 
sweetbeats said:
Recording 1k at 17.46ips will play back at 1k. The trick is to find out if the deck is running at 15ips (or whatever speed you are setting it for...maybe you WANT it to run at 17.46ips) and get it adjusted to 15ips. That's what Ethan is presenting here...same reason MRL sells speed test tapes...otherwise it'd be a sham.
Click to expand...


Well then if it's really that important to anybody that the machine has to run at absolutely, positively at 15 ips, I have a brand spanking new 1/4" Tascam wow and flutter tape here I'll sell rather cheap.





EDIT: sorry, it's a Tascam 1/2" full track W&F that I have left. But it's still brand new.
 
Last edited:
miroslav said:
I don't think he is worried about "variations" in his recorder's speed...rather if his recorder is even at a standard/accurate speed to begin with.

1kHz at 13ips is still 1kHz at 13ips...but that recorder would NOT be running at an accurate speed compared to the 15ips standard.
Click to expand...


How about timing leader tape and a stop watch ??
 
pianodano said:
Phased lock loop motors should run at 15 ips. If not, why care if it's running at 14.95 ips as long as the wow and flutter is acceptable ie a stable frequency count over about 1 minute.
Click to expand...

Exactly!

I'm not quite sure what (if anything) he's really concerned about with his deck...
...or is this just one of those fun lab experiments to do on a rainy day....? ;)
 
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