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Author Topic: Tone generators circa 1978  (Read 3764 times)

Jim Sam

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Tone generators circa 1978
« on: January 27, 2011, 01:48:14 pm »

Does anyone know how much test tone generators varied circa 1978?

I know not to expect digital accuracy.  For example, I would not
expect a 10.00 kHz tone, but how much leeway was there?  +/- 0.25kHz,
+/-0.5 kHz, +/-1.0 kHz, etc.?

I'm asking because I'm working with some tapes of that vintage.  They play back at 7.5 ips, and appear to have been recorded at 250 nWb/m.  The tone sequence is not normal, where the tones change abruptly as if a button on the recorder were pressed, rather than a dial rotated.

Thanks!
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John Roberts {JR}

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Re: Tone generators circa 1978
« Reply #1 on: January 27, 2011, 04:05:43 pm »

I wouldn't expect much accuracy.  The line up and slate/tone generators I generally put in consoles back then was typically based on a cheap function generator chip (like the 8038). So accuracy was limited by component value tolerances and IC process.

The tones changing abruptly sounds like a master section based tone source that had several different frequency push buttons. Ironically the LF (30 Hz) slate tone I used was divided down from the mains frequency so it would be reliable, but only there briefly to make an audible beep when spooling tape at high speed.

The tones were mainly used to confirm head alignment and bias, so tight pitch accuracy was not a concern.

JR

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Mac Kerr

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Re: Tone generators circa 1978
« Reply #2 on: January 27, 2011, 04:30:35 pm »

Does anyone know how much test tone generators varied circa 1978?

I know not to expect digital accuracy.  For example, I would not
expect a 10.00 kHz tone, but how much leeway was there?  +/- 0.25kHz,
+/-0.5 kHz, +/-1.0 kHz, etc.?

I'm asking because I'm working with some tapes of that vintage.  They play back at 7.5 ips, and appear to have been recorded at 250 nWb/m.  The tone sequence is not normal, where the tones change abruptly as if a button on the recorder were pressed, rather than a dial rotated.

Thanks!

I would expect the alignment tones at the head of an old tape to be switched between tones. At 7.5ips there should probably be a 1k tone@0Vu, possibly with a 100hz tone to check eq, and a 10kHz tone at -10Vu for azimuth alignment. At high tape speeds all the tones would be at 0Vu, but at 7.5ips tape speed 0Vu was a little optimistic for 10k.

The 7.5ips Ampex alignment tape of the day had a 700hz tone at 0Vu (Ampex operating level) and all other tones at -10dB from that. MRL alignment tapes may have had different tones and levels.

Mac
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Geoff Doane

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Re: Tone generators circa 1978
« Reply #3 on: January 27, 2011, 04:59:36 pm »

Does anyone know how much test tone generators varied circa 1978?

I know not to expect digital accuracy.  For example, I would not
expect a 10.00 kHz tone, but how much leeway was there?  +/- 0.25kHz,
+/-0.5 kHz, +/-1.0 kHz, etc.?

I'm asking because I'm working with some tapes of that vintage.  They play back at 7.5 ips, and appear to have been recorded at 250 nWb/m.  The tone sequence is not normal, where the tones change abruptly as if a button on the recorder were pressed, rather than a dial rotated.

Thanks!

As JR said, these tones are not frequency accurate at all.  Don't varispeed your playback machine to make the 1K tone exactly 1.000 kHz.  A duplicating house did that with some master tapes mixed with an old MCI JH-636 (late '70s vintage, I think) that I used to maintain.  They made a thousand cassette tapes of somebody's fiddle album at half a tone sharp!

Frequencies changing abrubtly is perfectly normal when using the built-in generator on a console.  You may also notice that the lower frequencies take a while to settle after they are selected.

It was good practice to note the levels of the tones on the box, but probably didn't always get done.  The company I work for had an odd standard of pink noise @ -6 dB, 1K @ 0, 10K @ -6, and then 100 @ -6.  There were special automated generators to create this tone sequence.  Fluxivity for 0 VU depended on the tape type, and how serious the tech that set the machine up was.  For lower levels on high output tape (250 nWb/m on 456) this wasn't too bad.  There was high frequency compression on regular tape, and the pink noise NEVER came back at the same level.

Oh yeah, are you sure you have the correct EQ?  It could be NAB or IEC, and that will make a huge difference.

Have you had any sticky tape and oxide shed problems yet?

GTD
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drknob

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Re: Tone generators circa 1978
« Reply #4 on: January 28, 2011, 09:33:18 am »

Hmmm... you're lucky you have tones. In my day, people who were tweaky would stop the tape between tones, change oscillator frequency and readjust the level before printing the next frequency. The source was almost always a built in console oscillator with only the most approximate performance. So, if your 1k tone is playing back closer to 1k than 500 or 2k, you're probably running at the correct speed.  ;D
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Harold Kilianski
CIRMMT, McGill University

michaeldtech

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Re: Tone generators circa 1978
« Reply #5 on: March 14, 2011, 03:33:48 pm »

I was taught to overshoot 2 seconds, stop tape, rewind, check new tone level, and punch the next tone at the proper start point.  Usually this was because the tape machines were far from the oscar, and there was a slight difference in level at different frequencies.
Are the tapes from a different country?  That can account for the difference as can the age.
I would not sweat it much and make sure that the tracks sound correct going down.  2 or 24 track you will be sweetening them anyway.
Mike
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