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Old 21st May 2017, 7:27 pm   #1
TonyDuell
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Default Wow and flutter meter calibration

I am not sure if this should be here or under tape recorders, so could the mods please move it if I have got it wrong.

One of the items to come home with me from the recent(ish) musuem sale was an ME103 wow and flutter meter. It is pretty similar to the ME101 on the radiomuseum site and works in much the same way.

My problem is how to calibrate it. There is an internal test oscillator (also used to record a signal on a tape, later to be played back to check the wow and flutter of the recorder). This oscillator normally runs at 3150Hz, it can be pushed up by 2% (to check the 'drift' function) or frequency modulated at 50Hz to give 0.3% wow and flutter. The idea is you use that to check the instrument.

But there are many presets inside. Not only for setting up the measurement side, but also 3 for the oscillator (normal frequency, +2% frequency and 0.3% modulation). Since I don't know if anything is correct I can't use that oscillator as a calibrator.

I would guess that the clock jitter (essentially the modern equivalent of wow and flutter) in any reasonable digital audio device would be minimal compared to the wow and flutter figures of a tape recorder (say). So I was wondering if there was a MP3 file or CD available of the 3150Hz tone with a known amount of wow and flutter added. Play that into the device and use it to set up the measuement side, then set the oscillator against it.

But I can't find such a file or CD. Does it exist? Any other ideas?
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Old 21st May 2017, 7:32 pm   #2
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Default Re: Wow and flutter meter calibration

I produced the following Audacity plug in to produce known amounts of W&F:

https://www.vintage-radio.net/forum/...d.php?p=815595
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Old 21st May 2017, 7:42 pm   #3
TonyDuell
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Default Re: Wow and flutter meter calibration

I will look into that. I've never tried Audacity, perhaps this is a good excuse to start...

Thanks.
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Old 25th May 2017, 6:37 pm   #4
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Default Re: Wow and flutter meter calibration

Thanks for that. I've downloaded Audacity and the plugin, and it certainly produces the right sort of sound from my PC speakers. Now to try it with the meter...
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Old 25th May 2017, 10:42 pm   #5
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Default Re: Wow and flutter meter calibration

Sounds promising, let me know how you get on.

Once it's calibrated what do you plan to test?
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Old 26th May 2017, 5:09 am   #6
TonyDuell
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Default Re: Wow and flutter meter calibration

One thing I am not sure of, yet...

As I understand it, the frequency variation for 1% Wow and Flutter depends on the modulation frequency, so if I select, say, 1% Wow and Flutter on audacity/plugin, is that corrected for the modulation frequency?

My meter has a 2 settings for the 'filter'. One is 'linear' where the indication does not depend on the modulation frequency, just on deviation of the signal, the other is an RC network which I assume gives the standard weighting curve. Which do I use?

As for what I intend to test, I collect tape recorders from the very poor to the not-too-bad and I would like to see what their performance is. The fact that I also collect obscure test instruments is part of it too.
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Old 26th May 2017, 5:25 pm   #7
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Default Re: Wow and flutter meter calibration

The plugin output should be correct.

If you use weighting then the displayed value will be the input attenuated by the appropriate frequency dependent amount. Best to leave this turned off for now.

As your meter is running at 3150Hz it's probably testing to DIN standards with a pk-pk detector so 1% on the plugin should give 1% on the meter.

What reading are you seeing?
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Old 26th May 2017, 7:04 pm   #8
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Default Re: Wow and flutter meter calibration

I've not tried it yet. But I have no reason to believe it is anywhere close to being in calibration. That is why I am looking for a signal source where I know what the meter should read. Not one that 'might' give the right result.

The meter I have is German so I would guess it does use the DIN standard. It's an ME103, very similar to the ME101 on the radiomuseum site (there are differences, but the details there will be enough to see what is going on).

So I should select 'linear' mode (RC filter switched out)? I will try that and see what I measure (without twiddling any presets are this point).
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Old 26th May 2017, 10:19 pm   #9
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Default Re: Wow and flutter meter calibration

Yes don't twiddle anything until any faults are cleared first.

In the case of my Leader LFM-39A there was a meter problem. Once this was fixed cal was fine on most ranges, only one pot needed a slight tweak.

Like you at the start I needed something to generate a known signal. Back in the day Leader issued a calibration CD containing known tones, but I couldn't obtain one hence writing the plug-in.

Try 'linear' and see what you get. Make sure the output voltage level from your soundcard is within the meter's input range.
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Old 27th May 2017, 5:12 am   #10
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Default Re: Wow and flutter meter calibration

I am well aware you don't adjust anything untill it works . The only exception I have to that is that in some cases seeing the effect of an adjustment can help trace the fault. But only if you have a way to re-adjust it properly.

The instrument seems to work on the internal test oscillator. If I press the 'test' button I can centre the 'drift' meter using the appropriate front panel control. Pressing the '+2%' button then causes the 'drift' meter to go up to 2%. Pressing the '+/-0.3%' button causes the wow and flutter meter to read 0.3%, as far as I can see on all ranges.

So I don't think there are any major faults. But as I am testing it against an oscillator that is as old as the instrument (it's part of it...), I am not convinced that it is a suitable 'standard'. Some of the presets are accessible through labelled holes on the back, possibly a previous owner tweaked those to get it to give the right results on the internal oscillator.
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Old 27th May 2017, 6:51 am   #11
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Default Re: Wow and flutter meter calibration

the problem you will have is making a decent test tape with which to test your various machines.

The ME103 was used with a test tape that was based on a metal frame and had a smooth action to minimise W&F. Much like the later metal bias tapes made by TDK.

At Thorns we used ME103 on the line but made our own tapes on a high quality mechanism due to the high usage. In the labs we used the correct tapes, I still have one somewhere as well as a ME103 but one meter went o/c so I had to graft something in
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Old 27th May 2017, 11:49 am   #12
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Default Re: Wow and flutter meter calibration

Quote:
Originally Posted by TonyDuell View Post
I am well aware you don't adjust anything untill it works .
When I first saw that, my initial reaction was "gosh, that's rather prickly - he was agreeing with your earlier comment, not teaching you to suck eggs". But then it occurred to me that perhaps you fell foul of the Smiley Problem?



Quote:
Originally Posted by TonyDuell View Post
That is why I am looking for a signal source where I know what the meter should read. Not one that 'might' give the right result.
I can confirm that there is no "might" about it.

The test signal gives perfect results on 3 separate bits of gear that I have here:

1. Lindos LA1
2. Lindos LA102
3. Woelke Magnetbandtechnik ME105

This is not surprising - it is a time-domain measurement, so not something a PC will struggle to generate. If it was a voltage-domain measurement, then yes, there might well be valid questions about calibration.

So, rather than scepticism, I would like to extend my appreciation and compliments to Paul for producing that plug-in. I missed it before this thread, so thank you for bringing my attention to it. I use Audacity a lot for engineering tasks, so this is a really useful addition to my setup

Regarding my ME105, which has been languishing in the attic until this morning, I was pleasantly surprised when I took the lid off. Good German engineering. The repair - such that is was - was nothing more advanced that switch cleaner! That's good, because when I last looked, I didn't spot any sign of service data anywhere online (I only have an instruction manual here). I don't fancy trying to reverse-engineer this - I've never seen so many transistors packed so densely! All BC108s or similar, plus a few metal-can op-amps. Every resistor is standing upright with beautifully formed folds at the top. Just lovely

Anyway, if there's any questions about it, I'd be happy to help. That said, Google took me to your Flickr photos, so I see that it is rather different to your ME105. If nothing else, I hope you can now be confident in Paul's test signals.

All the best,

Mark
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Old 27th May 2017, 3:52 pm   #13
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Default Re: Wow and flutter meter calibration

Yes I did fall foul of the smiley problem.

And I too am _very_ grateful for this plug-in. My problem is learning to use it correctly. I don't doubt it generates accurate test signals, but I have to know how to set my meter and what reading I should get. Please don't think I am criticising the work in any way. I think this is a misunderstanding based on the use of a forum rather than face-to-face communications. In other words I am confident in Paul's work, but not in my ability to use it.

The ME103 is not too complicated inside (I did reverse-engineer mine), it's all discrete transistors. Some of the metal cans -- look like TO1 germanium transistors -- only have 2 leads and are diodes. Once I'd spotted that it was easy. It's also very nice to work on, the main PCB can be hinged down by removing 2 screws and loosening 2 more.

And amazingly the only fault was dirty switch contacts which responded to a shot of propan-2-ol.
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Old 27th May 2017, 5:12 pm   #14
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Default Re: Wow and flutter meter calibration

Of all the various audio measurements, I admit that I'm least familiar with W&F. But it pretty much amounts to an FM detector, the output of which is converted to DC with a quasi-peak rectifier. There is the option to weight the results to give a number that correlates more closely with human perception - as is very common in other audio measurements - but that seems to be the nub of it. The Wikipedia entry is quite good: https://en.wikipedia.org/wiki/Wow_an...er_measurement

For more info about weighting (which I think will answer your question about "linear" vs the "RC network), you could try the handbook for the Lindos LA100: http://www.lindos.co.uk/la100m.pdf - I picked that simply because I have a copy in front of me; no-doubt other/better texts are available... On page 25 are some words, but page 187 shows the IEC386 weighting curve. Actually, the same graph is on that Wikipedia article (drawn by the same person!). At 4Hz, it's unity gain, so switching the filter in and out should make no difference to the reading. At the 6Hz that Paul's plugin defaults to, the difference should about to less than a dB (<10%). Of course, who knows which standard the ME103 uses

Regarding your concerns about the built in oscillator, it's worth saying that it's only the frequency that matters, and so this is extremely easy to measure with more precision than is needed with a simple frequency counter. The ME105, being newer, uses a quartz oscillator. I'm assuming that it's only really critical for the Drift indication. Certainly, playing around with a function generator feeding into mine, it seems that the Drift meter is fed with the DC component of the recovered FM, and the Flutter meter is the amplitude of the AC component.

Sorry I can't be more specific about your model, but perhaps these observations help? As you've reverse-engineered yours, I'd love to see copies. They might not bear much resemblance to mine, but it's still interesting to see how these things are implemented.

Cheers,

Mark

PS: The LA100 manual only includes partial schematics, but the LA1 manual does include the complete information. The W&F discriminator is on page 12.
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Old 27th May 2017, 5:35 pm   #15
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Default Re: Wow and flutter meter calibration

If you PM me with your e-mail address I will be happy to send you the handdrawn reverse-engineered circuits of the ME103.

It is similar to the ME101 on the Radiomuseum site, there are differences in the range switching, etc, but that's really all. Possibly different transistor types.

I am also totally new to W&F measurement. As some of you know I collect unusual measuring instruments and I decided to buy this one at the Dulwich museum sale. I will use it, but I need to learn how.

The internal oscillator is only used for testing/self calibration and for making a test tape. I will try to give an overall description. The oscillator (a mulitvibrator) runs at 3150Hz (set by a preset), it can be shifted up by 2% (again that is set by a preset) or frequency-modulated at 50Hz to give a signal with what is said to be 0.3% wow&flutter.

The output of the oscillator goes to a pair of 4mm sockets on the panel and to pin 1 of a DIN socket on the back labelled 'diode cable' (to go to the 'diode' socket on a tape recorder).

Input to the measuing part comes from another pair of 4mm sockets or pin 3 of that DIN socket. The test switch disconnects that and connects the oscillator output instead, so that you can measure the oscillator signal.

That signal is clipped, amplfiied, filtered. And then full-wave rectified. I was surprised it wasn't an FM discriminator, but it certainly doesn't look like one. So the voltage at the output of that stage depends on the frequency of the input. This goes to the drift meter (via an amplifer with a set-zero control) and via the measurement filter (the one that is swithed out by the 'linear' button) to an amplfiier for the W&F meter. It is also possible to use an external filter (connected to another DIN socket on the back), I am not going to try this for the moment.

So yes. Drift is the DC component of the output of the voltage-to-frequency converter (i.e. how far the signal is off the nominal frequency), W&F is the AC component (how much does it 'wobble about'.

My guess is that the internal filter approximates to the DIN or IEC characteristic (which I believe are fairly similar).

The oscillator signal is not used in the measurment part at all, other than for testing. I can easily set the 'base' frequency of 3150Hz and the +2% frequency using a counter (the crystal oscillator of any of my counters would be accurate enough for this -- easily). But setting the modulation depth for the 0.3% W&F test setting is a bit harder, as it's modulated at 50Hz, by a clipped, half-wave rectified, sine wave. My idea was to calibrate the measurement side against Paul's Audacity plug-in, then switch the instrument to test mode, 0.3% W&F and adjust the appropriate preset to get a 0.3% reading. Of course as I said before, I will not adjust _anything_ untill I know what is going on, what I measure at the moment.
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Old 27th May 2017, 6:15 pm   #16
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Default Re: Wow and flutter meter calibration

I've just tried it...

Made up a cable from my laptop's headphone socket to the DIN 'diode cable' socket on the ME103.

First got Audacity to generate a steady 3150Hz tone. I can zero the drift meter on that. Then tried a tone 2% higher, and get very close to 2% reading on the drift meter (perhaps 1.9%, but as I said, close). They Paul's plugin, meter set to 'linear' filter. 0.3% W&F gives a reading very close to that on all ranges.

And the internal oscillator gives the right readings in 'test' mode.

I think it's very unlikely that I have misunderstood how to use Audacity/the Plugin _and_ the meter has drifted by just enough to give the right results if you see what I mean. So my conclusion is that it works, and is close to being calibrated. I think I am going to leave well alone (since W&F measurements don't have to be exact) and just use the instrument.

Thanks to all who have helped me. Perhaps this thread can be kept open if people have something to add or I have more comments.
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Old 27th May 2017, 6:32 pm   #17
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Default Re: Wow and flutter meter calibration

Yes, do please leave it open - it's an interesting topic.

I'm thinking about some of the things said in the last couple of posts because I'm not clear about the crystal oscillator in mine now. I think that the crystal oscillator is only connected to the demodulator when there's no signal - and at that point you null the drift meter to the centre of the meter - that's almost certainly just a DC offset. After that, it's not connected. The instructions imply that if you reduce the input level enough to make the signal lamp go out, then the crystal oscillator is automatically re-connected.

Also, I get exactly 3.15kHz from the output socket unless I press the 0.1% CAL button. In this mode, an oscillator is running at 3.15kHz (close enough), but modulated by a 50Hz oscillator. There is a pre-set to adjust this one to get it to 3.15kHz - I assume it's an RC type oscillator. All in all, I'd love to see a service manual.

I'm working on a more mundane problem now. This unit has loose handles, and the only way to fix that is to take apart the side extrusions (hard to explain, but easy to see). This is the only poorly designed part of the instrument as far as I can see. Otherwise, there's lots to love.

I'll PM you my email address in a moment.

Mark
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Old 27th May 2017, 6:45 pm   #18
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Default Re: Wow and flutter meter calibration

Your 0.1% CAL button sounds much the same as the +/-0.3% button on mine. The oscillator in mine (ME103) is a simple multivibrator. There is a preset to set the frequency, another preset which is swithed in by the +2% button (to shift the frequency to check the drift meter) and a preset via which a 50Hz signal is injected when the +/-0.3% button is pressed (to check the W&F meter). That 50Hz signal comes from a secondary on the mains transformer, connected to a resistor/zener network. So one half-cycle is clipped at 0.7V (forward bias of a silicon diode, here the zener) the other half cycle is clipped to 8.2V (zener voltage).

On the ME103 (the ME105 might be similar) there are some presets that can be adjusted through labelled holes in the back, but many more on the PCB. So there might be things you can adjust that are not obvious without a circuit diagram/service manual.
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Old 27th May 2017, 7:06 pm   #19
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Default Re: Wow and flutter meter calibration

Yes, there are 5 holes in the base which are nicely labelled and explained briefly in the instructions, plus another 3 internally that aren't labelled. I don't think the 50Hz comes from the transformer as the instructions say that it's independent of the mains frequency. I lost count of the number of transistors, but it's a lot more complicated than the ME101/103 - they were obviously "going for broke" with this unit

Can anyone fill in some background about the company? There's really not much out there...
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