A tech question...

[DamnYankee]

Hey Fellas,

I have a 2 channel Tektronix Oscilloscope that I want to insert (or rather, have it monitor/measure waveforms for clipping) between my 24x8x2 mixer and my tape deck.

Have any of you done this and can you recommend a decent signal generator?

Thanks!

DY

[SAnthony]

HEY DY, I have used this method mostly for troubleshooting a circuit. You know find the bad IC..etc... What about a simple test tone CD, you can probably find one on Ebay. Then I would think establish your monitoring points, Maybe a 1/4" adapter to the channel insert for a prefader reading,
Another at the Channel direct out, a third at the 8 buss input/output and finally a 4th at the stereo out. Could maybe do a 5th point at the recorder. Infact if you get a really good Test tone CD you should be able to monitor Phase through the mixer as well as distortion...... I hope this helps....
Shawn

[DamnYankee]

Originally Posted by SAnthony

HEY DY, I have used this method mostly for troubleshooting a circuit. You know find the bad IC..etc... What about a simple test tone CD, you can probably find one on Ebay. Then I would think establish your monitoring points, Maybe a 1/4" adapter to the channel insert for a prefader reading,
Another at the Channel direct out, a third at the 8 buss input/output and finally a 4th at the stereo out. Could maybe do a 5th point at the recorder. Infact if you get a really good Test tone CD you should be able to monitor Phase through the mixer as well as distortion...... I hope this helps....
Shawn

Sounds good...but still, aren't I going to need a signal generator inbetween the inserts and the Scope?

Right now, I'm monitoring out of my stereo outs on the board directly to my Scope but it looks like spaghetti dancing around (I monitor the peaks but it's really difficult). I'd like to generate regular waveforms that would be easier to see/monitor.

[SAnthony]

Hey DY, A good test tone CD will have a range of different frequency waves for you to choose from. The one I have is on a Ear Training CD called "From Tin to Gold" and there is a section of pure waveforms to choose from. For instance I usually use the 1000 HZ when setting Bias on
my tube amps. You are basically going to use a CD player as the tone generator. You are going to input your CD into the line input of your mixer. Then you are going to play several different frequencies and check for clipping. Then you can monitor the 5 different points I mentioned above and set Gain and Cuts and Boosts accordingly to minimize distortion. HOWEVER, Remember, The test tone CD is recorded at Ideal levels and your program material you are recording or mixing might not be, so you can't use this test to set EQ! You really just need to make sure you get a Good quality Test tone CD. The Better ones will cover all octaves and may range from 40 HZ up to 20 KHz in reasonably small increments. Some will have a track or two to test Phase on your speakers and Phase through your signal path. That should do the trick I believe...... You just need to select the right CD...
Take Care,
Shawn

[DamnYankee]

Originally Posted by SAnthony

Hey DY, A good test tone CD will have a range of different frequency waves for you to choose from. The one I have is on a Ear Training CD called "From Tin to Gold" and there is a section of pure waveforms to choose from. For instance I usually use the 1000 HZ when setting Bias on
my tube amps. You are basically going to use a CD player as the tone generator. You really just need to make sure you get a Good quality Test tone CD. The Better ones will cover all octaves and may range from 40 HZ up to 20 KHz in reasonably small increments. Some will have a track or two to test Phase on your speakers and Phase through your signal path. That should do the trick I believe...... You just need to select the right CD...
Take Care,
Shawn

Hmmm...that's not what I have in mind; lemme explain what I'm trying to do:

When I mix down from my multitracker thru my board and down to my 2 track deck, I want monitor the signal between the board to my 2 track. I want the music to show up on the scope in a waveform so I can saturate the tape without distorting. When I see the waveform start to clip, then I know to back off on the signal to the 2 track deck.

That's what I want to do. Now, I'm positive I need a waveform generator to insert between my board and my Scope. So that's where I'm at...do I need a waveform generator to do what I'm trying to do...and if I do, what are some recommendations? I don't want to spring $1,000 for an audio waveform generator...

(I hope that makes sense...)

DY

[SAnthony]

Hey DY, I still think the CD may be able to give you the same results as the tone generator...... BUT, what you might look into is even simpler than
that.... Not sure what type of VU your 2 track has. But assuming you are doing Analog Tape 2 track, you might consider a Peak Level/VU meter....
My Tascam 112 Cassette deck has these. It is basically an old school VU meter with an onboard clip/peak Led indicator. At less saturated levels the VU needle will stay far out of the red zone (-10 dB) and the clip LED will light if it is clipping. However with like a 2:1 compression You can saturate the tape more VU travels closer to the red Zone (-3dB) before the clip Led lights up... I E. More Saturation and Louder recording as well.
A standard VU Meter can be modded with the Peak LED indicator and this can be added to your 2 track. It wouldn't be nearly as expensive...
Just a Thought....
Take Care,
Shawn

[David Kulka]

Originally Posted by DamnYankee

...When I mix down from my multitracker thru my board and down to my 2 track deck, I want monitor the signal between the board to my 2 track. I want the music to show up on the scope in a waveform so I can saturate the tape without distorting. When I see the waveform start to clip, then I know to back off on the signal to the 2 track deck...

Alas, it's not quite that simple. Tape recorder distortion is hard to see on a scope, especially when looking at music. Different frequencies will begin to distort at different levels, and transients, which are the biggest concern, are the hardest to see because they are so brief. Extreme distortion will be clearly visible on a scope but that doesn't really tell you much.

You can certainly try it by connecting an output of your recorder to an input of the scope, but the results might not be all that helpful. Going by ear might be better. You can start by recording way too hot, so that the distortion is obvious, then gradually back down and try to determine what your max level should be for a given type of program material.

[DamnYankee]

Originally Posted by David Kulka

....You can start by recording way too hot, so that the distortion is obvious, then gradually back down and try to determine what your max level should be for a given type of program material.

That is EXACTLY what I want to do - and observe on the scope the wavelengths - as it leaves the board TO the 2 track deck - so I can see the visual on the fly instead of having to finish recording and then listening to playback.

Like this...

[David Kulka]

Does not compute.

If you connect the scope to the signal leaving the board to the 2 track, you'll be seeing the input of the deck rather than the output, and the distortion or saturation or whatever won't be seen on the scope at all, so looking at it won't tell you anything.

But as I said, even if you do it the right way -- viewing the deck outputs on the scope -- it still won't be very helpful. Trust me, scopes are great but this idea won't really work.

If you isolate a vocal, keyboard, or drum track and make a loop you might be able create a test where the scope would be somewhat useful but in the end you'll still have to choose the best record level by using your your ears.

[DamnYankee]

Originally Posted by David Kulka

Does not compute.

If you connect the scope to the signal leaving the board to the 2 track, you'll be seeing the input of the deck rather than the output, and the distortion or saturation or whatever won't be seen on the scope at all, so looking at it won't tell you anything.

But as I said, even if you do it the right way -- viewing the deck outputs on the scope -- it still won't be very helpful. Trust me, scopes are great but this idea won't really work.

If you isolate a vocal, keyboard, or drum track and make a loop you might be able create a test where the scope would be somewhat useful but in the end you'll still have to choose the best record level by using your your ears.

I see what your saying...hmmm...I gotta think about this...

There has to be a way I can use Z (intensity) on the scope to monitor audio levels coming out of the board and into the 2 track. I can't afford a hardware waveform analyzer ($5-$10k) and I think the software versions require RECORDED music - not music that's being mixed on the fly...

*tapping chin...thinking...*

[David Kulka]

A Z axis input modulates the intensity of the waveform on the scope. You would want to use the X axis input, which is just the standard vertical input.

Maybe there's some confusion here about what type of distortion you want to look for. If you want to avoid exceeding the max output level of you mixer then yes, you should connect the scope to the output of the board. Clipping will be fairly easy to see. In fact, you could find the clipping point fairly easily and then just stick some white tape on the top and bottom of the screen to remind you where your headroom limits are.

But that has nothing with tape saturation. Tape saturation (or overload) is hard to see on a scope but if you want to try, you need to connect the scope to the output of the recorder.

[DamnYankee]

Originally Posted by David Kulka

A Z axis input modulates the intensity of the waveform on the scope. You would want to use the X axis input, which is just the standard vertical input.

Maybe there's some confusion here about what type of distortion you want to look for. If you want to avoid exceeding the max output level of you mixer then yes, you should connect the scope to the output of the board. Clipping will be fairly easy to see. In fact, you could find the clipping point fairly easily and then just stick some white tape on the top and bottom of the screen to remind you where your headroom limits are.

But that has nothing with tape saturation. Tape saturation (or overload) is hard to see on a scope but if you want to try, you need to connect the scope to the output of the recorder.

Thanks Dave...

Let's look at your first suggestion: do I need a signal generator between the output of the board and my scope to monitor clipping? The bottom line here is, I do not think the LEDs in Soundtracs' meterbridge is accurate. They are red-lining long before I start clipping/distorting. I want to push the board to the point of clipping and I need SOMETHING as a reference to alert me when the clipping occurs. I'm hoping my scope will do that.

Now the 2nd suggestion: after I've maxed out the board via the scope, I want to playback the recording thru my scope to see/hear how much the tape is driven. If this can't be done or is difficult to observe, then so be it.

Does all this make sense???

[Geoff_T]

Originally Posted by DamnYankee

The bottom line here is, I do not think the LEDs in Soundtracs' meterbridge is accurate. They are red-lining long before I start clipping/distorting. I want to push the board to the point of clipping and I need SOMETHING as a reference to alert me when the clipping occurs. I'm hoping my scope will do that.


Does all this make sense???

Hi

I haven't been following this thread but I just clicked on the most recent posting and saw the above assumption which, to answer your last question, is NO!

My guess is that the LEDs on the Soundtrac console are calibrated to read 0VU at +4dBu (1.228 volts) and the third red LED will be +7dBu. This is miles before the console clips because the headroom could be as much as 20dB above this point.

This is absolutely standard console procedure. Now why is it that you want to push the board to the point of clipping? Are you trying to hit digital 0dB?

[bongomania]

Originally Posted by DamnYankee

do I need a signal generator between the output of the board and my scope to monitor clipping?

No, your audio is your signal. Why would you need a second signal to monitor the first?

Originally Posted by DamnYankee

I want to push the board to the point of clipping and I need SOMETHING as a reference to alert me when the clipping occurs. I'm hoping my scope will do that.

Nope, the scope would be a clumsy and inaccurate tool for that job. There are devices designed to analyze distortion, and you need one of those, not a scope. One DIY hack approach would be to use a test-tone CD (or a signal generator, but really, don't bother) and a second set of meters that you believe to be accurate. Play the CD, raise the gain until you hear distortion, and calibrate your new meters to be "just going red" at the level where you just start to hear distortion. Then you can use those calibrated meters to alert you to any future distortion (using the same mixer- you'd need to recalibrate if you ever wanted to use them with any other desk). Does that sound like a pain in the butt? If so, then shell out for a distortion analyzer.

Originally Posted by DamnYankee

I want to playback the recording thru my scope to see/hear how much the tape is driven. If this can't be done or is difficult to observe, then so be it.

So be it! Sorry.

[Geoff_T]

Hi

I think an easier solution... that will still involve a lot of work... would be to make a small circuit board that drives a LED when the signal reaches a particular level. Then either drill the Soundtrac meter bridge or fit the LEDs to a separate box and set the LED threshold to come on just before clipping.

Something like this .... Velleman Inc.

But I still have to ask WHY?

Why do you want to drive the console at such a hot level that you won't be able to use any EQ or effects that involve gain because it will drive it past the clipping point?

Why not run the console at a level that provides the best headroom, won't overdrive the tape machine, and will enable you to crank in tons of EQ boost?

i.e. Around the level that the console meters are just going into the red...

[DamnYankee]

Originally Posted by Geoff_T

Hi

I think an easier solution... that will still involve a lot of work... would be to make a small circuit board that drives a LED when the signal reaches a particular level. Then either drill the Soundtrac meter bridge or fit the LEDs to a separate box and set the LED threshold to come on just before clipping.

Something like this .... Velleman Inc.

But I still have to ask WHY?

Why do you want to drive the console at such a hot level that you won't be able to use any EQ or effects that involve gain because it will drive it past the clipping point?

Why not run the console at a level that provides the best headroom, won't overdrive the tape machine, and will enable you to crank in tons of EQ boost?

i.e. Around the level that the console meters are just going into the red...

Hi Geoff,

Thank you kindly for your response. I can't see where the best headroom is because when the first red LED lights up on the bridge, it's like they all light up - the green LEDs do not seem to behave the same way. Therefore, I'm not trusting my bridge so I'm looking for another alternative - and I was hoping I could throw my 2 channel scope on the output and monitor that way. When I do that, I get two channel waveforms that I can measure, but it's very busy to watch (like the old McIntosh MI-3 if you're familiar with those)...

I think I'll look to see if I can find a pair of stereo VUs and go that route...

Anyways, thanks to all who responded!

DY

[Geoff_T]

Originally Posted by DamnYankee

Hi Geoff,

Thank you kindly for your response. I can't see where the best headroom is because when the first red LED lights up on the bridge, it's like they all light up - the green LEDs do not seem to behave the same way. Therefore, I'm not trusting my bridge so I'm looking for another alternative - and I was hoping I could throw my 2 channel scope on the output and monitor that way. When I do that, I get two channel waveforms that I can measure, but it's very busy to watch (like the old McIntosh MI-3 if you're familiar with those)...

I think I'll look to see if I can find a pair of stereo VUs and go that route...

Anyways, thanks to all who responded!

DY

Hi

One of the tricks you can do with an LM39** LED driver chip, that Soundtrac may have done... I have no knowledge of these consoles... is that the lower level LEDs can light up like a travelling spot and, when it hits the peak red LEDs, they all light up to warn of the overload.

If you have a digital multimeter, you can check the 0VU reference is 1.228 volts ac on the multimeter and this will prove that the VU's are accurate... or not.

The scope is a great tool in the hands of the knowledgeable and the idea of white tape across the screen at peak levels was a good one.

You just have to work out a sensible operating level where you don't need this information.

[DamnYankee]

Originally Posted by Geoff_T

Hi

One of the tricks you can do with an LM39** LED driver chip, that Soundtrac may have done... I have no knowledge of these consoles... is that the lower level LEDs can light up like a travelling spot and, when it hits the peak red LEDs, they all light up to warn of the overload.

If you have a digital multimeter, you can check the 0VU reference is 1.228 volts ac on the multimeter and this will prove that the VU's are accurate... or not.

The scope is a great tool in the hands of the knowledgeable and the idea of white tape across the screen at peak levels was a good one.

You just have to work out a sensible operating level where you don't need this information.

There is a mod for the meterbridge but damned if I can find it; it's a simple resistor swapout but I can't find the white paper that tells me the value of the resistor to swap to.

Anyways...thank you again for your expertise, Geoff - everyone in here greatly appreciates what you and the other experts contribute!

Take care,

DY