OT: Capturing 16mm Film sound via Projector

xberk wrote on 2/1/2016, 4:43 PM
I'm digitizing some of my old 16mm film productions. I captured the picture on my new RetroScan Universal machine. Sound was more problematic. For sound, all I have to work with is my old Singer Graflex Insta load 16mm Projector. It took me a bit of experimenting to find the right setup. At first, going directly from the Singer's speaker output jack into an H1 Zoom gave a bad result. Too hot. Distorted. Some mismatch of impedance ( I'm lame when it comes to audio stuff) Even the lowest volume on the projector was over driving. The output was meant for 8ohm speakers. I figured I need to somehow convert the signal to something the H1 Zoom can handle.

Finally I came up with this: From the Singer's speaker output jack to an old amp's AUX input -- then from the Tape Recorder output of the Amp directly into an H1 Zoom recorder to digitize the track as a WAV file. With the projector volume down fairly low. This works ok. I think the result was decent. I've ordered an attenuation cable that might help me eliminate the amp. And actually, I'm not sure what looping through the amp is doing?

Open to further suggestion.


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Comments

musicvid10 wrote on 2/1/2016, 4:54 PM
Most likely you will need a 70V -> Line Level Audio Transformer.
Maybe Radio Shack or Bernstein-Applebee.

Or maybe you can find instructions online to build a variable T-Pad for the purpose.

Bes] yet, run it through a 1/4" TS to an Ultra-DI (try to borrow one) and pad like -30dB to your sound card.
http://www.musiciansfriend.com/pro-audio/behringer-ultra-di-di100-direct-box?rNtt=ultra%20di&index=2

Whatever you choose, keep the audio level down from the projector. You're looking at a lot of analog distortion regardless.





xberk wrote on 2/1/2016, 6:03 PM
>>Best yet, run it through a 1/4" TS to an Ultra-DI

Hey thanks! .. This looks like a quality way to go.

So I'd need a 1/4" TS male to male cable from the projector to the ULTRA-DI and then out from the ULTRA-DI with some kind cable that goes from 1/4" TS down to 3.5MM into the H1 Zoom?

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Chienworks wrote on 2/1/2016, 6:22 PM
The H1 only has a mic input, i'm guessing? If that's true then even the aux output from the amplifier is probably too hot for it. The direct box puts out a mic level signal, which is why it's preferable.

I disagree with keeping the projector sound level low. I'd turn it up as high as i could get it without the audio distorting from the projector's own speaker. Once that is set then i'd use the projector's output jack into the direct box. Why? I'm sure the signal to noise ratio of the projector's circuitry is lousy. If you turn the volume control down you diminish the signal but not much of the noise, so the S/N ratio gets worse. Usually the best S/N ratio is when each stage of the chain is set to the highest level you can get without distortion so that you get more signal sitting above the noise floor.
rraud wrote on 2/2/2016, 11:01 AM
I concur with Kelly, you'll need a:
- DI that can handle a speaker level input. (the Countryman Type-10 or 85 comes to mind, but they ain't cheap).
- an adapter/cable to go from the speaker output to the DI's 1/4" TS input.
- an adapter/cable to go from the DI's balanced XLR mic level output to the Zoom's unbalanced 1/8" TRS mic level input.
xberk wrote on 2/2/2016, 1:27 PM
>> DI that can handle a speaker level input. (the Countryman Type-10 or 85 comes to mind, but they ain't cheap).

Thanks for the response. Countryman looks like great equipment but why wouldn't the Ultra DI first suggested above do just as well for what I need?

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musicvid10 wrote on 2/2/2016, 1:50 PM
I think CountryMan specs with a little less noise but that difference is quickly overwhelmed by analog hiss from the optical audio stripe. Either is perfectly good; Don't plug the projector into an h1's inputs.

Kelly, the projector's audoi output is likely 70 volts.
Turning it down to 25% will only drop it a few dB relative to a 1V nominal input.
I recorded a daily remote radio broadcast using 70V over a two-wire telephone line for three summers..

That kind of peak voltage is absolutely unnecessary when going from the projector source to a DI.
SecondWind-SK wrote on 2/2/2016, 1:51 PM
Or, if you're handy with a soldering iron, put an 8-ohm resistor across the speaker terminals of the projector and a 100 kilo ohm resister in series with one side of the projector out put then on to the Zoom. The other side of the zoom input goes directly to the ground side of the Zoom.

I tried ascii to show this, but that was a bust. What the circuit does is place an 8 ohm load on the projector's speaker output. Then, the 100k resister reduces the signal strength to the mic input of the Zoom at a proper level. And, if you want to get a little fancier, replace the 100 Kohm resistor with a 250 KOhm variable potentiometer to provide a way to adjust the level going to the Zoom. Obviously, you'll need the appropriate connectors for both the projector speaker out and the Zoom in. Anyone who does electronic repairs is likely to have these parts on hand, although I realize that it is becoming harder and harder to find someone who repairs electronic equipment.

As far as the signal to noise issue, it really depends on where the volume control is located in the projector's circuitry. Most of the noise in the circuit is generated by the early stages of the amplifier circuits, which means that the noise goes up and down with the volume control, which is typically located before the output amplifier circuits. On the other hand, if the volume control is located within one of the early stages, then the previous comment about running the projector volume at a normal/higher level is accurate.Old audio recording studio tech here.

By the way, the nature of the sound sample you posted is just about what I would expect from a vintage 16mm film. Optical sound tracks are noisy.
musicvid10 wrote on 2/2/2016, 2:17 PM
The output impedance of the projector is unlikely to be anything like 8 ohms unless it's a newer one.
So I wouln't load it like that.

OP should definitely find the specs FIRST if he doesn't have an o'scope.

We think of an actual load on a 70V line from an older projector to be in the neighborhood of 100 watts, or a little over one amp. You can do the math. The stepdown xformers I mentioned are a viable alternative, but they have a HF rolloff and you have to hunt for them.

The two DI's mentioned are failsafe, they are SOP in my and rraud's business; and the safety factor (up to 1000V across the inputs!) is built into the cost of the solution.
If you save one job by using them, you've paid for them.
farss wrote on 2/2/2016, 2:42 PM
[I]"Kelly, the projector's audoi output is likely 70 volts."[/I]

I'm inclined to doubt that.
70V and 100V "constant voltage" audio systems were / are used for sound distribution systems. The speakers have to be fitted with a transformer which adds cost. Never seen 70/100V systems use standard 1/4" jacks, too much risk of someone getting a shock. Such systems were hard wired or used different connectors.

One source of confusion is the difference between professional and consumer definitions of "line level". For pro audio it's +4dBm 600 ohm, for domestic it's around 100mV high impedance, sort of kind, kind of :(

Bob.
farss wrote on 2/2/2016, 2:50 PM
[I]"The output impedance of the projector is unlikely to be anything like 8 ohms unless it's a newer one. "[/I]

The manual for the projector in question says it has two 16 ohm speakers. Test procedure recommends connecting an 8 ohm 15 W dummy load.

http://www.acofs.org.au/part_5_files/Graflex/Tech%202100%20series.PDF

Bob.
xberk wrote on 2/2/2016, 3:06 PM
Thanks everyone!

So if I go with the ULTRA-DI 100 as mentioned above, I would also need the following

1/4" TS Male to Male to go from Projector Jack to DI unit input

XLR to 3.5mm adapter to go from DI unit XLR output to Line in on the H1 Zoom

Is that right?

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musicvid10 wrote on 2/2/2016, 3:06 PM
Well, that settles that question.

[EDIT]
Still, I don't think a 20-watt non-inductive resistor is a good Idea (see below)
[/EDIT]

So, we're back to the original, and most often-employed solutions:
a. A passive transformer; or,
b. An active DI
Both rraud and I go for the second suggestion, as I read.
john_dennis wrote on 2/2/2016, 3:40 PM
If you were handy with a soldering iron, you could bypass the power amplifier altogether. On page 1.3, one side of C3 would have the output of the projector preamp as it goes to the volume control, tone control and then to the power amp. A one transistor power amplifier with 27db of gain is probably not the lowest distortion, most linear part of the audio signal chain.

Be aware of the possibility of DC voltages and don't try this at home if you don't feel confident that you understand what you're doing. Advice that I've ignored many, many times with varying degrees of success.

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John_Cline wrote on 2/2/2016, 4:45 PM
First of all, there is virtually no chance that the speaker output of the projector is set drive a 70 volt line. End of that discussion.

Secondly, the audio output of the projector is unbalanced and that would feed into the DI just fine, however, the XLR output of the DI is balanced, the 3.5mm input jack on the H1 is probably set up to accept a stereo signal on a TRS plug (tip, ring, sleeve), if you used that XLR to 3.5mm adapter cable you linked to and fed the H1 with the balanced output of the DI, it would feed a mono signal into the H1 and be recorded on two channels but the channels would be 180 degrees out of phase. This is not good at all. This input stage overload was an issue on all the Zoom recorders until the H6 which has a proper analog volume control on the inputs before the gain stage.

What you need is a simple, passive, unbalanced L-pad attenuator consisting of two resistors to reduce what is basically a line level output of the projector to a level that the H1 can ingest without distortion. This can be constructed with a cheap 10k potentiometer and the appropriate cables and TS connectors. You would plug the 3.5mm mono TS plug into the H1 and it would record on one channel of the stereo pair on the H1 and you would just use Vegas to select that channel only. Here's a diagram I made of a dead simple L-pad audio attenuator:



Using this attenuator, you would set the volume control on the projector to maybe 1/3 of the way up and set the recording level on the H1 to maybe "5" and then use the potentiometer on the attenuator to set the levels with the VU meter on the H1. You may have to experiment, but the H1 has a gain stage before its record level setting, so it is easy to overload the input of the H1 and it will distort no matter what record level you use. The attenuator reduces the input level to below the H1's overload point.
xberk wrote on 2/2/2016, 7:35 PM
Holy Cow ... that's tremendous John .. I so appreciate the effort .. I wish I understood it better.
A few questions from a rank amateur electrician. I'd love to attempt building this box.

By RCA jacks do you mean female RCA phono plugs with one wire going to the 10K Pot #3 for Input and #2 for Output jack.

Is Terminal #1 the ground? If so, where do I solder the ground wire?

Why do I need two channels, the projector will only output one channel.

Input cable would then be 1/4" TS to RCA male -- Output cable RCA male to 3.5mm TRS male.

Also, I didn't understand OUTPUT to DC-30 ?

Paul

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Chienworks wrote on 2/2/2016, 9:48 PM
"First of all, there is virtually no chance that the speaker output of the projector is set drive a 70 volt line. End of that discussion."

We had a bunch of 16mm projectors of different brands, mostly Bell & Howell, at school back when i was a tender lad and the kid who always got picked for the A/V setups. We always connected a normal 8-ohm speaker directly to the 1/4" output jack on the projector with nothing more than a straight cable. If it was a 70 volt line it would have fried the speakers.

If i try hard enough, i can even picture in my mind the ring around the 1/4" jack being stamped with "8 to 16 ohm speaker load", though it's probably been 35 years since i last looked at one of them.
John_Cline wrote on 2/2/2016, 11:32 PM
Before I answer all your questions, you can just buy this prebuilt stereo audio attenuator from Amazon for $11. Just use one channel.

Stereo Variable Audio Attenuator


"By RCA jacks do you mean female RCA phono plugs with one wire going to the 10K Pot #3 for Input and #2 for Output jack."

Yes, exactly. The RCA jacks would be panel mount jacks that simply screw in through a hole on the box with a nut on the back.

RCA Jacks

Aluminum Project Box

"Is Terminal #1 the ground? If so, where do I solder the ground wire?"

If you use an aluminum box, just mounting the jacks to the box will take care of the ground for the jacks. The RCA jacks typically come with a washer with a soldering tab, just use one of these tabs on either jack and solder a wire from terminal #1 of the potentiometer to the tab. All the grounds go through the aluminum box, which also shields the audio.

10k audio taper potentiometer with knob

"Why do I need two channels, the projector will only output one channel."

You don't. I made the graphic years ago for another project. Ignore the DC-30 part, that would be the output to the H1.

"Input cable would then be 1/4" TS to RCA male -- Output cable RCA male to 3.5mm TRS male."

Yes, input via an RCA to 1/4" TS cable. You could use an RCA to TRS 3.5mm cable as long as the pin on the RCA is connected to BOTH the tip and ring of the 3.5mm plug. You could also use an RCA to mono TS 3.5mm plug and it would only record on one channel of the stereo pair on the H1, it would be a very simple matter of telling Vegas to only use the active channel and ignore the silent channel. (Right-click on the audio track in Vegas, go to "Channels" and select "Right Only" or "Left Only" depending on which channel got recorded.)

RCA to mono 3.5mm TS cable

1/4" to RCA cable
musicvid10 wrote on 2/3/2016, 7:39 AM
I've been biting my tongue because of enormous amount of respect for my neighbor to the south.
The 99c solution being proposed sounds like a royal hum-fest to me.

If such a thing worked, we would all be carrying potentiometers around in our kits, and licensed venues wouldn't have rules requiring isolated direct boxes for every connection, including videography, recording, projectors, and the musicians' instruments and amplifiers themselves.

Dedicated signal and ground isolation is the big (but not the only) reason. A direct box will tame the hum and buzz, but not eliminate it, by breaking the ground loop and sequestering the signal from rogue AC.

Another reason is safety; older equipment such as projectors and tube amps that share chassis with signal ground will rock your world if even one outlet in your home grid has the hot and neutral reversed. Looking at a document like the one Bob linked (but is blocked in the US), I can't honestly tell if yours is vacuum tube or solid state.

That all being said, feel free to try the solution mentioned. It won't cost you much (it will "probably" pass the smoke test). If it hums, borrow or purchase an ultra-di, be sure to set the ground lift, and attenuate to line level. For forty bucks, it's something you'll use again, and cheaper than a new sound interface. Heck, you could even use a cheap Rolls Matchbox to test and compare the options being offered here, but with a little more thd.

OTOH, if your sound interface doesn't have balanced XLR inputs, the only suggestion that might be worth a try is John Cline's.

You will learn a lot from the process, I promise you. The safest and most effective solution is the one that was designed for the job.
Or, you could simply outsource the job to someone with more experience.

It will be interesting to hear how this works out for you. Please post back and let us know.

@John Cline - before the 1970's, sound projectors were all tube amplified, mostly 70v (at least in auditoriums and theaters), and every speaker had a wart (step-down transformer) attached. You could string speakers all over the place, using only a small fraction of the wire runs necessary for deploying four-packs of series-parallel matched speakers. I first learned carbon-rod era projection technology as a school kid, and kept my interest through the years.


SecondWind-SK wrote on 2/3/2016, 9:49 AM
We had a couple of B&H 16 mm projectors and a Kodak 16 mm in our recording studio. All were 50s to 60s manufacture. All were 8 ohm transformer outputs. We used a bridging transformer with 10 k primary and 600 ohm secondary to control hum. But, remember the OP is going into a battery operated device. No ground loop issues there.
musicvid10 wrote on 2/3/2016, 10:33 AM
"
If there were any transformers, they were stepped down form 25v or 70v line audio to 8 ohms -- almost universal in those days..

"
Precisely -- your inputs see a balanced, isolated, attenuated signal -- exactly what I've been recommending.

"

Not exactly. The loop originates in the unbalanced audio shield, and acts as an antenna for stray EMI, computer and cell phone crud, and even local country/western stations.

Signal path loops still persist if the signal ground is not isolated as you so nicely described above. It's so important to realize that the isolation transformer or direct box performs two essential functions -- Attenuation and signal path isolation from the muck.

John_Cline wrote on 2/3/2016, 3:19 PM
"@John Cline - before the mid-1970's, sound projectors were all tube amplified, mostly 70v (at least in auditoriums and theaters), and every speaker had a wart (step-down transformer) attached."

I have been an audio engineer since 1968, so I have a lot of experience with 25v and 70v distributed speaker systems, it is the most practical way to have long speaker runs using relatively thin wire. (It's the same principle as the power grid, to move large amounts of power long distances, step up the voltage for transmission (115,000 volts) and reduce it when it gets to the destination. In the U.S. typically 7,500 volts for distribution around town and then drop it to 220 volt three-phase at the house.) You're also talking about old tube equipment, tubes swing high voltage at relatively low current, you have to use transformers to get them to drive low voltage, high current 8-ohm loads. The Singer Graflex Insta-Load 16mm projector we're talking about does not have a 70 volt output.

Direct boxes came about originally because of the need to hook up bass guitars to a PA system. The output of guitars requires an unbalanced very-high-impedance input to keep from loading the guitar pickup and affecting its frequency response. Audio mixing consoles (desks) typically have low impedance balanced inputs, plus unbalanced high-impedance sources don't travel the hundred feet or more through the multicable to the front-of-house mixer very well without picking up a lot of noise along the way and rolling off the high end because of cable capacitance.

"Precisely -- your inputs see a balanced, isolated, attenuated signal -- exactly what I've been recommending.

The problem we're discussing here is reducing the unbalanced output of the projector to feed the unbalanced input of the H1 without overloading the input stage of the H1. Your suggestion doesn't deal with unbalancing the output of the DI to feed the unbalanced input of the H1. Most people would assume that getting a 3-pin XLR to TRS 3.5mm cable to interface the balanced, mic level output of the DI to the unbalanced input of the H1 will work, it will not. The H1 has a single TRS 3.5mm input with one channel on the tip and the other on the ring, if you were to plug the balanced output of the DI into into the H1 using the TRS 3.5mm cable, sure the audio signal would show up on both channels of the H1 BUT the channels would be exactly 180 degrees OUT OF PHASE with each other, if you pull the resulting audio file into Vegas, it will sound like the audio is coming from far left and far right if you monitor in stereo instead of appearing to come from the center like a mono signal should. Most people won't really notice this until they attempt to listen to it on something with a single mono speaker, like a smartphone. The two channels will sum together and since they are 180 degrees out of phase, they will cancel each other out and you will hear NOTHING. You could just use one channel of the stereo file in Vegas or you could invert the phase of one of the channels but you have to remember to do this. My unbalanced L-pad potentiometer attenuator solution keeps it all unbalanced and this solution is way cheaper than a good direct box. If Xberk were feeding the projector into an audio mixer with balanced mic-level inputs, then a direct box would be the ideal solution. That's EXACTLY what the DI was designed to do.

You COULD use a direct box to feed the H1 but you would need to construct an unbalancing cable that ties pins 1 and 3 of the XLR to the sleeve of the 3.5mm plug, and ties pin 2 of the XLR to BOTH the tip and ring of the 3.5mm plug.
musicvid10 wrote on 2/3/2016, 3:30 PM
"

H1 doesn't have balanced inputs?
I'm surprised.

But the solution is simple.
Connect only one leg of the balanced DI mono output to either TS or RS, with the center tap connected to the sleeve..
Let the other leg float, or terminate (in practice, it makes very little difference, and yes, I've run the tests)..
Safer for the preamp, still less hum, and still plenty of signal; the attenuation is adjustable.

Most importantly, the floating center tap of the balanced output provides a stationary AC reference voltage to the sleeve, quite unlike the audio shield upstream of the DI..

I don't have any glaring issues with the rest of what you've said.
John_Cline wrote on 2/3/2016, 3:59 PM
"Connect only one leg of the balanced DI output to either TS or RS. Let the other leg float, or terminate (in practice, it makes very little difference)"

The ONLY solution of unbalancing the output of a direct box, particularly if it has a transformer output (which all the cheaper ones do), is to use the method I described at the very end of my last message. You certainly can't let just one leg float disconnected, since virtually no direct boxes are set up using the center tap of a transformer.



This entire discussion was about one specific problem that Xberk has, all of the Zoom recorders up until the H6 had a fixed gain stage at the input followed by a digitally-controlled gain stage for adjusting the volume, the input stage will overload and distort if fed with a signal much hotter than -4 dbv regardless of where you set the H1's digital volume control. All he needs to do is attenuate the unbalanced signal from the projector so he can set the output volume of the projector at a healthy enough level to maximize signal-to-noise ratio AND attenuate the resulting signal enough to keep the fixed-gain input stage of the H1 from distorting. My L-pad potentiometer solution does just that and no more, and it's cheaper than buying a direct box and constructing a special unbalancing cable to interface the DI to the H1.
musicvid10 wrote on 2/3/2016, 4:09 PM
I eagerly await the results of your tests.

Floating the center leg in a direct box, whether a transformer output or active DI, is the whole purpose of the ground-lift switch. If your concerned about your transformer, slap a 300-ohm carbon resistor across the wild leg and the tap.

[EDIT] WRT your latest diagram, whether you split the phase or not using a center tap, as long as the unbalanced input sleeve is isolated from the upstream audio shield, you are in better shape than you were before. I consider the employment of a center tap to be a best practice because it is a rock-steady reference to the audio signal.