ruffrecords

1KHz 0dBu pocket generator
« on: December 04, 2020, 06:06:01 PM »
I see lots of questions about signal and noise levels when people interface external mic pres with a DAW. The issue is usually they have no idea of how real world signal levels translate into dBFS readings in their DAW and the interface and DAW manufacturers don't make this any easier. So I wondered if there is a project for a DIY 1KHx 0dBu sine wave generator.? Fits in your pocket and powered by a PP3 or similar. You plug it into you interface and you instantly know how many dBFS equals 0dBu. Or maybe this exists aleady?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


EmRR

Re: 1KHz 0dBu pocket generator
« Reply #1 on: December 04, 2020, 06:34:14 PM »
I dunno, there are a lot of basic battery powered signal generators /  cable testers in the $30-$50 range already, and if people can't read the manual to see the 0dBFS rating of their interface, they're kinda hopelessly lost already, I don't think another device is going to ultimately help, not with that extremely basic level of operational understanding.   What does 0dBu mean in the field anyway?  It depends on the device feeding the converter anyway, and that's a moving target requiring another level of understanding that no simple tool will elucidate.  I have preamps with max headroom of about +6dBm, I have others that are +36dBm; it's another number from a manual they may not read. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Matador

Re: 1KHz 0dBu pocket generator
« Reply #2 on: December 04, 2020, 09:14:43 PM »
Raspberry Pi for $35?  It supports the ALSA standard, and has a 12-bit DAC chip that drives the stereo headphone/audio output.

Seems like a simple C program to calculate a basic sine table (or any arbitrary waveform) into a buffer, then pump the buffer out the audio jack (forever), and scale it properly for 0.775 Vrms output as measured at the jack.  You could likely send two different frequencies (one for the left, and another for the right) if that's what you wanted, or even send a balanced signal of opposite phases out to an XLR.

Probably less than 100 lines!

Re: 1KHz 0dBu pocket generator
« Reply #3 on: December 04, 2020, 10:23:17 PM »
Raspberry Pi for $35?  It supports the ALSA standard, and has a 12-bit DAC chip that drives the stereo headphone/audio output.

Seems like a simple C program to calculate a basic sine table (or any arbitrary waveform) into a buffer, then pump the buffer out the audio jack (forever), and scale it properly for 0.775 Vrms output as measured at the jack.  You could likely send two different frequencies (one for the left, and another for the right) if that's what you wanted, or even send a balanced signal of opposite phases out to an XLR.

Probably less than 100 lines!

Much cheaper to use an ICL8038 or XR2206, they are discontinued but still very much available, 0 lines of code. Or you can always build a good old Wien Bridge oscillator. The ICL8038 and XR2206 have the advantage of also being able to generate square waves and triangle waves besides a sine.
« Last Edit: December 04, 2020, 10:27:16 PM by Dualflip »

Whoops

Re: 1KHz 0dBu pocket generator
« Reply #4 on: December 05, 2020, 12:53:55 AM »
So I wondered if there is a project for a DIY 1KHx 0dBu sine wave generator.? Fits in your pocket and powered by a PP3 or similar.

Hi Ian, would it have a Balanced output?
« Last Edit: December 05, 2020, 07:50:31 AM by Whoops »

squarewave

Re: 1KHz 0dBu pocket generator
« Reply #5 on: December 05, 2020, 01:29:50 AM »
I just make a reference tone in Audacity, play that out and adjust the level relative to something like 0dB on a VU meter or whatever. After I establish what the level is I don't touch any knobs, I play the ref tone again while simultaneously recording 10s to another track. Then I pan the ref tone hard left, the recording hard right and export that to a stereo wav file with a filename that has the ref tone description / level in the name. Now I have an historical record of the DUT's performance that I can compare to later.

A digitally generated tone is probably going to be a little cleaner as it's really limited to what your DAC can do.

plimousse

Re: 1KHz 0dBu pocket generator
« Reply #6 on: December 05, 2020, 04:07:17 AM »
Just a little generator.
Cheers,
P.

ruffrecords

Re: 1KHz 0dBu pocket generator
« Reply #7 on: December 05, 2020, 04:24:35 AM »
Hi Ian, would be a Balanced output?

It would have to be to be useful for most interfaces. But it could be impedance balanced.

Edit: On the other hand, since it is battery operated it would be floating anyway so you could just connect the output to pins 2 and 3 of an XLR. The interface would not be able to tell it is not balanced.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

ruffrecords

Re: 1KHz 0dBu pocket generator
« Reply #8 on: December 05, 2020, 04:39:33 AM »
Much cheaper to use an ICL8038 or XR2206, they are discontinued but still very much available, 0 lines of code. Or you can always build a good old Wien Bridge oscillator. The ICL8038 and XR2206 have the advantage of also being able to generate square waves and triangle waves besides a sine.
I rememeber those. Unfortunately they both need a minimum 10V supply which is just outside the range of a PP3.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: 1KHz 0dBu pocket generator
« Reply #9 on: December 05, 2020, 04:51:13 AM »
Just a little generator.
Cheers,
P.

Thanks for posting!


ruffrecords

Re: 1KHz 0dBu pocket generator
« Reply #10 on: December 05, 2020, 04:53:44 AM »
Just a little generator.
Cheers,
P.
I think a cut down version of that is exactly what we need. Thanks for posting.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

gyraf

Re: 1KHz 0dBu pocket generator
« Reply #11 on: December 05, 2020, 06:43:39 AM »
IMO, what we need to calibrate is a given setup's output level, not its input sensitivity (although they often correlate and can be abstracted).

I usually advice to generate a 100Hz waw-file @0dB, play it back at maximum level, full scale, then measure output AC between xlr pins2&3 with a cheap multimeter. A 100hz range makes this measurable with needed precision on cheap auto-repair-type of multimeters

And then I advice to calibrate so that 0dBfs equals +14dBu (3.88V AC RMS/AVG) plus/minus 2dB according to taste

We decided on the approximate target level of +14 back in the 90'es when we did a listening survey of the whole bunch of amplifiers, processors and stuff that we had in our studios: Although most of all that is meant to be pro audio, and will happily work on +22 without clipping, it was really worrying to find how much of it did not taste well at all at those levels - it was clear that the bulk of analogue equipment sounded significantly better at +12 than at +20, despite NOT clipping. Only notable exceptions being the GML8200 and the Sontec we had briefly at the time.

Looking deeper into this at some later time we ended up concluding that the findings were most probably due to slewrate limiting when interfacing those very-high levels

Yes, I've been considering making a ultra-simple VU-meter, no fancy mechanism or time constants, to ship with purchase to all new customers. I do believe it would be worth it in hours spent getting this basic reference right..

/Jakob E.
..note to self: don't let Harman run your company..

Re: 1KHz 0dBu pocket generator
« Reply #12 on: December 05, 2020, 08:00:55 AM »
Hello


And then I advice to calibrate so that 0dBfs equals +14dBu (3.88V AC RMS/AVG) plus/minus 2dB according to taste


I'm surprised, this is very low !
What is your studio line level ? 10 dB below ?

Best
Zam

ruffrecords

Re: 1KHz 0dBu pocket generator
« Reply #13 on: December 05, 2020, 10:23:24 AM »
I have found a 14 pin PIC that costs about 80 pence that has an 6 bit DAC. It also contains an ADC so we could include a simple LED 0dBU input level indicator too - maybe 3 leds, -0.5dBu, 0dBu and +0.5dBu.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

JohnRoberts

Re: 1KHz 0dBu pocket generator
« Reply #14 on: December 05, 2020, 10:59:39 AM »
I see lots of questions about signal and noise levels when people interface external mic pres with a DAW. The issue is usually they have no idea of how real world signal levels translate into dBFS readings in their DAW and the interface and DAW manufacturers don't make this any easier. So I wondered if there is a project for a DIY 1KHx 0dBu sine wave generator.? Fits in your pocket and powered by a PP3 or similar. You plug it into you interface and you instantly know how many dBFS equals 0dBu. Or maybe this exists aleady?

Cheers

Ian
First I had to figure out what a PP3 is (9v battery).

More than enough PS voltage to make a conventional technology analog low distortion, amplitude stabilized, 0dBu audio signal (0.775Vac). In addition I would be tempted to add a low battery voltage indicator to increase trust in the signal integrity, since weak batteries could degrade output. 

I try to avoid hardware products that can be covered with a smartphone app but modern smart phones seem less serious about providing audio output signals.

====

I did a quick survey of VOM /multimeters to see if any provide a reference audio output, easy enough to include, the closest I found was one that provided a 20ma current loop output.

====

For today's TMI, over the decades I scratched out a lot of discrete design ideas after beer o'clock to make a simple multi-capability pocket audio troubleshooting tool. This would work from a single AA, or maybe AAA cell for compact size.

The basic concept is a simple astable multivibrator. When the probe leads are unloaded (open circuit) the circuit stops oscillating to conserve battery life. The frequency of oscillation is determined by the load resistance. In an ideal world for audio troubleshooting this would cover impedances between 4/8 ohm for speakers, 150-600 ohm for mics, 2k for mic preamp inputs, and 10k for line level inputs.

The output would basically be a square wave and the voltage would scale so the highest output level would occur for line inputs (10k), and significantly lower into mic inputs (2k).

The square wave would crudely excite loudspeakers with audible sound.

This is not for lining up 0VU levels, but crude troubleshooting to find open wire lead connections, and faulty signal paths.

Needless to say I never progressed to melting solder, and it seems like wishful thinking to cover loudspeaker impedances and line inputs without at least a range switch.

===
Now after going over to the dark side I need to see about microprocessors that will run from 1V battery power.

JR       

@Ian wrt PIC, they make many cheap ones with 8 bit a/d built in. I don't remember finding real audio DACs built in until the more serious DSPics (16b), but I haven't looked lately... I made lots of serviceable enough sine waves for my drum tuners using cheap PIC PWM outputs (smoothed by LPF). I need to see what they sell now that will run down to 1V battery.... actually never mind, I pass. 
Cancel the "cancel culture", do not participate in mob hatred.

rackmonkey

Re: 1KHz 0dBu pocket generator
« Reply #15 on: December 05, 2020, 11:20:43 AM »
There are numerous function generator apps for the iPhone, and probably for Android. Other than needing an external adapter to connect the phone, is there a reason why you wouldn’t go the simple route of finding the right app? What am I missing?
Whether you think you can or you think you can't, you're probably right.

abbey road d enfer

Re: 1KHz 0dBu pocket generator
« Reply #16 on: December 05, 2020, 12:05:57 PM »
I used an mp3 player for that.
I have loaded sinewaves and music. I used it quite often for line identification. Indeed, as Ian wrote, it is floating, so it's compatible with balanced and unbalanced lines.
Unfortunately, it let me down (battery dead), and since recording sessions are very few and far between, I have not yet bought a replacement.
Only issue is level is quite low.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: 1KHz 0dBu pocket generator
« Reply #17 on: December 05, 2020, 11:34:53 PM »
What's wrong with a Wien Bridge osc? you can make one with very little parts, you can add an AGC for low distortion and predictable output, and it takes very few parts. It can be used with a 9V battery.

ruffrecords

Re: 1KHz 0dBu pocket generator
« Reply #18 on: December 06, 2020, 03:19:13 AM »
What's wrong with a Wien Bridge osc? you can make one with very little parts, you can add an AGC for low distortion and predictable output, and it takes very few parts. It can be used with a 9V battery.
Nothing at all. See post #6

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

boji

Re: 1KHz 0dBu pocket generator
« Reply #19 on: December 06, 2020, 05:59:11 AM »
Need to engrave fader faceplates soon, among other calibrations.
Embarrassed to say I threw money at this problem by purchasing a NTi Minirator.

Timley thread! Thanks everyone.


 

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