Getting Audio right with Virtual Sacrament Meeetings

[russellhltn]

At the risk of flogging a horse, here's an article, VU AND PPM AUDIO METERS – AN ELEMENTARY EXPLANATION.

The line out, being an analog with old standards, the 316mV is more of a perceived loudness "target" (VU). Some overhead is allowed for peaks.

The phone on the other hand, with it's A/D hard limit, 0dbFS is more of a "never exceed" value.

The article suggests mapping 0 VU to -20dbFS. Which means you'd need a 40dB attenuator.

[CalS201]

Nice ASCII art. I'm stealing that to illustrate my DIY cable

That's plagiarism !

The iPhone/iPad devices have a maximum microphone level of about 30mv so a 20db pad (divide by 10) correctly reduces the 300mv signal from the record out jack.

Nit-pick, 0VU for consumer line-out is −10 dBV, or 0.316Vrms, 0.447Vpeak. Not 0.300V.

I never equated 0.300V to 0VU or -10dbV

But if the iPhone distorts at 35, that's not leaving you any overhead for peaks. (difference between VU and peak reading meters/peak light) I'd think you'd want at least an additional 10dB so to have some headroom. Especially since we don't know if there's any compressors or limiters in the chain.

While we want a good solid signal, what we really need is something our apps can work with comfortably without having to turn up the gain to the point we start noticing the noise floor.

At the risk of flogging a horse, here's an article, VU AND PPM AUDIO METERS – AN ELEMENTARY EXPLANATION...............The article suggests mapping 0 VU to -20dbFS. Which means you'd need a 40dB attenuator.

I will try to explain why a 20db (divide by 10) pad works better than a 40db pad (divide by 100) .

I first measured the peak and rms amplitude at the record out jack using an oscope on a 10 second sweep. See attachment. I measured the iphone mic input amplitude by using the recording app to record as I stepped a 1khz sine wave from 15mv to 20,25,30,35,40, and 45mv rms. I found the maximum level (before clipping) by playing back the recording and observing the results on an oscope. Clipping occurs between 35-40 mvrms, which is 50-57mv pk. The ascii art diagram shows the results.

Code: Select all

RecordOut                                      iPhone  Android Android
  Jack                                          iPad      1       2
                                                mvFS     mvFS    mvFS 
    
325/650mv------  peak--------------* *---------- 50       35      18
 |   |                          *       *
 |   |                                           35rms    25      13
 |   |                        *           *
 |   |
 57  113rms*                *              *
 |   |   -----------------------------------------
 |   |      *              *                *
 |   |
 |   |       *            *                  *
 |   |
 |   |        *          *
 |   |         *        *
 |   |            *  *
 |   |
 |   --Speaking overly loud 2" from pulpit mic, 
 |     saying "test, test, one, two, three"
 |
 --Estimated normal loud speaking at normal distance

Arriving at the attentuator division ratio that will cut the RecordOut Jack level down to mic level for the phone is pretty simple when you know the maximum peak level "expected" at the RecordOut jack and the peak level "allowed" (before clipping) into the phone.


FOR IPHONE
For loud speech at normal distance from pulpit mic its 325mv pk / 50mv pk = 6.5 (about 16db)
For overly loud speech 2" from pulpit mic its 650mv pk / 50mv pk = 13 (about 22db)

A purchased 20db pad (divide by 10) or the DIY pad I suggested earlier (22db,divide by 11.5) should work OK,

FOR ANDROID #2
For loud speech at normal distance from pulpit mic its 325mv pk / 18mv pk = 18 (about 25db)
For overly loud speech 2" from pulpit mic its 650mv pk / 18mv pk = 36 (about 31db)

A purchased or DIY 30 pad should work OK.


Using a 40db pad (divide by 100) with either phone results in a signal level less than a tenth of the phone's fullscale input, which forces you to somehow raise the gain (and noise) by 10X.


First plagiarism , then nitpicking , now flogging horses . What has happened to these forums? Maybe I need to contact the forum administrator.

[russellhltn]

I've figured out the discrepancy. It turns out the phone I was using for development (Pixel 1) is very, very sensitive. 40dB attenuation is a good start, but still pretty strong. The Pixel 3a is not quite as sensitive, but still works well with -40. The Droid Maxx seems to do OK with it, but could use a little more volume. But -40dB is way too much attenuation for my laptop. (Adjusting for the right plug).

Clearly, one attenuator that works for everyone isn't possible.

Also, I was mis-remembering the value I was using. It's -40dB, not -50.

[ElderMarkClark@pm.me]

99% of the time, the RCA jacks are an input outlet on Church PA systems.

Mark Clark
STS Gresham Stake Oregon

[snihipali99]

Here is another way of reducing the line-level signal from record output jack to mic-level that is required by most laptops and iphones. It also presents the proper impedance to the computer for it to recognize the external mic signal. It uses the EJ8 (crab box) that many bldgs already have. It perfroms the same functions as the "Headset Buddy Line-Level Adapter" https://www.amazon.com/Headset-Buddy-Li ... B00OAW85ZG

See attached PDF.

Thinking of trying this out, so to clarify those 1/4" plugs can be used as an output of they are both used and combined?

I have 2 buildings with the same setup using the record out to a usb audio adapter for the computer, streaming using OBS. One building which is actually older the sound is clear and no issues. The other building there is some distortion and seems to be too loud. I tried using OBS audio filters which help some but if speakers get too close to the microphone or talk loud then the distortion can be heard. I want to use the EJ8 box to project music as some wards don't have a pianist and play music instead which I have it setup to do. But trying to see if I could use it as well to control the record out for the one building with audio issues.

[CalS201]

Referring to the EJ8 "quick-but-dirty lashup" I mentioned in earlier post.

Thinking of trying this out, so to clarify those 1/4" plugs can be used as an output if they are both used and combined?

Yes, any and all of the 1/4" and 1/8" jacks on the EJ8 can be used as inputs or as outputs, but combining 2 outputs is only needed when you have to connect to a phone or laptop with TRRS jack that needs to see a particular impedance(about 1-2K ohm).

I have 2 buildings with the same setup using the record out to a usb audio adapter for the computer, streaming using OBS. One building which is actually older the sound is clear and no issues. The other building there is some distortion and seems to be too loud. I tried using OBS audio filters which help some but if speakers get too close to the microphone or talk loud then the distortion can be heard.

Have you checked and lowered the USB Audio Adapter MIC "gain" in the laptop's ControlPanel/Sound settings? Doing so may solve your distortion issue. Having said that, the EJ8 lashup will lower the RecordOut jack signal level. Just connect the RecordOut Jack to one of the 1/8" EJ8 jacks (with a TS or TRS cable) and connect the USB Audio Adapter MIC jack to one of the 1/4" (you don't need to combine 2 jacks) EJ8 jacks(with a TRS cable). The signal at the USB adapter should then be less than 1/6 of the RecordOut level.

An alternative to the EJ8 lashup is the https://www.amazon.com/Movo-MV-RC300-Mi ... B073HR6SY4.

I want to use the EJ8 box to project music as some wards don't have a pianist and play music instead which I have it setup to do. But trying to see if I could use it as well to control the record out for the one building with audio issues.

You can use the EJ8 to input music to the PA system "OR" use it to output the PA system to your laptop. You cannot do both at the same time with one EJ8.

[lajackson]

The other building there is some distortion and seems to be too loud.

Is it possible that the building sound system itself is overdriving the record out line?

[snihipali99]

An alternative to the EJ8 lashup is the https://www.amazon.com/Movo-MV-RC300-Mi ... B073HR6SY4.

Thanks, I ordered this to try out. I was able to test with the EJ-8 and the sound was great, hopefully this attenuator will perform the same.

On another note the building where the audio is good, it is running a laptop on Windows 10 while the building where it is not is a desktop running Windows 7. I tried going through the sound settings in the control panel for the computer running Windows 7 and the levels don't seem to have an affect on the sound. It made me think that maybe Windows 10 or the laptop is better at controlling the sound.

[CalS201]

On another note the building where the audio is good, it is running a laptop on Windows 10 while the building where it is not is a desktop running Windows 7. I tried going through the sound settings in the control panel for the computer running Windows 7 and the levels don't seem to have an affect on the sound. It made me think that maybe Windows 10 or the laptop is better at controlling the sound.

Drilling down to the correct sound setting can be confusing on a Windows computer. You are looking for the "INPUT" settings of the "USB Audio Device". If your computer has a built-in audio device then the "USB Audio Device" is a 2nd device, so make sure you select/confirm which device you are adjusting. For example, my laptop has 3 audio input devices and 2 audio output devices:

INPUTS
Microphone (USB Audio Device)
Microphone (Logitech Pro for Notebooks)
Microphone (Realtek High Definition Audio)

OUTPUTS
Speakers(Realtek High Definition Audio)
Speakers(USB Audio Device)

Selecting the wrong one might explain why your adjustments have no effect.

[CalS201]

On another note the building where the audio is good, it is running a laptop on Windows 10 while the building where it is not is a desktop running Windows 7. I tried going through the sound settings in the control panel for the computer running Windows 7 and the levels don't seem to have an affect on the sound. It made me think that maybe Windows 10 or the laptop is better at controlling the sound.

Drilling down to the correct sound setting can be confusing on a Windows computer. .............................................
Selecting the wrong one might explain why your adjustments have no effect.

This forum post about the USB adapter and settings on the ControlPanel may be helpful to you.
viewtopic.php?f=27&t=37779#p216128