Microphone spec levels, generation, and calibration

[gx1400]

The OHIS spec calls out

The Microphone signal is a typical electret microphone signal: -45dBV +/- 3dbV into 600 to 1k ohm

Then goes on to describe the amplification needed for a dynamic mic

1. Should the spec section 3.3 be more clear about what an acceptable mic signal level is? Specifically " -48dBV to -42 dBV into a 1000 ohm resistive load"

I wonder if later in the spec you could outline the intent and methods for achieving those levels, for example the reference circuitry.

2. Are these dBV measurements measured as Peak voltage or RMS?

3. Do you have any feedback on how to reliably generate audio into the mic to verify those levels? Or would you ultimately expect the end user to verify audio levels with signal reports and adjusting mic gain settings?

I think I noticed a mic level adjust pot on the Halibut user adapter product. I was curious how a "OHIS compliant/calibrated mic setup might vary from user to user based on talking loudness and mic handling (distance from mic, etc)

5. Have you considered a recommended calibration procedure for a given mic and user adapter? I thought it would be cool to have a reference circuit that has a OHIS mic input, a bnc input for an signal/tone generator for calibration, maybe a fixed load and an oscilloscope output.

Then an end user could use a VU meter (even a few leds or an led bar segment, similar to an audio compressor meter) could help provide visual feedback that mic levels are appropriate for their hardware and talking style

[SmittyHalibut]

1. I think I did? Maybe I should make it more clear. But the spec is: "Electret level." Meaning that it's the normal level an electret microphone element puts out. The amplifier is to bring a dynamic mic up to the same level as an electret mic. Really, the numbers are a bit wishy-woshy and depend so much on how the user places the microphone from their mouth, the DC bias voltage given to the electret mic, etc. So there's a fair bit of play in these numbers. But they're all pretty darn close. Reference circuits are a good idea.
2. RMS. Thank you, I'll clarify that.
3. "Reliably" with microphones is either a) difficult and expensive to accomplish, or b) very hand-wave-y. In ham radio, most systems either have a mic gain adjustment (but never explain to the user how to properly adjust it sigh), or have a fixed gain and users just learn how close-to or far-away-from their mounts to hold the mic. There's generally about a 5 to 10dBV variance from all this, so there's not a lot of precision in these adjustments. Most hams just do their thing until and unless someone complains to them that their transmit audio is crummy.
   Point being, the OHIS spec is way more precise than pretty much anything else in ham radio. In the "worst" case, I think the user can rely on signal reports for adjustment. But the best case would be to include the tools to help them adjust their levels correctly. I'm going to add a trim-pot to the Open Source User Adapter design (Design Open Source User Adapter user-adapter#1 (comment)), but adding a VU meter is more complexity than I want on this design. I might make a VU meter in-line tester as a separate thing though. That's test equipment, not something that's needed all the time.
4. There is NO RULE NUMBER SIX!
5. Calibrating the mic level requires the individual user, plus the microphone, and any mic pre-amp, to be included in the calibration. Using a signal generator won't help because the electronics are all fixed anyway. What you're calibrating are the variances in the user's speaking volume, the microphone's sensitivity, and the placement of the microphone on that user's head. So there's no need for a signal generator. But, a VU meter would be very handy. One can easily argue a VU meter for the headphones would also be useful, though it's assumed that the output volume will be adjusted in real time by the user turning the volume knob on the radio. Adjusting the volume out of the radio toward the headphones is more about equalizing the radio's speaker output and headphone output so that when you pull the OHIS connector out of the radio and it switches to the speaker, you don't get a sudden loud burst of noise.

See #16

[gx1400]

1. User error on my side, but I do think there is some redundancy and conflicting information between section 3.3 and 5.3. I think the spec is a little confusing in that 3.3 describes a 'typical' signal out into a load with a range of 600-1000 ohms. I think Section 5.3 is again describing the impedance of the microphone element itself. I wonder if your intent here was to provide contextual information, which is great but I think as a technical end user of a spec, it can muddy the "requirements" and lead to misinterpretation.

   3.3. Electrical Specification
   
   Microphone: The Microphone signal is a typical electret microphone signal: -45dBV +/- 3dbV into 600 to 1k
   ohm. The Radio device provides a DC bias voltage to power the microphone element, about 0.5mA across a
   2vDC microphone element (eg: 5vDC through 6.8k ohm resistor). Dynamic microphone elements require about
   10 to 15db of amplification, and for the DC bias to be blocked; this Standard provides a simple 5 component
   microphone pre-amp, powered by the DC bias, to bring a Dynamic microphone element up to Electret signal
   levels.
   

   I would suggest simplifying to:

   3.3 Electrical specification
   
   Microphone: The Microphone signal level shall be: -45dBV +/- 3dbV RMS into 600 ohm. 
   
   This level is typical of electet microphones. See section 7.1 for notes on amplifying Dynamic element microphones.
   

   Or even better, I might suggest to nix Section 3.3 completely and allow for 5.3 to be the sole place you give actual numbers. Having it in multiple places can be confusing in a technical document.

2. Thanks for the consideration!
3. Nix this comment. Good feedback, entirely dependent on user and their setup. Per the VU meter, I'll comment over on #16
4. Sorry auto formatting and retyping sections in markdown is messy, I believe I typed this on my phone.
5. No additional comments!

[SmittyHalibut]

1. Finally back at my computer not my phone, I actually reviewed 3.3 and 5.3, and I agree with you. I'll greatly simplify Section 3.3 (not just Microphone) to be just a very quick summary, and point to 5.3 for full details. However, I will include the DC bias voltage in the summary, not just the signal level. But I won't go into details, I'll just say that its needed.
2. (nothing)
3. (stupid markdown)
4. I never pass up an opportunity to throw in a good Monty Python quote. :-)